Atoms of elements in the same group have the same number of . This is why they have similar chemical properties.

Same number of electrons in their outer shell.

Explanation:

Whe working out their electronic configuration ( by looking at the atomic number) , one will find out that the number of electrons in their outer shell is also the same so the elements have to lose or gain the same amount of electrons to become stable.

its , valence electrons  I just did it