The period to which an atom belongs can tell how many energy levels it has.
The period table of an element are categorised into horizontal rows (periods) and vertical columns (groups).
Each elements falls under a particular row (i.e period) and column (i.e group).
The group number talks about the number of valence electron(s) in the outermost shell of the atom while the period number talks about the number of energy levels (or number of shells) the atom has.
This is the most reason why elements in the same group have the same number of valence electron(s) while elements in the same period have the same number of energy levels
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