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Potassium

1s2 2s2 2p6 3s2 3p6 4s1

Explanation:

The atom having only one electron its outermost shell must belong to an element in group one of the periodic table.

Having noted that, we proceed to find out what element in group one that has the atom just described in the question.

That atom must belong to an element in the fourth period. The only group 1 element in the fourth period is potassium.

The electron configuration of potassium is;

1s2 2s2 2p6 3s2 3p6 4s1

we can conclude that the melting point of table salt is above room temperature (it's actually 801 deg c). the melting point of mercury must be below room temperature (- 39 deg c).