Magnesium and calcium have similar chemical properties because a magnesium atom and a calcium atom have the same

(1) atomic number

(2) mass number

(3) total number of electron shells

(4) total number of valence electrons

The correct answer is (4) total number of valence electrons. This means that they have the same number of electrons in their final electron shell and the same number of electrons that are prepared to establish bonds with electrons from other elements. No elements have the same atomic number and the same mass number.