Answer:
Elements on the left of the period 3 have higher melting and boiling points than those on the right. These low melting and boiling points are typical of non-metals. Argon, chlorine, sulfur and phosphorus are non-metals. On the left hand side of period 3 we find the elements sodium, magnesium and aluminium.
Explanation:
In the whole of period 3, the outer electrons are in 3-level orbitals. These are all the same sort of distances from the nucleus, and are screened by the same electrons in the first and second levels. The major difference is the increasing number of protons in the nucleus as you go from sodium across to argon.
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