{"id":4304,"date":"2011-06-05T09:37:55","date_gmt":"2011-06-05T09:37:55","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=4304"},"modified":"2023-10-04T08:23:17","modified_gmt":"2023-10-04T07:23:17","slug":"hafnium-and-niels-bohr","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4304","title":{"rendered":"Hafnium and Niels Bohr"},"content":{"rendered":"
\n

In 1923, Coster and von Hevesy[1]<\/a><\/span> claimed discovery of the element\u00a0Hafnium<\/strong>, atomic number 72<\/strong> (latin Hafnia, meaning Copenhagen, where the authors worked) on the basis of six lines in its X-ray spectrum.\u00a0The debate had long raged as to whether (undiscovered) element 72 belonged to the rare-earth group 3 of the periodic table\u00a0below yttrium, or whether it should be placed in\u00a0group 4 below zirconium.\u00a0Establishing its chemical properties finally placed it in group 4. Why is this apparently arcane and obscure re-assignment historically significant? Because, in June 1922, in G\u00f6ttingen, Niels Bohr had given a famous series of lectures now known as the Bohr Festspiele<\/em> on the topic of his electron shell theory of the atom. Prior to giving these lectures he had submitted his collected thoughts in January 1922[2]<\/a><\/span>.<\/p>\n

Like Mendeleev<\/a> before, who had predicted ekasilicon<\/em>, ekaaluminium<\/em> and ekaboron (<\/em>eventually discovered as\u00a0germanium<\/a>,\u00a0gallium<\/a> and\u00a0scandium<\/a>),\u00a0Bohr had used his electron shell theory to (correctly) predict the properties of element 72. In modern terms, he had concluded that its electron shell structure must be 2.8.18.32.10.2 or [Xe].4f14<\/sup>.5d2<\/sup>.6s2<\/sup>. Classification as a rare earth would have resulted in the 4f shell having 15 electrons, impossible in Bohr’s theory. Coster and von Hevesy note in their article that Bohr’s <\/a>striking prediction<\/a><\/em> was now verified. Bohr latter told this story in his \u00a0Nobel \u00a0prize lecture<\/a>.<\/p>\n

Why I am writing all of this? For various reasons:<\/p>\n

    \n
  1. Unlike Mendeleev, Bohr’s prediction of the properties of a (then uncharacterized) element, whilst famous at the time, is nowadays largely forgotten by chemists. It is one of the great achievements of the then new quantum theory.<\/li>\n
  2. Reading the 67 pages of Bohr’s article[2]<\/a><\/span> on the topic reveals no discussion of element 72 (articles of this era are nowadays only available as scanned images, not full text, and one must rely on a human visual scan of all 67 pages, which of course may not be reliable) but its (absence) in the table below is striking. Here VI means the 6th row of the periodic table.\n

  3. Notice the only other missing elements, Technetium (43), Promethium (61), Astatine (85), Francium (87) and\u00a0Rhenium (75, the only non-radioactive one remaining to be discovered),<\/li>\n
  4. I must presume that Bohr introduced his discussion of element 72 into his June lectures to make an impact with his audience! One might have hoped that tracking down what happened between January 1922, when Bohr fails to make much of the missing element 72, and June in the same year would be possible from\u00a0Coster and von Hevesy’s citation of Bohr in 1923. But it was the practice of the time to rarely cite one’s sources. Thus they give no published citation to Bohr, and one might conclude that they might instead be quoting Bohr from his lectures rather than his writings (poor old Bury, now forgotten![3]<\/a><\/span>).\n

  5. Bohr’s own 1922 article on the topic is also visually striking. It contains in its 67 pages:\n
      \n
    1. 13 (short) equations<\/li>\n
    2. Two figures (the second a variation on the first)<\/li>\n
    3. One table (above).<\/li>\n
    4. and lots of text (in German).<\/li>\n
    5. No citations at the end, not even one, although many people are acknowledged in the text itself.<\/li>\n
    6. No explicit statement of shell structures as e.g.\u00a02.8.18.32.10.2 or [Xe].4f14<\/sup>.5d2<\/sup>.6s2<\/sup>.<\/li>\n<\/ol>\n

      Given that Bohr’s article can be regarded as one of the most influential of the 20th century (even prior to its being placed on a firm theoretical footing by\u00a0solution of the Schroedinger equation for the hydrogen atom), I find it interesting how quickly it achieved this status (Bohr won the Nobel prize in 1922 as well). One might conclude that reputations were made as much\u00a0via<\/em> verbal presentations as by the immediate visual impact of the associated publications.<\/li>\n<\/ol>\n

      Finally, I note the striking contrast between Bohr’s article[2]<\/a><\/span> and Langmuir’s[4]<\/a><\/span>, written about a year earlier in 1921, although first set out in 1919<\/a>. Here, Langmuir[4]<\/a><\/span> sets out some postulates, the first of which is shown below (and first explained in 1919<\/a>)<\/p>\n

      \"\"<\/a>

      Langmuir’s 1921 postulate.<\/p><\/div>\n

      The filled electron shells are clearly set out here (much more clearly than in Bohr’s 1922 article[2]<\/a><\/span>). But yet again, we remain uncertain as to how Langmuir arrived at this postulate (perhaps here<\/a>) Although he (very briefly) mentions Bohr in his own paper, it is only in the context of speculating about what prevents the electrons from falling into the nucleus, and few citations are again given (a notable exception is to Pease[5]<\/a><\/span> for suggesting the triple bond). We may only suspect that Langmuir had heard Bohr talking about his theory, and had extended G. N. Lewis’ concept (also not directly cited) of (filled) valence shells for his own theory of chemical bonding.<\/p>\n

      Well, in a little less than 90 years, we have progressed from finding almost no sources cited in some of the most influential papers of the 20th century, to the DOI <\/a>(or URL<\/a>) embedded in everything. I think that when the history of the present era is written, the introduction of the DOI\/URL will take its place in the pantheon of great scientific events. Its the connections that matter, stupid!<\/p>\n

      \n

      Postscript<\/strong>. Hevesy in this review[6]<\/a><\/span> written in 1925 sets out a good history of Hafnium. This article contains (on p7) a clear statement of the electron shell structure of Hafnium as 2.8.18.32.8.2.2, which is cited as Bohr’s result.<\/em> Hevesy quotes Bohr via<\/em> reference 12, which is in fact to a book Bohr published in 1924. There is no mention of Langmuir in Hevesy’s review.<\/p>\n

      Postscript1:<\/strong> Hafnium (as its oxide) is now an essential element to the ever smaller fabrication of silicon chips (32nm and smaller). It is one of 14 elements considered essential to the future green technologies (six of which, but not including \u00a0Hafnium, are considered in critical risk of supply disruption by 2015).<\/p>\n

      References<\/h2>\n
      1. D. COSTER, and G. HEVESY, \"On the Missing Element of Atomic Number 72\", Nature<\/i>, vol. 111, pp. 79-79, 1923. https:\/\/doi.org\/10.1038\/111079a0<\/a>\n\n<\/li>\n
      2. N. Bohr, \"Der Bau der Atome und die physikalischen und chemischen Eigenschaften der Elemente\", Zeitschrift f\ufffdr Physik<\/i>, vol. 9, pp. 1-67, 1922. https:\/\/doi.org\/10.1007\/bf01326955<\/a>\n\n<\/li>\n
      3. C.R. Bury, \"LANGMUIR'S THEORY OF THE ARRANGEMENT OF ELECTRONS IN ATOMS AND MOLECULES.\", Journal of the American Chemical Society<\/i>, vol. 43, pp. 1602-1609, 1921. https:\/\/doi.org\/10.1021\/ja01440a023<\/a>\n\n<\/li>\n
      4. I. Langmuir, \"Types of Valence\", Science<\/i>, vol. 54, pp. 59-67, 1921. https:\/\/doi.org\/10.1126\/science.54.1386.59<\/a>\n\n<\/li>\n
      5. R.N. Pease, \"AN ANALYSIS OF MOLECULAR VOLUMES FROM THE POINT OF VIEW OF THE LEWIS-LANGMUIR THEORY OF MOLECULAR STRUCTURE.\", Journal of the American Chemical Society<\/i>, vol. 43, pp. 991-1004, 1921. https:\/\/doi.org\/10.1021\/ja01438a003<\/a>\n\n<\/li>\n
      6. G. Hevesy, \"The Discovery and Properties of Hafnium.\", Chemical Reviews<\/i>, vol. 2, pp. 1-41, 1925. https:\/\/doi.org\/10.1021\/cr60005a001<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

        In 1923, Coster and von Hevesy claimed discovery of the element\u00a0Hafnium, atomic number 72 (latin Hafnia, meaning Copenhagen, where the authors worked) on the basis of six lines in its X-ray spectrum.\u00a0The debate had long raged as to whether (undiscovered) element 72 belonged to the rare-earth group 3 of the periodic table\u00a0below yttrium, or whether […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":5,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"","footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[2,1],"tags":[549,555,557,558,556,569,551,2651,550,553,568,554,42],"ppma_author":[2661],"class_list":["post-4304","post","type-post","status-publish","format-standard","hentry","category-chemical-it","category-general","tag-bohr","tag-bury","tag-chemical-bonding","tag-chemical-properties","tag-copenhagen","tag-green-technologies","tag-hafnium","tag-historical","tag-langmuir","tag-niels-bohr","tag-silicon-chips","tag-technetium","tag-x-ray"],"yoast_head":"\nHafnium and Niels Bohr - 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