3.4: Classification of Elements and Compounds

Pure substances consist of only one type of matter. A pure substance can be an element or a compound. An element consists of only one type of atom, while a compound consists of two or more types of atoms held together by a chemical bond. Elements are classified as atomic or molecular based on the nature of their basic units.

Compounds are pure substances composed of two or more elements in fixed, definite proportions. Compounds are classified as ionic or molecular (covalent) based on the bonds present in them.

Molecular Compounds

Molecular compounds (or covalent compounds) result when two or more different nonmetal atoms share electrons to form covalent bonds. The basic units of molecular compounds are discrete neutral molecules composed of different constituent atoms. For example, the molecular compound carbon monoxide is composed of CO molecules containing covalently bonded carbon and oxygen atoms. Similarly, methanol contains CH₃OH molecules as base units, constituting one carbon atom, one oxygen atom, and four hydrogen atoms, all of which are covalently connected.

Molecular compounds can be identified based on their physical properties. Under normal conditions, molecular compounds often exist as gases, low-boiling liquids, and low-melting solids, although exceptions exist.

Ionic Compounds

When an element composed of atoms that readily lose electrons (a metal) reacts with an element composed of atoms that readily gain electrons (a nonmetal), a transfer of electrons usually occurs, producing ions. The compound formed by this transfer is stabilized by the electrostatic attractions (ionic bonds) between the ions of opposite charge present in the compound. For example, when each sodium atom in a sample of sodium metal (group 1) gives up one electron to form a sodium cation, Na⁺, and each chlorine atom in a sample of chlorine gas (group 17) accepts one electron to form a chloride anion, Cl⁻, the resulting compound, NaCl, is composed of sodium ions and chloride ions in the ratio of one Na⁺ ion for each Cl⁻ ion.

A compound that contains ions and is held together by ionic bonds is called an ionic compound. Ionic compounds are solids that typically melt at high temperatures and boil at even higher temperatures. In solid form, an ionic compound is not electrically conductive because its ions are unable to flow. When molten, however, it can conduct electricity because its ions can move freely through the liquid.

In every ionic compound, the total number of positive charges of the cations equals the total number of negative charges of the anions. Thus, ionic compounds are electrically neutral overall, even though they contain positive and negative ions.

Many ionic compounds contain polyatomic ions as the cation, the anion, or both. As with simple ionic compounds, these compounds must also be electrically neutral, so their formulas can be predicted by treating the polyatomic ions as discrete units. We use parentheses in a formula to indicate a group of atoms that behave as a unit. For example, the formula for calcium phosphate, one of the minerals in our bones, is Ca₃(PO₄)₂. This formula indicates that there are three calcium ions (Ca²⁺) for every two phosphate (PO₄)³⁻ groups. The (PO₄)³⁻ groups are discrete units, each consisting of one phosphorus atom and four oxygen atoms, and having an overall charge of 3−. The compound is electrically neutral, and its formula shows a total count of three Ca, two P, and eight O atoms.

This text is adapted from Openstax, Chemistry 2e, Section 2.6: Molecular and Ionic Compounds.

In nature, pure substances exist as either elements or compounds. Depending on the basic unit, elements exist in either atomic or molecular form, while compounds can be subcategorized as either molecular or ionic.

Atomic elements have single atoms as their basic units. Helium, neon, and krypton are examples of monatomic elements that can exist as stable individual atoms.

Molecular elements, on the other hand, contain two or more atoms bonded together, forming a molecule as the basic unit. Diatomic hydrogen with two hydrogen atoms and polyatomic sulfur with eight sulfur atoms are examples of molecular elements.

In comparison to molecular elements, molecular compounds are formed when atoms of two or more nonmetallic elements combine by sharing valence electrons through covalent bonds. 

The combination lowers the overall potential energy of the system, thereby driving the formation of a stable molecule. Hence, the basic units of a molecular compound are discrete molecules composed of constituent atoms.

Ethanol is a molecular compound, which contains basic units of distinct ethanol molecules comprising two carbon, six hydrogen, and one oxygen atoms.

Ionic compounds are formed when atoms of metallic elements combine with atoms of one or more nonmetallic elements. The metal atoms readily lose their valence electrons to form positively charged cations while the non-metal atoms gain electrons forming negatively charged anions.

Strong electrostatic attractions or ionic bonds are formed between the oppositely charged ions, thereby lowering the total energy of the system and driving the assembly of stable, ordered, three-dimensional ionic lattice structures. 

The smallest array of ions that form electrically neutral basic units of an ionic compound is called the formula unit. Table salt is an ionic compound with a formula unit NaCl containing an alternating three-dimensional array of sodium ions and chloride ions.

However, many ionic compounds are composed of ions that are not singular, like sodium or chloride, but made of multiple covalently bonded atoms with a net negative or positive charge — called polyatomic ions. 

For instance, sodium acetate is composed of positively charged sodium ions and polyatomic acetate ions carrying a net negative charge.