Which of the listed properties is the same for the elements beryllium (Be), magnesium (Mg), and calcium (Ca) ?

reactivity

atomic radii

number of energy levels

number of valence electrons

Answer is valence electron, they all have total of 2 valence electron.

remember group number = valence electron

Answer: The correct answer is number of valence electrons.

Explanation:

Elements are distributed into 7 Periods and 18 groups in periodic table.

Elements that belong to same group show similar chemical properties because they have same number of valence electrons.

Beryllium belongs to Group 2 and period 2 of the periodic table.

Magnesium belongs to Group 2 and period 3 of the periodic table.

Calcium belongs to Group 2 and period 4 of the periodic table.

Hence, the correct answer is number of valence electrons.