Which statement best explains why calcium and chlorine combine in a 1:2 ratio?

1. Chlorine can accept twice as many electrons as calcium has in its outer shell.
2. Calcium has two valence electrons, and chlorine can accept one electron in its outer shell.
3. Chlorine’s atomic number is twice calcium’s atomic number.
4. Calcium has one electron shell, and chlorine has two electron shells.

Option 2. Calcium has two valence electrons, and chlorine can accept one electron in its outer shell.