اوگانسون

از ویکی‌پدیا، دانشنامهٔ آزاد
پرش به ناوبری پرش به جستجو
فارسیEnglish
تنسیناوگانسونآن‌ان‌نیوم
Rn

Og

(Uho)
ظاهر
ویژگی‌های کلی
نام، نماد، عدد اوگانسون، Og،‏ 118
تلفظ به انگلیسی گوش دهیدi‎/nnˈɒktiəm/‎
oon-oon-OK-tee-əm
گروه، دوره، بلوک ۱۸، ۷، p
جرم اتمی استاندارد [294] گرم بر مول
آرایش الکترونی (predicted) [Rn] 5f14 6d10 7s2 7p6[۱]
الکترون به لایه (predicted) 2, 8, 18, 32, 32, 18, 8[۱]
ویژگی‌های فیزیکی
چگالی (نزدیک به دمای اتاق) (predicted) 13.65[۲] g·cm−۳
نقطه جوش (extrapolated) 350±30[۱] K،‎ 80±30 °C،‎ 170±50 °F
نقطه بحرانی (extrapolated) 439[۳] K، 6.8[۳] MPa
گرمای هم‌جوشی (extrapolated) 23.5[۳] کیلوژول بر مول
گرمای تبخیر (extrapolated) 19.4[۳] کیلوژول بر مول
ویژگی‌های اتمی
وضعیت اکسید شدن 0, +2[۴], +4[۴]
(predicted)
انرژی‌های یونش نخستین: (extrapolated) 975±155[۱] ر
دومین: (extrapolated) 1450[۵] کیلوژول بر مول
شعاع اتمی (predicted) 152[۲] pm
شعاع کووالانسی (extrapolated) 230[۵] pm
متفرقه
عدد کاس 54144-19-3[۶]
پایدارترین ایزوتوپ‌ها
مقاله اصلی ایزوتوپ‌های اوگانسون
ایزوتوپ NA نیمه‌عمر DM DE (MeV) DP
294Og [۷] syn ~0.89 ms α 11.65 ± 0.06 290لیورموریوم

اوگانسون نام رسمی برای عنصر فوق سنگین با عدد اتمی ۱۱۸ و نماد شیمیایی Og است. هم چنین با اکارادون یا عنصر ۱۱۸ شناخته می‌شود و در جدول تناوبی عناصر، یک عنصر بلوک p و آخرین عنصر دوره هفتم است. اوگانسون در حال حاضر تنها عضو مصنوعی گروه ۱۸ می‌باشد. بزرگترین عدد اتمی و جرم اتمی بین عناصر کشف شده تاکنون را دارد.

پرتوزایی اتم اوگانسون، بعلت جرم زیادش، بسیار ناپایدار است و از ۲۰۰۵، فقط سه یا احتمالاً چهار اتم از ایزوتوپ‌های 294Og شناخته شده‌است.[۸] با وجود مشخصات خیلی کم تجربی مربوط به خواص و ترکیب‌های ممکن آن، محاسبات نظری منجر به پیش‌بینی‌های متعدد و بعضاً غیرمنتظره‌ای شده‌اند. برای مثال، اگرچه اوگانسون عنصر گروه ۱۸ است، ممکن است برخلاف دیگر عناصر گروه ۱۸، یک گاز نجیب نباشد.[۱] پیشتر تصور می‌شد تحت شرایط استاندارد یک گاز باشد اما اکنون به علت اثرات نسبیتی، یک جامد شناخته می‌شود.[۱]

تاریخچه[ویرایش]

تلاش‌های ناموفق ترکیب[ویرایش]

در اواخر ۱۹۹۸، فیزیکدان لهستانی روبرت اسمولانژوک محاسبات مربوط به همجوشی هسته اتم برای ترکیب اتم‌های فوق سنگین شامل اوگانسون را منتشر کرد.[۹] محاسبات او نشان داد که می‌توان اوگانسون را با همجوشی سرب با کریپتون در شرایط کنترل شدهٔ دقیق، به وجود آورد.[۹]

در ۱۹۹۹، محققان آزمایشگاه ملی لارنس برکلی از این نتایج استفاده نمودند و خیلی زود بعد از انتشار نتایج در ساینس،[۱۰] در مطلبی منتشر شده توسط Physical Review Letters اکتشاف لیورموریم و اوگانسون را اعلام کردند.[۱۱] محققان وقوع واکنش زیر را گزارش دادند.

86
36
Kr
+ 208
82
Pb
293
118
Uuo
+ n

سال بعد، وقتی پژوهشگران دیگر آزمایشگاه‌ها قادر به تکرار نتایج نشدند و آزمایشگاه برکلی هم به خوبی نتوانست نتایج را تکرار کند، آن‌ها اقدام به عذرخواهی نمودند.[۱۲] در ژوئن ۲۰۰۲، مدیر آزمایشگاه اعلام کرد که ادعای اصلی کشف این دو عنصر بر اساس اطلاعات جعل شده توسط مؤلف اصلی ویکتور نیکوف بوده‌است.[۱۳][۱۴]

گزارش‌های اکتشاف[ویرایش]

نخستین مشاهدهٔ زوال اتم‌های اوگانسون در ۲۰۰۲ و در مؤسسه مشترک تحقیقات هسته‌ای (JINR) توسط یوری اوگانسیان و گروهش در دوبنا روسیه انجام گرفت.[۱۵] در ۹ اکتبر ۲۰۰۶، محققان JINR و آزمایشگاه ملی لارنس لیورمور کالیفرنیا آمریکا، که در JINR دوبنا مشغول به فعالیت هستند، اعلام کردند[۱۶] که به شکل غیرمستقیم مجموعاً سه (احتمالاً چهار) هستهٔ اوگانسون-۲۹۴ (یک یا دو در 2002[۱۷] و دو تای دیگر در ۲۰۰۵) حاصل از برخورد اتم‌های کالیفرنیم-۲۴۹ و یون‌های کلسیم-۴۸ را شناسایی کرده‌اند.[۱۷] and two more in 2005) produced via collisions of کالیفرنیم-249 atoms and calcium-48 ions.[۱۸][۱۹][۲۰][۲۱][۲۲]

249
98
Cf
+ 48
20
Ca
294
118
Uuo
+ ۳ n
مسیرهای واپاشی پرتوزایی ایزوتوپ اوگانسون-۲۹۴. @ انرژی واپاشی و نیمه‌عمر متوسط برای ایزوتوپ همراه و هر ایزوتوپ دختر داده شده‌اند. شکستگی اتم‌ها تحت شکافت خود به خود (SF) به رنگ سبز نشان داده شده‌است.

در ۲۰۱۱، آیوپاک نتایج همکاری ۲۰۰۶ دوبنا-لیورمور را مورد بررسی قرار داد و نتیجه گرفت: «سه مورد گزارش شده برای ایزوتوپ Z = ۱۱۸ فراوانی داخلی خیلی خوبی دارند ولی هیچ لنگری برای هستهٔ مشخص نیست تا در حد معیار اکتشاف خود را نشان دهد.»[۲۳]

به علت احتمال خیلی پایین واکنش همجوشی (مقطع همجوشی حدود 0.3-0.6 pb است) آزمایش چهار ماه طول کشید و دوز پرتو ۴×۱۰۱۹ یون کلسیم را که باید به کالیفرنیم شلیک می‌شدند، به کار گرفت تا اولین واقعهٔ ثبت شده به عنوان ترکیب اوگانسون به وقوع بپیوندد.[۶] با این وجود، محققان اطمینان زیادی دارند که نتایج مثبت نیستند، چون احتمال اینکه تشخیص‌ها تصادفی نبوده باشند، کمتر از یک در ۱۰۰۰۰۰ تخمین زده شده بود.[۲۴]

در آزمایش‌ها، واپاشی آلفای سه اتم اوگانسون مشاهده شدند. واپاشی چهارمی با شکافت خود به خود مستقیم نیز مطرح شد. نیمه عمر ۰٫۸۹ میلی‌ثانیه محاسبه شد: 294Og توسط واپاشی آلفا به 290Og واپاشیده می‌شود. چون فقط سه هسته وجود دارد، نیمه عمر مشتق شده از طول عمرها عدم قطعیت بالایی دارد: ۰٫۸۹+۱٫۰۷
−۰٫۳۱
 ms
.[۱۶]

294
118
Uuo
290
116
Lv
+ 4
2
He

شناسایی هستهٔ 294Og مستقیماً با بمباران 245Cm با یون‌های 48Ca و ایجاد منحصراً 290Lv به اصطلاح هستهٔ دختر،

245
96
Cm
+ 48
20
Ca
290
116
Lv
+ ۳ n,

و با بررسی اینکه واپاشی 290Og با زنجیره واپاشی هسته 294Og همخوانی داشت، مورد تأیید قرار گرفت.[۱۶] 290Lv یا هستهٔ دختر بسیار نا پایدار است و با طول عمر ۱۴ میلی‌ثانیه به 286Fl واپاشیده می‌گردد و ممکن است شکافت خود به خود یا واپاشی آلفا به 282Cn که در واقع نتیجهٔ شکافت خود به خود است، نیز روی دهند.[۱۶]

در مدل تونل-کوانتومی، نیمه عمر واپاشی آلفای 294Og مقدار ۰٫۶۶+۰٫۲۳
−۰٫۱۸
 ms
با انرژی تولیدی (Q-Value) تجربی در ۲۰۰۴ منتشر شد.[۲۵] محاسبه با میزان انرژی (Q-Value)های نظری از مدل ماکروسکوپی-میکروسکوپی مونتیان-هافمن-پاتیک-سوبیچفسکی نتایج تقریباً پایین ولی قابل مقایسه‌ای می‌دهد.

نامگذاری[ویرایش]

تا دههٔ ۱۹۶۰ اوگانسون با عنوان اکا-اماناتیون (اماناتیون نام قدیمی رادون است) شناخته می‌شد.[۲۶] در ۱۹۷۹ آیوپاک توصیه نامه‌هایی در خصوص نامگذاری عنصر با نام آن‌ان‌اکتیم، یک نام با قاعده ی جایگزین تا زمانی که اکتشاف عنصر تأیید شود و آیوپاک اقدام به نامگذاری آن کند، منتشر نمود.[۲۷]

قبل از عذرخواهی ۲۰۰۲، محققان برکلی نام گیورسیم (Gh) را به خاطر آلبرت گیورسو (عضو برجستهٔ تیم تحقیقاتی) در نظر گرفته بودند.[۲۸]

مکتشفان روسی در سال ۲۰۰۶ خبر از کار خود دادند. در ۲۰۰۷، رئیس مؤسسه روسی عنوان کرد که تیم دو نام را برای عنصر جدید لحاظ می‌کند: فلایوریم به افتخار گئورگی فلیرف، مؤسس آزمایشگاه تحقیقاتی در دوبنا؛ و مسکوویم جهت شناساندن استان مسکو، جایی که دوبنا در آن قرار دارد.[۲۹] او هم چنین عنوان نمود که اگرچه عنصر در یک همکاری آمریکایی، منجر به کالیفورنیم، به کشف رسیده‌است، اما به درستی، عنصر باید به افتخار روسیه نامیده شود زیرا که آزمایشگاه واکنش‌های هسته‌ای فلروف در JINR تنها جایی در دنیا بود که می‌توانست امکانات این کشف را فراهم کند.[۳۰] این نام‌ها بعد تر برای فلروویم (فلروویم) و لیورموریم (مسکوویم) مطرح شدند.[۳۱] البته، نام نهایی مطرح شده برای عنصر ۱۱۶ لیورموریم بود.[۳۲]

هیچ نامی به‌طور رسمی برای عنصر پیشنهاد نشده‌است زیرا که هیچ ادعایی مبنی بر کشف آن توسط آیوپاک قبول نشده‌است. طبق دستورالعمل‌های فعلی آیوپاک، نام نهایی تمام عناصر باید به «-یم» ختم شود که این یعنی نام آن‌ان‌اکتیم تقریباً به‌طور قطع به «-یم» ختم می‌شود و نه «-ئون»، حتی اگر که اوگانسون یک گاز نجیب شناخته شود، که همگی به‌طور سنتی نام‌های ختم به «-ئون» دارند (با استثنای هلیم، آن هم به این علت که وقتی کشف شد به عنوان یک گاز نجیب محسوب نمی‌شد).[۳۳]

مشخصات[ویرایش]

پایداری هسته و ایزوتوپ‌ها[ویرایش]

اوگانسون (ردیف ۱۱۸) کمی بالاتر از «جزیرهٔ ثبات» (دایرهٔ سفید) است و در نتیجه هستهٔ آن کمی پایدارتر از میزان مورد انتظار می‌باشد.

پایداری هسته با افزایش عدد اتمی پس از پلوتونیم، سنگین‌ترین عنصر دیرینه به شدت کاهش می‌یابد و تمام ایزوتوپ‌های با عدد اتمی بالای ۱۰۱، به استثنای دوبنیم-۲۶۸، با نیمه عمر زیر یک روز، پرتوزایی می‌کنند. هیچ عنصری با عدد اتمی بالای ۸۲ (بعد از سرب) ایزوتوپ پایدار ندارد.[۳۴] با این وجود، به علت برخی دلایل که هنوز به خوبی درک نشده‌اند، کمی افزایش پایداری حول اعداد اتمی ۱۱۰ تا ۱۱۴ وجود دارد که منجر به ظهور مفهومی در فیزیک هسته‌ای با عنوان «جزیرهٔ ثبات» می‌گردد. این مفهوم، مطرح شده توسط استاد دانشگاه کالیفرنیا، گلن سیبورگ، شرح می‌دهد که چرا عناصر فوق سنگین بیشتر از حد انتظار باقی می‌مانند.[۳۵] اوگانسون پرتوزا است و نیمه عمری ظاهراً کمتر از یک میلی ثانیه دارد. با این وجود، این مقدار بیشتر از مقادیر پیش‌بینی شده‌است،[۳۶] بنابراین ایدهٔ مفهوم «جزیرهٔ ثبات» را تقویت می‌کند.[۳۷]

محاسبات با استفاده از مدل تونل-کوانتوم وجود ایزوتوپ‌های غنی از نوترون اوگانسون را با نیمه عمرهای واپاشی آلفای نزدیک به ۱ میلی‌ثانیه پیش‌بینی می‌کنند.

محاسبات نظری انجام گرفته بر مسیرهای ترکیب و نیمه عمرهای دیگر ایزوتوپ‌ها نشان می‌دهند که برخی می‌توانند کمی پایدارتر از ایزوتوپ ترکیب شدهٔ 294Og باشند، احتمالاً 293Og, 295Og, 296Og, 297Og, 298Og, 300Og و 302Og. از موارد مذکور، 297Og می‌تواند بیشترین احتمال برای کسب عنوان هستهٔ جاویدتر داشته باشد، و بنابراین می‌تواند محل تمرکز اقدامات بعدی حول این عنصر قرار بگیرد. برخی ایزوتوپ‌ها با نوترون‌های زیاد، مانند برخی که حول 313Og قرار دارند نیز می‌تواند هستهٔ پایدارتری داشته باشند.[۳۸]

خواص محاسبه شدهٔ اتمی و فیزیکی[ویرایش]

اوگانسون عضوی از گروه ۱۸ است، عناصر با ظرفیت شیمیایی صفر. اعضای این گروه معمولاً نسبت به اکثر واکنش‌های شیمیایی معمول بی‌اثر هستند (برای مثال، سوختن) زیرا که لایه ظرفیت بیرونی کاملاً با هشت الکترون پر شده‌است. این منجر به انرژی پیکربندی کمینه و پایداری می‌شود که الکترون‌های بیرونی به سختی با هم پیوند دارند.[۳۹] به‌طور مشابه تصور می‌شود، اوگانسون لایه ظرفیت بیرونی پر دارد که الکترون‌های ظرفیت آن در آرایش الکترونی 7s27p6 قرار گرفته‌اند.[۱]

در نتیجه، برخی انتظار دارند اوگانسون خواصی شیمیایی و فیزیک مشابه با دیگر اعضای این گروه و در جدول تناوبی بیش از همه شبیه به گاز نجیب بالایی اش یعنی رادون، داشته باشد.[۴۰] طبق روند تناوبی انتظار می‌رود اوگانسون کمی فعالتر از رادون باشد. البته، محاسبات نظری نشان می‌دهند که کاملاً فعال است تا جایی که نمی‌توان آن را یک گاز نجیب نامید.[۴] علاوه بر فعالتر بودن نسبت به رادون، اوگانسون می‌تواند حتی فعال تر از فلروویم و کوپرنیسیم هم باشد.v علت افزایش ظاهری فعالیت شیمیایی اوگانسون نسبت به رادون بی‌ثباتی مربوط به انرژی و گسترشی شعاعی آخرین زیرلایه ی اشغال شده یا 7p است.[۱] به‌طور دقیق تر، تعاملات اسپین – مداری قابل توجه بین الکترون‌های 7p با الکترون‌های بی‌اثر 7s2، به‌طور مؤثری باعث ایجاد لایهٔ الکترونی دومی نزدیک به فلروویم و کاهش قابل ملاحظه‌ای در پایداری لایه‌های پر عنصر ۱۱۸ می‌شوند.[۱] همچنین نتیجه‌ای به دست آمده‌است در خصوص اینکه اوگانسون، برخلاف دیگر گازهای نجیب، با یک الکترون ضمن آزادسازی انرژی پیوند برقرار می‌کند – یا به عبارت دیگر الکترون خواهی مثبت را به نمایش می‌گذارد.[۴۱][۴۲]

تاکنون انتظار می‌رود اوگانسون گسترده‌ترین قطبش‌پذیری را بین تمام عناصر ماقبل خودش (دوبرابر رادون) در جدول تناوبی داشته باشد.[۱] با برونیابی از دیگر گازهای نجیب، انتظار داریم اوگانسون نقطه جوشی بین ۳۲۰ و ۳۸۰ کلوین داشته باشد[۱] که خیلی با مقادیر تخمین زده شدهٔ قبلی یعنی ۲۶۳ کلوین[۵] یا ۲۴۷ کلوین[۴۳] فاصله دارد. حتی با وجود عدم قطعیت‌های زیادی در محاسبات، خیلی بعید به نظر می‌رسد که در شرایط استاندارد یک گاز باشد،[۱] و چون محدودهٔ مایع بودن دیگر گازها خیلی کوچک است، یعنی بین ۲ تا ۹ کلوین، این عنصر در شرایط استاندارد باید جامد باشد. با این حال اگر اوگانسون در شرایط استاندارد به حالت گاز باشد، یکی از چگال‌ترین مواد گازی در شرایط استاندارد خواهد بود (حتی اگر مثل دیگر گازهای نجیب تک اتمی باشد).

به علت قطبش پذیری فوق‌العاده اش، انتظار می‌رود اوگانسون برخلاف قاعده انرژی یونش پایین (مشابه با سرب که ۷۰٪ رادون و به‌طور قابل ملاحظه‌ای از فلروویم کمتر است[۴۴]) و فاز ماده چگال استانداردی داشته باشد.[۱]

ترکیبات پیش‌بینی شده[ویرایش]

XeF4 آرایش هندسی مربعی دارد.

هیچ ترکیبی از اوگانسون تاکنون به دست نیامده، اما محاسبات روی ترکیبات نظری از ۱۹۶۴ آغاز شده‌اند.[۲۶] پیش‌بینی شده‌است که اگر انرژی یونش این عنصر به قدر کافی بالا باشد، اکسایش آن دشوار شده و در نتیجه عدد اکسایش رایج آن ۰ خواهد بود (مثل دیگر گازهای نجیب)؛[۴۵] با این حال، ظاهراً این نظریه درست‌ترین نیست.[۴۶]

محاسبات بر روی مولکول دواتمی Og2 نشانگر وجود یک پیوند تقریباً معادل با میزان محاسبه شده برای Hg2، و انرژی تفکیک پیوندی برابر 6 kJ/mol، حدوداً ۴ برابر Rn2 می‌باشند. اما چشمگیرتر آن است که طول پیوند کوتاهتری از Rn2 به میزان ۰٫۱۶ آنگستروم محاسبه شده که نشان دهندهٔ ساختار پیوند قابل ملاحظه‌ای است.[۱] از سوی دیگر، ترکیب OgH+ انرژی تفکیک پیوندی (به عبارت دیگر پروتون خواهی) کمتر از RnH+ نشان می‌دهد.[۱]

پیوند بین اوگانسون و هیدروژن در OgH بسیار شل در نظر گرفته می‌شود و می‌توان آن را به عنوان یک نیروی اصیل واندروالسی در نظر گرفت تا یک پیوند شیمیایی واقعی. در سوی دیگر، با وجود عناصر الکترونگاتیو بالا، به نظر می‌رسد اوگانسون مثلاً نسبت به کوپرنیسیم یا فلروویم ترکیباتی پایدارتری تشکیل دهد. اعداد اکسایش پایدار +۲ و +۴ برای فلوریدها در ترکیب‌های OgF2 و OgF4 پیش‌بینی شده‌اند. عدد +۶ به خاطر پیوند قوی در زیر لایهٔ 7p1/۲ پایداری کمتری دارد.[۴۶] این نتیجهٔ تعاملات مدار – اسپینی است که اوگانسون را معمولاً واکنش پذیر و فعال می‌نماید. برای مثال، نشان داده شد که واکنش اوگانسون با F2 برای تشکیل OgF2 انرژی 106 kcal/mol را از 46 kcal/mol که از این گونه تعاملات حاصل می‌شود، آزاد می‌کند. برای مقایسه، تعامل مدار – اسپینی برای مولکول مشابه RnF2 تقریباً برابر 10 kcal/mol از انرژی ساختار 49 kcal/mol است. تعامل یکسان آرایش چهاروجهی Td را برای OgF4، برخلاف آرایش هندسی مربعی D4h برای XeF4 و هم چنین RnF4 تثبیت می‌کند. پیوند Og-F به احتمال زیادی یونی باشد تا یک پیوند کووالانسی که باعث غیرفرار بودن ترکیبات OgFn شود.[۴][۴۷] انتظار می‌رود OgF2 به علت الکتروپوزیتیوی بالای اوگانسون نسبتاً یونی باشد.[۴۸] برخلاف دیگر گازهای نجیب (احتمالاً به جز زنون[۴۹][۵۰] اوگانسون به قدر کافی الکترون دهنده[۴۸] تصور می‌شد که می‌تواند پیوند Og-Cl با کلر تشکیل بدهد.[۴]

منابع[ویرایش]

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پیوند به بیرون[ویرایش]

Oganesson, 118Og
Oganesson
Pronunciation
Mass number[294] (unconfirmed: 295)
Oganesson in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Rn

Og

(Usb)
tennessineoganessonununennium
Atomic number (Z)118
Groupgroup 18
Periodperiod 7
Blockp-block
Element category  Unknown chemical properties, was expected to be a noble gas; now predicted to be metallic-looking reactive solid, and either a semiconductor (possibly a metalloid) or a post-transition metal[2][3]
Electron configuration[Rn] 5f14 6d10 7s2 7p6 (predicted)[4][5]
Electrons per shell2, 8, 18, 32, 32, 18, 8 (predicted)
Physical properties
Phase at STPsolid (predicted)[4]
Melting point320 K ​(50 °C, ​120 °F) (predicted)[6]
Boiling point350±30 K ​(80±30 °C, ​170±50 °F) (extrapolated)[4]
Density (near r.t.)4.9–5.1 g/cm3 (predicted)[7]
Critical point439 K, 6.8 MPa (extrapolated)[8]
Heat of fusion23.5 kJ/mol (extrapolated)[8]
Heat of vaporization19.4 kJ/mol (extrapolated)[8]
Atomic properties
Oxidation states(−1),[5] (0), (+1),[9] (+2),[10] (+4),[10] (+6)[5] (predicted)
Ionization energies
  • 1st: 860.1 kJ/mol (predicted)[11]
  • 2nd: 1560 kJ/mol (predicted)[12]
Covalent radius157 pm (predicted)[13]
Other properties
Natural occurrencesynthetic
Crystal structureface-centered cubic (fcc)
Face-centered cubic crystal structure for oganesson

(extrapolated)[14]
CAS Number54144-19-3
History
Namingafter Yuri Oganessian
PredictionHans Peter Jørgen Julius Thomsen (1895)
DiscoveryJoint Institute for Nuclear Research and Lawrence Livermore National Laboratory (2002)
Main isotopes of oganesson
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
294Og[15] syn 0.69 ms[16] α 290Lv
SF
295Og[17] syn 181 ms? α 291Lv
| references

Oganesson is a synthetic chemical element with the symbol Og and atomic number 118. It was first synthesized in 2002 at the Joint Institute for Nuclear Research (JINR) in Dubna, near Moscow in Russia, by a joint team of Russian and American scientists. In December 2015, it was recognized as one of four new elements by the Joint Working Party of the international scientific bodies IUPAC and IUPAP. It was formally named on 28 November 2016.[18][19] The name is in line with the tradition of honoring a scientist, in this case the nuclear physicist Yuri Oganessian, who has played a leading role in the discovery of the heaviest elements in the periodic table. It is one of only two elements named after a person who was alive at the time of naming, the other being seaborgium, and the only element whose namesake is alive today.[20]

Oganesson has the highest atomic number and highest atomic mass of all known elements. The radioactive oganesson atom is very unstable, and since 2005, only five (possibly six) atoms of the nuclide 294Og have been detected.[21] Although this allowed very little experimental characterization of its properties and possible compounds, theoretical calculations have resulted in many predictions, including some surprising ones. For example, although oganesson is a member of group 18 – the first synthetic element to be so – it may be significantly reactive, unlike all the other elements of that group (the noble gases).[4] It was formerly thought to be a gas under normal conditions but is now predicted to be a solid due to relativistic effects.[4] On the periodic table of the elements it is a p-block element and the last one of period 7.

History

Early speculation

The possibility of a seventh noble gas, after helium, neon, argon, krypton, xenon, and radon was considered almost as soon as the noble gas group was discovered. The Danish chemist Hans Peter Jørgen Julius Thomsen predicted in April 1895, the year after the discovery of argon, that there was a whole series of chemically inert gases similar to argon that would bridge the halogen and alkali metal groups: he expected that the seventh of this series would end a 32-element period which contained thorium and uranium and have an atomic weight of 292, close to the 294 now known for the first and only confirmed isotope of oganesson.[22] Niels Bohr noted in 1922 that this seventh noble gas should have atomic number 118 and predicted its electronic structure as 2, 8, 18, 32, 32, 18, 8, matching modern predictions.[23] Following this, Aristid von Grosse wrote an article in 1965 predicting the likely properties of element 118. It was 107 years from Thomsen's prediction before oganesson was successfully synthesised, although its chemical properties have not been investigated to determine if it behaves as the heavier congener of radon.[12]

Unconfirmed discovery claims

In late 1998, Polish physicist Robert Smolańczuk published calculations on the fusion of atomic nuclei towards the synthesis of superheavy atoms, including oganesson.[24] His calculations suggested that it might be possible to make oganesson by fusing lead with krypton under carefully controlled conditions, and that the fusion probability (cross section) of that reaction would be close to the lead–chromium reaction that had produced element 106, seaborgium. This contradicted predictions that the cross sections for reactions with lead or bismuth targets would go down exponentially as the atomic number of the resulting elements increased.[24]

In 1999, researchers at Lawrence Berkeley National Laboratory made use of these predictions and announced the discovery of livermorium and oganesson, in a paper published in Physical Review Letters,[25] and very soon after the results were reported in Science.[26] The researchers reported that they had performed the reaction

208
82
Pb
+ 86
36
Kr
293
118
Og
+
n
.

The following year, they published a retraction after researchers at other laboratories were unable to duplicate the results and the Berkeley lab could not duplicate them either.[27] In June 2002, the director of the lab announced that the original claim of the discovery of these two elements had been based on data fabricated by principal author Victor Ninov.[28][29] Newer experimental results and theoretical predictions have confirmed the exponential decrease in cross sections with lead and bismuth targets as the atomic number of the resulting nuclide increases.[30]

Discovery reports

The first genuine decay of atoms of oganesson was observed in 2002 at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, by a joint team of Russian and American scientists. Headed by Yuri Oganessian, a Russian nuclear physicist of Armenian ethnicity, the team included American scientists of the Lawrence Livermore National Laboratory, California.[31] The discovery was not announced immediately, because the decay energy of 294Og matched that of 212mPo, a common impurity produced in fusion reactions aimed at producing superheavy elements, and thus announcement was delayed until after a 2005 confirmatory experiment aimed at producing more oganesson atoms.[32] On 9 October 2006, the researchers announced[15] that they had indirectly detected a total of three (possibly four) nuclei of oganesson-294 (one or two in 2002[33] and two more in 2005) produced via collisions of californium-249 atoms and calcium-48 ions.[34][35][36][37][38]

249
98
Cf
+ 48
20
Ca
294
118
Og
+ 3
n
.
Schematic diagram of oganesson-294 alpha decay, with a half-life of 0.89 ms and a decay energy of 11.65 MeV. The resulting livermorium-290 decays by alpha decay, with a half-life of 10.0 ms and a decay energy of 10.80 MeV, to flerovium-286. Flerovium-286 has a half-life of 0.16 s and a decay energy of 10.16 MeV, and undergoes alpha decay to copernicium-282 with a 0.7 rate of spontaneous fission. Copernicium-282 itself has a half-life of only 1.9 ms and has a 1.0 rate of spontaneous fission.
Radioactive decay pathway of the isotope oganesson-294.[15] The decay energy and average half-life is given for the parent isotope and each daughter isotope. The fraction of atoms undergoing spontaneous fission (SF) is given in green.

In 2011, IUPAC evaluated the 2006 results of the Dubna–Livermore collaboration and concluded: "The three events reported for the Z = 118 isotope have very good internal redundancy but with no anchor to known nuclei do not satisfy the criteria for discovery".[39]

Because of the very small fusion reaction probability (the fusion cross section is ~0.3–0.6 pb or (3–6)×10−41 m2) the experiment took four months and involved a beam dose of 2.5×1019 calcium ions that had to be shot at the californium target to produce the first recorded event believed to be the synthesis of oganesson.[40] Nevertheless, researchers were highly confident that the results were not a false positive, since the chance that the detections were random events was estimated to be less than one part in 100000.[41]

In the experiments, the alpha-decay of three atoms of oganesson was observed. A fourth decay by direct spontaneous fission was also proposed. A half-life of 0.89 ms was calculated: 294
Og
decays into 290
Lv
by alpha decay. Since there were only three nuclei, the half-life derived from observed lifetimes has a large uncertainty: 0.89+1.07
−0.31
 ms
.[15]

294
118
Og
290
116
Lv
+ 4
2
He

The identification of the 294
Og
nuclei was verified by separately creating the putative daughter nucleus 290
Lv
directly by means of a bombardment of 245
Cm
with 48
Ca
ions,

245
96
Cm
+ 48
20
Ca
290
116
Lv
+ 3
n
,

and checking that the 290
Lv
decay matched the decay chain of the 294
Og
nuclei.[15] The daughter nucleus 290
Lv
is very unstable, decaying with a lifetime of 14 milliseconds into 286
Fl
, which may experience either spontaneous fission or alpha decay into 282
Cn
, which will undergo spontaneous fission.[15]

In a quantum-tunneling model, the alpha decay half-life of 294
Og
was predicted to be 0.66+0.23
−0.18
 ms
[42] with the experimental Q-value published in 2004.[43] Calculation with theoretical Q-values from the macroscopic-microscopic model of Muntian–Hofman–Patyk–Sobiczewski gives somewhat lower but comparable results.[44]

Confirmation

One atom of the heavier isotope 295Og may have been seen in a 2011 experiment at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany aimed at the synthesis of element 120 in the reaction 248Cm+54Cr, but uncertainties in the data meant that the observed chain cannot be definitely assigned to 299120 and 295Og: the data indicates for 295Og a half-life of 181 milliseconds, longer than that of 294Og, which is 0.7 milliseconds.[17]

In December 2015, the Joint Working Party of international scientific bodies International Union of Pure and Applied Chemistry (IUPAC) and International Union of Pure and Applied Physics (IUPAP) recognized the element's discovery and assigned the priority of the discovery to the Dubna–Livermore collaboration.[45] This was on account of two 2009 and 2010 confirmations of the properties of the granddaughter of 294Og, 286Fl, at the Lawrence Berkeley National Laboratory, as well as the observation of another consistent decay chain of 294Og by the Dubna group in 2012. The goal of that experiment had been the synthesis of 294Ts via the reaction 249Bk(48Ca,3n), but the short half-life of 249Bk resulted in a significant quantity of the target having decayed to 249Cf, resulting in the synthesis of oganesson instead of tennessine.[46]

From 1 October 2015 to 6 April 2016, the Dubna team performed a similar experiment with 48Ca projectiles aimed at a mixed-isotope californium target containing 249Cf, 250Cf, and 251Cf, with the aim of producing the heavier oganesson isotopes 295Og and 296Og. Two beam energies at 252 MeV and 258 MeV were used. Only one atom was seen at the lower beam energy, whose decay chain fitted the previously known one of 294Og (terminating with spontaneous fission of 286Fl), and none were seen at the higher beam energy. The experiment was then halted, as the glue from the sector frames covered the target and blocked evaporation residues from escaping to the detectors. The Dubna team planned to repeat this experiment in 2017–2020.[47] The production of 293Og and its daughter 289Lv, as well as the even heavier isotope 297Og, is also possible using this reaction. The isotopes 295Og and 296Og may also be produced in the fusion of 248Cm with 50Ti projectiles, a reaction planned at the JINR and at RIKEN in 2017–2018.[47][48][49] A search beginning in summer 2016 at RIKEN for 295Og in the 3n channel of this reaction was unsuccessful, though the study is planned to resume; a detailed analysis and cross section limit were not provided. These heavier and likely more stable isotopes may be useful in probing the chemistry of oganesson.[50][51]

Naming

Element 118 was named after Yuri Oganessian, a pioneer in the discovery of synthetic elements, with the name oganesson (Og). Oganessian and the decay chain of oganesson-294 were pictured on a stamp of Armenia issued on 28 December 2017.

Using Mendeleev's nomenclature for unnamed and undiscovered elements, oganesson is sometimes known as eka-radon (until the 1960s as eka-emanation, emanation being the old name for radon).[14] In 1979, IUPAC assigned the systematic placeholder name ununoctium to the undiscovered element, with the corresponding symbol of Uuo,[52] and recommended that it be used until after confirmed discovery of the element.[53] Although widely used in the chemical community on all levels, from chemistry classrooms to advanced textbooks, the recommendations were mostly ignored among scientists in the field, who called it "element 118", with the symbol of E118, (118), or even simply 118.[5]

Before the retraction in 2001, the researchers from Berkeley had intended to name the element ghiorsium (Gh), after Albert Ghiorso (a leading member of the research team).[54]

The Russian discoverers reported their synthesis in 2006. According to IUPAC recommendations, the discoverers of a new element have the right to suggest a name.[55] In 2007, the head of the Russian institute stated the team were considering two names for the new element: flyorium, in honor of Georgy Flyorov, the founder of the research laboratory in Dubna; and moskovium, in recognition of the Moscow Oblast where Dubna is located.[56] He also stated that although the element was discovered as an American collaboration, who provided the californium target, the element should rightly be named in honor of Russia since the Flyorov Laboratory of Nuclear Reactions at JINR was the only facility in the world which could achieve this result.[57] These names were later suggested for element 114 (flerovium) and element 116 (moscovium).[58] Flerovium became the name of element 114; the final name proposed for element 116 was instead livermorium,[59] with moscovium later being proposed and accepted for element 115 instead.[20]

Traditionally, the names of all noble gases end in "-on", with the exception of helium, which was not known to be a noble gas when discovered. The IUPAC guidelines valid at the moment of the discovery approval however required all new elements be named with the ending "-ium", even if they turned out to be halogens (traditionally ending in "-ine") or noble gases (traditionally ending in "-on").[60] While the provisional name ununoctium followed this convention, a new IUPAC recommendation published in 2016 recommended using the "-on" ending for new group 18 elements, regardless of whether they turn out to have the chemical properties of a noble gas.[61]

The scientists involved in the discovery of element 118, as well as those of 117 and 115, held a conference call in March 2016. Element 118 was the last to be decided upon; after Oganessian was asked to leave the call, the remaining scientists unanimously decided to have the element "oganesson" after him. Oganessian was a pioneer in superheavy element research for sixty years reaching back to the field's foundation: his team and his proposed techniques had led directly to the synthesis of elements 107 through 118. Mark Stoyer, nuclear chemist at the LLNL, later recalled, "We had intended to propose that name from Livermore, and things kind of got proposed at the same time from multiple places. I don’t know if we can claim that we actually proposed the name, but we had intended it."[62] In June 2016, IUPAC announced that the discoverers planned to give the element the name oganesson (symbol: Og). The name became official on 28 November 2016.[20] Oganessian later commented on the naming:[63]

For me, it is an honour. The discovery of element 118 was by scientists at the Joint Institute for Nuclear Research in Russia and at the Lawrence Livermore National Laboratory in the US, and it was my colleagues who proposed the name oganesson. My children and grandchildren have been living in the US for decades, but my daughter wrote to me to say that she did not sleep the night she heard because she was crying.[63]

— Yuri Oganessian

The naming ceremony for moscovium, tennessine, and oganesson was held on 2 March 2017 at the Russian Academy of Sciences in Moscow.[64]

Characteristics

Nuclear stability and isotopes

Oganesson (row 118) is slightly above the "island of stability" (white circle) and thus its nuclei are slightly more stable than otherwise predicted.

The stability of nuclei quickly decreases with the increase in atomic number after curium, element 96, whose half-life is four orders of magnitude longer than that of any subsequent element. All isotopes with an atomic number above 101 undergo radioactive decay with half-lives of less than 30 hours. No elements with atomic numbers above 82 (after lead) have stable isotopes.[65] This is because of the ever-increasing Coulomb repulsion of protons, so that the strong nuclear force cannot hold the nucleus together against spontaneous fission for long. Calculations suggest that in the absence of other stabilizing factors, elements with more than 104 protons should not exist.[66] However, researchers in the 1960s suggested that the closed nuclear shells around 114 protons and 184 neutrons should counteract this instability, creating an island of stability in which nuclides could have half-lives reaching thousands or millions of years. While scientists have still not reached the island, the mere existence of the superheavy elements (including oganesson) confirms that this stabilizing effect is real, and in general the known superheavy nuclides become exponentially longer-lived as they approach the predicted location of the island.[67][68] Oganesson is radioactive and has a half-life that appears to be less than a millisecond. Nonetheless, this is still longer than some predicted values,[42][69] thus giving further support to the idea of the island of stability.[70]

Calculations using a quantum-tunneling model predict the existence of several neutron-rich isotopes of oganesson with alpha-decay half-lives close to 1 ms.[71][72]

Theoretical calculations done on the synthetic pathways for, and the half-life of, other isotopes have shown that some could be slightly more stable than the synthesized isotope 294Og, most likely 293Og, 295Og, 296Og, 297Og, 298Og, 300Og and 302Og (the last reaching the N = 184 shell closure).[42][73] Of these, 297Og might provide the best chances for obtaining longer-lived nuclei,[42][73] and thus might become the focus of future work with this element. Some isotopes with many more neutrons, such as some located around 313Og, could also provide longer-lived nuclei.[74]

Calculated atomic and physical properties

Oganesson is a member of group 18, the zero-valence elements. The members of this group are usually inert to most common chemical reactions (for example, combustion) because the outer valence shell is completely filled with eight electrons. This produces a stable, minimum energy configuration in which the outer electrons are tightly bound.[75] It is thought that similarly, oganesson has a closed outer valence shell in which its valence electrons are arranged in a 7s27p6 configuration.[4]

Consequently, some expect oganesson to have similar physical and chemical properties to other members of its group, most closely resembling the noble gas above it in the periodic table, radon.[76] Following the periodic trend, oganesson would be expected to be slightly more reactive than radon. However, theoretical calculations have shown that it could be significantly more reactive.[10] In addition to being far more reactive than radon, oganesson may be even more reactive than the elements flerovium and copernicium, which are heavier homologs of the more chemically active elements lead and mercury respectively.[4] The reason for the possible enhancement of the chemical activity of oganesson relative to radon is an energetic destabilization and a radial expansion of the last occupied 7p-subshell.[4] More precisely, considerable spin–orbit interactions between the 7p electrons and the inert 7s electrons effectively lead to a second valence shell closing at flerovium, and a significant decrease in stabilization of the closed shell of oganesson.[4] It has also been calculated that oganesson, unlike the other noble gases, binds an electron with release of energy, or in other words, it exhibits positive electron affinity,[77][78] due to the relativistically stabilized 8s energy level and the destabilized 7p3/2 level,[79] whereas copernicium and flerovium are predicted to have no electron affinity.[80][81] Nevertheless, quantum electrodynamic corrections have been shown to be quite significant in reducing this affinity by decreasing the binding in the anion Og by 9%, thus confirming the importance of these corrections in superheavy elements.[77]

Oganesson is expected to have an extremely broad polarizability, almost double that of radon.[4] By extrapolating from the other noble gases, it is expected that oganesson has a melting point of approximately 320 K[6] and a boiling point between 320 and 380 K.[4] This is very different from the previously estimated values of 263 K[82] or 247 K for the boiling point.[83] It thus seems highly unlikely that oganesson would be a gas under standard conditions,[4] and as the liquid range of the other gases is very narrow, between 2 and 9 kelvins, this element should be solid at standard conditions. Nevertheless, if oganesson forms a gas under standard conditions, it would be one of the densest gaseous substances at standard conditions, even if it is monatomic like the other noble gases.

Because of its tremendous polarizability, oganesson is expected to have an anomalously low first ionization energy of 860.1 kJ/mol, similar to that of cadmium and less than those of iridium, platinum, and gold. This is significantly smaller than the values predicted for darmstadtium, roentgenium, and copernicium, although it is greater than that predicted for flerovium.[84] Even the shell structure in the nucleus and electron cloud of oganesson is strongly impacted by relativistic effects: the valence and core electron subshells in oganesson are expected to be "smeared out" in a homogeneous Fermi gas of electrons, unlike those of the "less relativistic" radon and xenon (although there is some incipient delocalisation in radon), due to the very strong spin-orbit splitting of the 7p orbital in oganesson.[85] A similar effect for nucleons, particularly neutrons, is incipient in the closed-neutron-shell nucleus 302Og and is strongly in force at the hypothetical superheavy closed-shell nucleus 472164, with 164 protons and 308 neutrons.[85] Moreover, spin-orbit effects may cause bulk oganesson to be a semiconductor while all the lighter noble gases are insulators, with a band gap of 1.5±0.6 V predicted (that for radon should be 7.1±0.5 V).[2]

Predicted compounds

Skeletal model of a planar molecule with a central atom symmetrically bonded to four peripheral (fluorine) atoms.
XeF
4
has a square planar molecular geometry.
Skeletal model of a terahedral molecule with a central atom (oganesson) symmetrically bonded to four peripheral (fluorine) atoms.
OgF
4
is predicted to have a tetrahedral molecular geometry.

The only confirmed isotope of oganesson, 294Og, has much too short a half-life to be chemically investigated experimentally. Therefore, no compounds of oganesson have been synthesized yet.[32] Nevertheless, calculations on theoretical compounds have been performed since 1964.[14] It is expected that if the ionization energy of the element is high enough, it will be difficult to oxidize and therefore, the most common oxidation state would be 0 (as for the noble gases);[86] nevertheless, this appears not to be the case.[12]

Calculations on the diatomic molecule Og
2
showed a bonding interaction roughly equivalent to that calculated for Hg
2
, and a dissociation energy of 6 kJ/mol, roughly 4 times of that of Rn
2
.[4] Most strikingly, it was calculated to have a bond length shorter than in Rn
2
by 0.16 Å, which would be indicative of a significant bonding interaction.[4] On the other hand, the compound OgH+ exhibits a dissociation energy (in other words proton affinity of oganesson) that is smaller than that of RnH+.[4]

The bonding between oganesson and hydrogen in OgH is predicted to be very weak and can be regarded as a pure van der Waals interaction rather than a true chemical bond.[9] On the other hand, with highly electronegative elements, oganesson seems to form more stable compounds than for example copernicium or flerovium.[9] The stable oxidation states +2 and +4 have been predicted to exist in the fluorides OgF
2
and OgF
4
.[87] The +6 state would be less stable due to the strong binding of the 7p1/2 subshell.[12] This is a result of the same spin-orbit interactions that make oganesson unusually reactive. For example, it was shown that the reaction of oganesson with F
2
to form the compound OgF
2
would release an energy of 106 kcal/mol of which about 46 kcal/mol come from these interactions.[9] For comparison, the spin-orbit interaction for the similar molecule RnF
2
is about 10 kcal/mol out of a formation energy of 49 kcal/mol.[9] The same interaction stabilizes the tetrahedral Td configuration for OgF
4
, as distinct from the square planar D4h one of XeF
4
, which RnF
4
is also expected to have;[87] this is because OgF4 is expected to have two inert electron pairs (7s and 7p1/2). As such, OgF6 is expected to be unbound, continuing an expected trend in the destabilisation of the +6 oxidation state (RnF6 is likewise expected to be much less stable than XeF6).[88][89] The Og–F bond will most probably be ionic rather than covalent, rendering the oganesson fluorides non-volatile.[10][90] OgF2 is predicted to be partially ionic due to oganesson's high electropositivity.[91] Unlike the other noble gases (except possibly xenon and radon),[92][93] oganesson is predicted to be sufficiently electropositive[91] to form an Og–Cl bond with chlorine.[10]

See also

References

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