What is the common about the elements of a period

The common about the elements of a period is that they have the same number of atomic orbitals.

Explanation:

The period of an element corresponds to the principal quantum number of the valence shell.

They are a total of 7 periods in the periodic table.

The periods represent the energetic levels that an atom has.

The periods are ordered in 7 horizontal rows.

For example, in the first period we have the Hydrogen (H) and the Helium (He).

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explanation:

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