Covalent Compound Naming Rules

Overview

This lecture explains how to name covalent (molecular) compounds, focusing on compounds with two non-metal elements and the rules for using prefixes and naming conventions.

Covalent Compounds Basics

• Covalent compounds, also called molecules, consist only of non-metals.
• Covalent compounds cannot contain metals.

Diatomic Elements

• Seven diatomic elements: hydrogen (H₂), nitrogen (N₂), oxygen (O₂), fluorine (F₂), chlorine (Cl₂), bromine (Br₂), iodine (I₂).
• Diatomic elements naturally exist as pairs, not single atoms.

Monoatomic vs. Diatomic Elements

• Monoatomic elements: lithium (Li), sodium (Na), silver (Ag).
• Diatomic elements: oxygen (O₂), iodine (I₂).

Common Names to Memorize

• H₂O is water.
• NH₃ is ammonia.
• CH₄ is methane.

Naming Binary Covalent Compounds

• Only name compounds with two different non-metal elements.
• Do not use the prefix "mono-" for the first element.
• Always use a prefix for the second element if needed (e.g., CO₂: carbon dioxide).
• If a prefix ends in a vowel and the element begins with a vowel, drop one vowel (e.g., monoxide, not monooxide).

Prefixes and Examples

• CO: carbon monoxide (not monocarbon monoxide).
• SF₄: silicon tetrafluoride.
• P₂O₅: diphosphorus pentoxide.
• SO₃: sulfur trioxide.

Subscript Rules and Reductions

• Do not reduce subscripts in covalent compound formulas (e.g., H₂O₂ stays as hydrogen peroxide).
• Ionic compounds require reduced subscripts to the simplest whole number ratio (e.g., Na₂Cl₂ becomes NaCl).

Using Prefixes vs. Charges

• Use prefixes to determine covalent compound formulas, not charges.
• Charges are used only for naming ionic compounds.

Ionic vs. Covalent Naming Summary

• Ionic: Metal + non-metal, use charges, no prefixes, reduce subscripts.
• Covalent: Non-metals only, use prefixes, do not reduce subscripts.

Key Terms & Definitions

• Covalent compound — molecule made of only non-metal elements.
• Diatomic element — element found as pairs of atoms (e.g., O₂).
• Monoatomic element — element found as single atoms (e.g., Na).
• Binary covalent compound — compound with two different non-metal elements.
• Prefix — word part indicating the number of atoms (e.g., di-, tri-, tetra-).
• Subscript — small number in a formula showing atom count.

Action Items / Next Steps

• Memorize the seven diatomic elements and their formulas.
• Learn the three common covalent compound names (water, ammonia, methane).
• Practice naming and writing formulas for binary covalent compounds using prefixes.