Answer:
Same number of valence electrons.
Explanation:
Elements in periodic table are arranged in such a way that the elements having same chemical properties lies in the same vertical columns called as groups. The reason for same chemical properties is the presence of same number of valence electrons in the valence shell.
Examples:
Sodium and Potassium are both Alkali Metals. They almost show same chemical properties. The electronic configuration both elements is as follow,
Na = 11 = 1s², 2s², 2p⁶, 3s¹
K = 19 = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s¹
It is observed that in both cases the valence shells (i.e. 3rd in Na and 4rth in K) contains same number of valence electrons. Therefore, it is concluded in main group elements the family share same number of valence electrons.
