Classify each compound as ionic or molecular. a. CO2 b. NiCl2 c. NaI d. PCl3

Understanding the difference between ionic and molecular compounds starts with a grasp of chemical bonding, the force that holds atoms together in compounds. Atoms bond to achieve a more stable electron configuration, which normally comes in the form of a full outer electron shell. There are several types of bonds, but the two most relevant to our exercise are ionic and covalent bonds.

Ionic bonding occurs when there is a complete transfer of electrons from one atom (usually a metal) to another (usually a nonmetal). This transfer creates ions; the metal becomes a positively charged cation, while the nonmetal becomes a negatively charged anion. The electrostatic attraction between these oppositely charged ions forms an ionic bond.

Covalent bonding, on the other hand, involves the sharing of electrons between atoms, typically nonmetals. Instead of transferring electrons to achieve stability, these atoms share one or more pairs of electrons in areas known as molecular orbitals. The resulting structure is what we know as a molecular compound, which exhibits different properties from ionic compounds.

When distinguishing between ionic and molecular compounds, the nature of the elements involved plays a critical role. Compounds composed of metals and nonmetals tend to be ionic. For instance, table salt (NaCl) comprises a metal (sodium) and a nonmetal (chlorine), forming an ionic bond as electrons are transferred from sodium to chlorine.

Ions have strong attractions due to the significant difference in electronegativity— a measure of an atom's ability to attract and hold onto electrons—between metals and nonmetals. Metals have lower electronegativity and are more likely to lose electrons, while nonmetals have higher electronegativity and are inclined to gain them, making the formation of ionic compounds energetically favorable.

Molecular compounds, also known as covalent compounds, can be classified based on their constituent atoms. Since these compounds involve nonmetals, which have similar electronegativities, they share electrons to form covalent bonds. The molecular compounds are further classified by the types of nonmetals that make them up, and the capacity of the bonding nonmetals to share electrons.

For example, a compound like CO2 has two nonmetals—carbon and oxygen. These atoms share electrons to fill their outer shell, resulting in a stable covalent bond. Understanding the specific nonmetals and their properties, such as electronegativity and valence electrons, allows for accurate prediction of molecular structure and the behavior of the compound under various physical and chemical conditions.