Question

what does it mean if elements are in the same group? what does it mean if elements are in the same period? number of electrons energy level

Explanation:

Elements in the same period have the same principal energy level. This is because the period number in the periodic table corresponds to the highest - occupied principal energy level (n - value) of the electrons in the atoms of those elements. Elements in the same group have the same number of valence electrons. Valence electrons are the electrons in the outermost shell of an atom and are responsible for the chemical properties of the element.

Answer:

If elements are in the same period, it means they have the same principal energy level. If elements are in the same group, it means they have the same number of valence electrons.