Question

True or false? The maximum covalency in 2nd-period elements is `4`. Why is this true if fluorine can only make one bond?

Answer 1

Notice what the "rule" says: the maximum covalency of 2nd-period elements. It doesn't say for all elements in the 2nd period, but rather, when considering each element.

Carbon is the atom that can make four bonds at once, e.g. in `"CH"_4` and other hydrocarbons. Not fluorine.

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On the other hand, `"Br"` and `"I"` can make more than four bonds, as they can pool in `(n-1)d` orbitals to form hypervalent compounds such as `"BrF"_5` and this beast (periodinane).