L17. Naming Molecular (Covalent) Compounds

Chemical Names and Formulas

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L17. Naming Molecular (Covalent) Compounds

Why do CO and CO2 have similar formulas but very different names — and why do the names tell you how many atoms are in each molecule?

A quick real-world hook

Imagine opening a can of soda and feeling tiny bubbles burst — that fizz comes from carbon dioxide gas (CO2) leaving the liquid. Now imagine a different invisible gas, carbon monoxide (CO), which can form in a car engine and is dangerous in a closed garage. Both gases are made of carbon and oxygen, but one has two oxygen atoms and a different name. This lesson shows how names for binary molecular compounds (two different nonmetal elements joined together) tell you the number of atoms of each element and help you distinguish similar substances in everyday life.

• Phenomenon: Same two elements can make different molecules (CO vs CO2) with different properties. Goal: Learn the naming rules that communicate how many atoms are in a molecule and form clear names for binary molecular compounds.

Rules for naming binary molecular compounds

Use these simple rules for two-element molecular compounds (both are nonmetals). Start with the name of the first element, then name the second element with an -ide ending. Use Greek prefixes to show how many atoms of each element are present. The exception: omit the prefix mono- when the first element has only one atom (we still say mono- for the second element if needed).

• Prefixes: 1 mono-, 2 di-, 3 tri-, 4 tetra-, 5 penta-, 6 hexa-, 7 hepta-, 8 octa-. Format: [prefix for first element if >1] + name of first element, then [prefix for second element if any] + root of second element + -ide. Example pattern: CO2 → carbon (no mono-) + di- + oxide → carbon dioxide.

Worked examples and a small practice model

Write names from formulas and check the particle picture in your head. For PCl3: phosphorus (first element) + tri- + chloride → phosphorus trichloride. For SF6: sulfur + hexa- + fluoride → sulfur hexafluoride. For N2O: di- + nitrogen appears first, then mono- + oxide → dinitrogen monoxide (also called nitrous oxide in common usage, but the systematic name uses prefixes). Practice by drawing the molecule as connected atoms to match the name: if the name says tetra- or di-, your drawing must show that many atoms.

• PCl5 → phosphorus pentachloride (5 chlorides). CO → carbon monoxide (no prefix on first element; one oxygen so mono- is used only for the second element implicitly, but we write carbon monoxide). Quick check: Names and formulas must agree on atom counts — the name tells how many of each atom are present.

Tips, common names, and safety notes

Some molecular substances have traditional or common names you will also hear (water, ammonia, nitrous oxide). For chemistry class, rely on systematic prefix names so you can tell atom counts. Remember safety: similar-looking names can mean very different safety risks (CO is poisonous; CO2 is not poisonous at low concentrations and gives fizz in drinks). When you later learn to write formulas, these names will guide the formula construction because the prefixes directly tell you how many atoms to include.

• Use systematic names in class; learn a few common exceptions as vocabulary (water = H2O, ammonia = NH3). Always check that the name’s prefixes match the intended atom counts before using a formula in a calculation or safety label.

Big idea synthesis

Systematic names for binary molecular compounds communicate the number of each type of atom by using Greek prefixes and the -ide ending on the second element. Memorize the common prefixes and the rule that the first-element mono- is usually omitted.

These names let you compare similar substances (for example CO versus CO2) and infer practical differences — a single change in atom count can change chemical behavior and safety. Use naming as a reliable map from words to the particle-level picture you'll draw later when you write formulas and Lewis models.