Which atom is likely to be more chemically reactive, an atom with a partly filled energy level or an atom with all energy levels completely filled? explain?

The correct option is an atom with a partly filled energy level

Explanation:

An atom with a partially filled energy level means the atom has a partially filled outermost shell which in turn means the atom as valence electrons. The presence of valence electrons in atoms makes them undergo chemical bonding with other atoms of the same/different elements (during chemical reactions).

Atoms with completely filled energy levels (such as the noble gases) have a completely filled outermost shell and hence do not have valence electrons which can take part in bond formation during chemical reactions and hence these group of atoms have low chemical reactivity.

It is more reactive the atom with a partly filled energy level because is less stable than the other case. Stability is inversely proportional to reactivity.