Covalent Compound Naming Rules

Overview

This lecture explains how to identify and name binary covalent compounds, including rules for their formulas and the use of Greek prefixes.

A covalent compound is formed when atoms are held together by covalent bonds (shared pairs of valence electrons).
Covalent bonds restrict the number and arrangement of atoms, creating discrete molecules.
The formula of a covalent compound represents the actual number of atoms in each molecule.
Unlike ionic compounds, covalent compound formulas are not ratios but reflect real molecular units (e.g., H₂O, CH₄).
Multiple unique compounds can exist for the same two elements (e.g., nitrogen and oxygen), so distinct names are necessary.

A compound is covalent if it consists of two nonmetals bonded together.
Metalloids usually form covalent compounds but can sometimes form ionic compounds.
The formal test is an electronegativity difference less than 1.9, but the two-nonmetal rule is simpler for most cases.

Step 1: Write the name of the first element as is.
Step 2: Write the name of the second element, changing its ending to -ide.
Step 3: Use Greek prefixes to indicate the number of each atom (except don't use "mono-" for the first element).
Always use a prefix on the second element, even if there is only one atom (e.g., carbon monoxide).
Example names: CO = carbon monoxide; CO₂ = carbon dioxide; B₂Cl₄ = diboron tetrachloride; N₂O₅ = dinitrogen pentoxide; CF₄ = carbon tetrafluoride.

Identify elements and their counts from the prefixes in the compound name.
Write the correct elemental symbol and use subscripts for the number indicated by the prefix.
Example: dinitrogen monoxide = N₂O; sulfur hexafluoride = SF₆; tetraphosphorus trisulfide = P₄S₃.

Covalent Bond — a pair of shared electrons between two atoms.
Molecule — the smallest unit of a covalent compound that retains its properties.
Greek Prefixes — words used to indicate the number of each atom in a molecule.