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The Wikipedia article on metals describes them as elements whose polyatomic configurations* exhibit any one of a number of patterns. I could not discern any property that characterized all the patterns.

To the best of my knowledge, there are two main categories of elements: metals, non-metals. There's a third category, metalloids, whose members are those that satisfy some of the criteria for inclusion in both main categories.

It would seem very odd if there were nothing that characterized all, and only, metals. In fact it would seem that metals are better conceived of as an arbitrary grouping of what is actually a number of other categories, each of which would include all, and only, the elements that exhibit a single type of polyatomic configuration*.

Is there a characteristic that characterizes all, and only, metals?

*(I'm using the phrase 'polyatomic configuration' in order to express 'the way that atoms of a particular element stick together when you have a number of atoms of that element connected together'. Is there a standard term that expresses that idea?)

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  • $\begingroup$ Metals will crystallize in a wide variety of Bravais lattices, much less point groups. And, the elements are divided into many finer shades of grey than metals and non-metals. I believe that your question rests on unfounded assumptions. $\endgroup$
    – Jon Custer
    May 8, 2015 at 18:20
  • $\begingroup$ @Hal Have you looked at conductivities? That is the way I would classify metals, semiconductors, insulators and superconductors... If you would like to know more about this I would be happy to write an answer but I'm not sure if you are aware of this already? $\endgroup$
    – AngusTheMan
    May 8, 2015 at 20:07
  • $\begingroup$ @AngusTheMan I've been reading about the periodic table and metalicness (?) on Wikipedia, looking for the answer. It did seem that conductivity was the underlying property, and that the metallic-nonmetallic dichotomy isn't best thought of as a pair of categories, but rather as opposite extremes of a continuum. I also learned that there are some more fundamental properties of elements that explain why certain patterns appear in the table. Is there a reason why conductivity correlates with the other characteristics of metals (opacity, hardness, etc.)? $\endgroup$
    – Hal
    May 8, 2015 at 20:58
  • $\begingroup$ One quality shared by metals is a "sea" of valence electrons, though as you state, this valence band is shared to some extent by metaloids (aka semiconductors). Many of the properties of metals are dependent on the fact that electrons, to a greater or lesser extent, are not localized. $\endgroup$
    – DrMoishe Pippik
    May 8, 2015 at 21:20
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    $\begingroup$ I'm afraid there's no such characteristics and even such characteristics can't exist as there is no sharp boundary between metallic and other substances. $\endgroup$
    – Mithoron
    May 8, 2015 at 23:20

2 Answers 2

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The Universe is full of continuous and smoothly-transitioning properties, yet which we often stubbornly try to assign a discrete yes/no answer in order to make the world easier to cope with. Metallicity is one of them. The Van Arkel-Ketelaar triangle is a diagram which attempts to show the transition between covalent, ionic and metallic types of bonding. Here is one populated by a variety of compounds:

The closer a substance is to one of the vertices, the more that vertex is important in describing the nature of bonding in the material. As you can see, the entire triangle is populated quite uniformly. There are certainly many more materials which would fit in the gaps between the materials shown, and even simply varying the ambient conditions (pressure, temperature, etc), a same substance can be put in different regions of the triangle.

Given this behaviour, it should be clear that any attempt to rigorously define a line dividing substances between metals and non-metals will be fraught with difficulty and likely not useful in the first place. No matter where the line is drawn, there will be some compounds that will look strange for "barely being a metal", while others will be strange for "barely being a non-metal".

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Possibly there is such a single characteristic.

I say "possibly" because in periodic table terms there is no universal agreement as to where metals stop and nonmetals (or metalloids) start.

That aside, Johnson (2007, p. 15) defines a metal as any element having an electrical conductivity at or below room temperature of at least 3 x 105 S m-1 along any direction in a single crystal of any known form of the element.

In practical terms, most authors agree that boron, silicon, germanium, and antimony are metalloids, so that all elements to the left of these are metals. There is some uncertainty as to the status of polonium and astatine. Both allotropes of polonium are metallic conductors; and astatine has been predicted to be a metal. So it is probably reasonable to regard polonium and astatine as metals.

Johnson's 2007 definition appears to be in agreement with these observations but for astatine, which he shows as a semi-metal or metalloid. However the prediction of astatine as a metal was made in 2013, so there is no real contradiction here.

In summary, as earlier Users noted, electrical conductivity may fit the bill.

  • Johnson D (ed) 2007, Metals and chemical change, RSC Publishing, Cambridge
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  • $\begingroup$ I can't think of anything better. +1. You probably know that Si and Ge actually become metallic when melted (they contract as they do so), and just about anything becomes metallic under enough pressure. $\endgroup$
    – Oscar Lanzi
    Apr 27 at 19:10

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