Which Period 3 element would you expect to have the highest electrical and thermal conductivity?
A. Chlorine (CI)
B. Phosphorus (P)
C. Sulfur (S)
D. Sodium (Na)

The correct option is D

Explanation:

Generally, thermal and electrical conductivity is affected by valence electrons in an atom. These valence electrons carry electric charges (or conduct heat as the case may be) throughout the atom. The fewer the valence electrons, the higher its conductivity - this is as a result of free movement within the atom. The higher the valence electrons, the lesser its conductivity and when the outermost electrons have completely filled the outermost shell, the atom would be an insulator as there won't be carriers of electric charges.

Hence, from the above, sodium would be expected to have the highest electrical and  thermal conductivity because it has just one electron in it's outermost shell

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