Answer:
4. Valence electrons
Explanation:
Strontium and barium have similar chemical properties because atoms of these elements have the same number of valence electrons.
The valence electrons are the outer shell electrons within an atom. Both strontium and barium belongs to the group of alkali -earth metals on the periodic table.
This group consists of elements with two valence electrons.
The number of valence electrons determines chemical properties.
