Complex sentence: A univalent (monovalent) atom, ion or group has a valence of one and thus can form one covalent bond; a divalent molecular entity has a valence of two and can form two sigma bonds or one sigma bond plus one pi bond.
Split into simpler sentences: A univalent (monovalent) atom, ion or group has only one valence electron. A divalent molecular entity has a valence of two and can form two sigma bonds or one sigma bond plus one pi bond.