Covalent Compounds:
Covalent compounds are formed between two nonmetals and involve the sharing of electrons. In a covalent bond, the electrons are shared between the atoms, resulting in the formation of molecules. Let's analyze each compound to determine if it is covalent.

(a) H2:
H2 represents a molecule of hydrogen gas. It consists of two hydrogen atoms that share electrons to form a covalent bond. Since hydrogen is a nonmetal, H2 is a covalent compound.

(b) CaO:
CaO represents calcium oxide. Calcium (Ca) is a metal, while oxygen (O) is a nonmetal. The bond formed between a metal and a nonmetal is typically ionic, not covalent. Therefore, CaO is an ionic compound.

(c) KCl:
KCl represents potassium chloride. Potassium (K) is a metal, while chlorine (Cl) is a nonmetal. Similar to the previous case, the bond between a metal and a nonmetal is generally ionic. Hence, KCl is an ionic compound.

(d) Na2S:
Na2S represents sodium sulfide. Sodium (Na) is a metal, and sulfur (S) is a nonmetal. Again, the bond between a metal and a nonmetal is typically ionic. Therefore, Na2S is an ionic compound.

Summary:
- H2 is a covalent compound.
- CaO, KCl, and Na2S are ionic compounds.

Covalent compounds involve the sharing of electrons between nonmetals, while ionic compounds result from the transfer of electrons between metals and nonmetals.

H2..