Helium's placement in the Periodic Table from a crystal structure viewpoint

Abstract

The present paper indicates that helium has more in common with the alkaline earths than is often considered. Not only does it share an analogous electron configuration but it also has an analogous crystal structure to that of beryllium and magnesium which is, in contrast, characteristic of the bulk material rather than of isolated atoms.

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IUCrJ (2020). 7, 577–578 https://doi.org/10.1107/S2052252520007769 577
IUCrJ
ISSN 2052-2525
CHEMISTRYjCRYSTENG
Keywords: Periodic Table; helium; crystal
structure.
Helium’s placement in the Periodic Table from a
crystal structure viewpoint
Mikhail Kurushkin*
Chemistry Education Research and Practice Laboratory, SCAMT Institute, ITMO University, 9 Lomonosova Str., Saint
Petersburg, Saint Petersburg 191002, Russian Federation. *Correspondence e-mail: kurushkin@sca mt-itmo.ru
By 2019, one hundred and fifty years after Dmitry Mendeleev published the first
successful version of the Periodic Table of the chemical elements, there was still no
universal agreement regarding what a chemical element is. A notable indication of the
ongoing ambiguity is IUPAC’s Gold Book, which allows two different versions of the
term ‘chemical element’: (1) a species of atoms – all atoms with the same number of
protons in the atomic nucleus; (2) a pure chemical substance composed of atoms with the
same number of protons in the atomic nucleus. W. H. E. Schwarz in his 2007 paper
(Schwarz, 2007) argued that there are in fact three different definitions of a chemical
element usually encountered: (1) a basic chemical element; (2) a metallurgical element or
simple material; (3) an astrophysical spectroscopic element or elemental atom. W. B.
Jensen suggested a definition (Jensen, 1998) which focuses on atomic nuclei rather on
neutral atoms: (1) a class of nuclei, all of which have the same atomic number. The one
question that permanently accompanies the definition of the chemical element is the
representation of the Periodic Table itself.
The most common version of the revered icon of chemistry is the IUPAC Periodic
Table of the Elements. Whichever representation of the periodic system is argued to be
the optimal one (Leigh, 2009; Scerri, 2009), consistency of representation is the criterion
that has to be met.
The IUPAC Periodic Table has four blocks of chemical elements: the s-, p-, d- and f-
blocks, hence its whole body is based on electron configurations. One hundred and
seventeen of the known elements fit into those blocks; however, there is only one
element, helium, placed on top of the p-block as it is a noble gas. Hence, the repre-
sentation becomes inconsistent overall because the Periodic Table simultaneously adopts
two different definitions of the chemical element. According to the Gold Book, we then
have one hundred and seventeen species of atoms and one pure chemical substance.
Thus, such a placement of helium transforms the Periodic Table of Chemical Elements
into the Periodic Table of Pure Substances.
Switching to the Periodic Table of Pure Substances would inevitably make us consider
two further questions: (1) states of aggregation; (2) allotropes. As E. R. Scerri fairly
states, in case of the Periodic Table of Pure Substances, ‘one would probably not consider
grouping together fluorine and chlorine, two green–yellow gases, along with a brown
liquid bromine and a violet–black solid such as iodine’ (Scerri, 2005). Numerous similar
examples can be provided. Which temperature and pressure do we choose for the
representation? Which allotrope(s) do we prefer? As a matter of convention, we can
choose standard temperature and pressure. But what about allotropes, the physical forms
of chemical elements? We would need to either choose one of the allotropes or incor-
porate them all in one place, which does not seem rational. It can be seen that the choice
of a Periodic Table of Pure Substances over the Periodic Table of Chemical Elements
would probably cause an overcomplicated representation.
We now return to helium, the noble gas. In the majority of the common versions of the
Periodic Table one can always find elements classified as solids, liquids and gases.
However, for the sake of the observation in this paper, it is suggested that all the pure
substances are considered in their solid state so that van der Waals forces become
pronounced. The question will then be, would solid helium above solid beryllium be
regarded as equally irregular as the case of gaseous helium above solid beryllium? To be
more specific, in the solid state, would we still support the idea of putting helium above
neon because they are both noble gases? It is well known that solid helium has been

obtained and characterized with its crystal structure being
hexagonal close-packed (Donohue, 1959).
Next, it might be considered surprising that solid helium,
beryllium and magnesium all have the same crystal structure
(Sluiter, 2007; Luo et al., 2012), which is hexagonal close-
packed (h.c.p.); while neon, argon, krypton and xenon all have
a face-centred cubic (f.c.c.) crystal structure (Sonnenblick et
al., 1982; Moyano et al., 2007) (Fig. 1).
Most recent publications dedicated to the placement of
helium clearly demonstrate that the topic has never been more
relevant (Labarca & Srivaths, 2016, 2017; Cvetkovic
´&
Petrus
ˇevski, 2017; Grochala, 2018). In his most recent essay,
Scerri highlights the two most common opposing views
regarding the placement of helium: (1) it should be grouped
with the rest of the noble gases; (2) it should be grouped with
the alkaline earths because of an s
2
configuration, but that
means the reduction of chemistry to quantum mechanics
(Scerri, 2019b). In another 2019 paper, however, Scerri
(2019a) theorizes that a ‘deep dive into quantum mechanics’
might actually facilitate our understanding of the fundamental
aspects of the periodic system.
For instance, the latter is usually the argument of Left-step
Periodic Table supporters (Scerri, 2012b; Kurushkin, 2017).
However, such an approach has not found recognition among
chemists due to the very low reactivity of helium (Scerri,
2012a), a view that might be reversed in the near future as new
exotic stable compounds of helium (Na
2
He) are being
discovered thanks to the ab initio evolutionary algorithm
USPEX (Dong et al., 2017). Furthermore, unusual helium-
bearing compounds (FeHe, FeHe
2
,FeO
2
He), stable under
extreme conditions, have also been reported recently (Zhang
et al., 2018; Monserrat et al., 2018).
Helium turns out to have more in common with the alkaline
earths than is often considered. Not only does it share an
analogous electron configuration, but it also has an analogous
crystal structure to that of beryllium and magnesium which is,
in contrast, characteristic of the bulk material rather than of
isolated atoms. The latter, importantly, means that the place-
ment of helium above beryllium is not solely a reduction to
quantum mechanics. Last but not least, recent ab initio
calculations (Bakai et al., 2011) have shown substantial He–Be
bonding in h.c.p.-beryllium.
Acknowledgements
The author acknowledges Fedor F. Grekov for his brilliant
book on crystal chemistry and Maria Lavrentieva for creating
the graphic.
References
Bakai, A. S., Timoshevskii, A. N. & Yanchitsky, B. Z. (2011). Low
Temp. Phys. 37, 791–797.
Cvetkovic
´, J. & Petrus
ˇevski, V. M. (2017). Chemistry (Easton),26,
167–170.
Dong, X., Oganov, A. R., Goncharov, A. F., Stavrou, E., Lobanov, S.,
Saleh, G., Qian, G.-R., Zhu, Q., Gatti, C., Deringer, V. L.,
Dronskowski, R., Zhou, X.-F., Prakapenka, V. B., Kono
ˆpkova
´,Z.,
Popov, I. A., Boldyrev, A. I. & Wang, H.-T. (2017). Nat. Chem. 9,
440–445.
Donohue, J. (1959). Phys. Rev. 114, 1009.
Grochala, W. (2018). Found. Chem. 20, 191–207.
Jensen, W. B. (1998). J. Chem. Educ. 75, 817–828.
Kurushkin, M. (2017). J. Chem. Educ. 94, 976–979.
Labarca, M. & Srivaths, A. (2016). Chemistry (Easton),25, 514–530.
Labarca, M. & Srivaths, A. (2017). Chemistry (Easton),26, 663–666.
Leigh, G. J. (2009). Chem. Int. 31, 4–6.
Luo, F., Cai, L.-C., Chen, X.-R., Jing, F.-Q. & Alfe
`, D. (2012). J. Appl.
Phys. 111, 053503.
Monserrat, B., Martinez-Canales, M., Needs, R. J. & Pickard, C. J.
(2018). Phys. Rev. Lett. 121, 015301.
Moyano, G. E., Schwerdtfeger, P. & Rosciszewski, K. (2007). Phys.
Rev. B,75, 024101.
Scerri, E. (2012a). Educ. Chem. 49, 13–17.
Scerri, E. (2019a). Nature,565, 557–559.
Scerri, E. R. (2005). Educ. Chem. 42, 135–136.
Scerri, E. R. (2009). Int. J. Quantum Chem. 109, 959–971.
Scerri, E. R. (2012b). Found. Chem. 14, 69–81.
Scerri, E. R. (2019b). Chem. Eur. J. 25, 7410–7415.
Schwarz, W. H. E. (2007). Found. Chem. 9, 139–188.
Sluiter, M. H. F. (2007). Phase Transit. 80, 299–309.
Sonnenblick, Y., Kalman, Z. H. & Steinberger, I. T. (1982). J. Cryst.
Growth,58, 143–151.
Zhang, J., Lv, J., Li, H., Feng, X., Lu, C., Redfern, S. A. T., Liu, H.,
Chen, C. & Ma, Y. (2018). Phys. Rev. Lett. 121, 255703.
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578 Mikhail Kurushkin Helium’s placement in the Periodic Table IUCrJ (2020). 7, 577–578
Figure 1
Face-centred cubic (neon, argon, krypton and xenon) and hexagonal
close-packed (helium, beryllium and magnesium) crystal structures.