Answer:
The number of electrons in there outermost shell
Explanation:
The periodic table consists of elements that are arranged according to there atomic number. Elements are arranged into vertical columns (called groups) and horizontal rows (called periods).
Elements in the same group have the same number of electron(s) in there outermost shell (hence have the same chemical properties). For example, elements in group 1 have one electron in there outermost shell and elements in group 17 have seven electrons in there outermost shell.
Elements in the same period have the same number of electron shell(s).
