"Atomic volume decreases along a period, reaches a minimum at the middle, and then increases for the rest of the period"

Why does the atomic volume, along a period, initially decrease, reach a minimum and then increase?

The atomic radius decreases along a period, so I would have thought that the atomic volume would also decrease along a period.

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  • $\begingroup$ Could you please provide the source of the quote? It seems like a really dubious statement... As you said, atomic radius decreases across the period. $\endgroup$ – Tan Yong Boon Jun 23 at 11:09
  • $\begingroup$ @Tan Yong Boon, I don't quite remember where exactly I had read it (it was online). But a quick Google search for the graph of atomic volume versus atomic number verifies this claim. I have added an image to the question. $\endgroup$ – Michael Faraday Jun 23 at 11:24
  • $\begingroup$ Perhaps it's because of the way atomic volume is defined. Atomic volume is defined as the ratio of gram atomic mass to Density. That's probably why it has a different trend to atomic size. But I do not know why exactly does atomic volume follow this trend. $\endgroup$ – Michael Faraday Jun 23 at 11:26
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    $\begingroup$ I believe molar volume is a more "macroscopic" property that depends on the structure that the element exists in. Covalent bonds tend to be relatively short between the atoms of non-metals while the interatomic distances are longer in metals. Hence, we see the greatest interatomic distances coming from group 1 metals and hence the greatest molar volumes from these elements. $\endgroup$ – Tan Yong Boon Jun 23 at 13:47
  • $\begingroup$ @Tan Yong Boon, why would chlorine have a larger atomic volume than sulphur? $\endgroup$ – Michael Faraday Jun 23 at 17:35

Firstly, we need to clearly define "atomic volume" and "molar volume". Typically, one would understand "atomic volume" to be the volume of an atom of the element and it is directly related to the radius of the atom. Molar volume is a macroscopic property that is defined by Wikipedia as the ratio of the molar mass to the mass density of the substance at a certain temperature and pressure. As such, molar volume would not only be dependent on atomic radius but would also depend on the structure and bonding of the atoms.

With this understanding, it is easier to explain the trend that you have provided. The molar masses of the group 1 metals are considerably large but yet they have a relatively low mass density. As such, they are seen as spikes in the graph, with the highest molar volumes in their respective periods. Mass density generally increases from group 1 to group 13 resulting in the fall in molar volume across the period initially while for the transition metals, they stay roughly constant and take relatively large values. However, once we reach the non-metals, the trend seems to reverse. The density values for the non-metals seem to display some variability so I am not sure how the their molar volume trend can be explained. Obviously, their molar volume would be higher than that of the transition metals that come before them as those are significantly more dense.

Density data for the elements can be found here. More detailed molar volume data for the elements can also be found on another page at the same website.

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