Which property describes an atom's combining power based on electrons it can gain or give up?

• A. Mass number
• B. Electronegativity
• C. Valence
• D. Bond energy

Answer: (C)

Valence is the number of bonds an atom tends to form, reflecting how many electrons it can gain, lose, or share to reach a stable, low-energy arrangement. This combining power is determined by the electrons in the outermost shell and how far the atom is from fulfilling an octet (or duet for hydrogen). For many main-group elements, valence corresponds to how many electrons are needed to complete the outer shell, so atoms with one, two, or four valence electrons typically form one, two, or four bonds, respectively. This is why carbon, with four valence electrons, tends to form four bonds; hydrogen, with one, tends to form one bond; and oxygen, with six outer electrons, tends to form two bonds. The other terms describe different ideas: mass number is just total protons and neutrons and doesn’t predict bonding; electronegativity describes how strongly an atom pulls electrons in a bond, not how many bonds it forms; bond energy measures the energy needed to break a bond, not an atom’s bonding capacity.