Chemical Composition, Formulas, and the Mole Explained

Posted on Feb 21, 2026 in Chemistry

Compounds Display Constant Composition

Another indicator that can be used to distinguish compounds from mixtures is the proportions of elements.

Law of Constant Composition

The law of constant composition: All samples of a given compound have the same proportions of their constituent elements. Water will always have 1 oxygen for every 2 hydrogen atoms (H₂O).

Chemical Formulas: How to Represent Compounds

Chemical formula

Chemical formula: Indicates the elements present in a compound and the relative number of atoms of each. It indicates atomic ratio, not mass ratio. For example, water consists of hydrogen and oxygen atoms in a 2:1 ratio.

Order of elements in formulas

Chemical formulas typically list the more metallic elements first. In compounds that do not include a metal, the more metal-like element is listed first. Among nonmetals, those to the left of the periodic table are more metal-like. Within a single column, elements toward the bottom are more metal-like (example: SO₂).

Polyatomic ions

Polyatomic ions: A polyatomic ion is a group of two or more atoms that are bonded together and carry an overall electric charge. They act as one single unit in chemical reactions and formulas. Parentheses with a subscript are used if several groups of the same kind are present.

Molecular models

3D representations of molecules
Illustrate the geometry of the molecule
Two types of molecular models: ball-and-stick and space-filling

A Molecular View of Elements and Compounds

Subcategories of elements and compounds:

Atomic element: Exist in nature with single atoms as their basic units.
Molecular elements: Exist in nature as mostly diatomic molecules — two atoms of the element bonded together.
Molecular compounds: Formed from two or more nonmetals.
Ionic compounds: A combination of cation(s) and anion(s); usually cations are metals and anions are nonmetals.

Writing Formulas for Ionic Compounds

Formula unit

Formula unit: The basic unit of an ionic compound. It is the smallest electrically neutral collection of ions. Formula units do not exist as single entities, but as part of a three-dimensional array.

Nomenclature: Naming Compounds

Systematic naming approach

Systematic naming approach creates a unique name for every single compound.

To name a compound, first determine what kind of compound it is by looking at the elements in the formula:

If it contains a metal and a nonmetal, it is ionic (examples include NaCl, Fe₂S₃, and K₂SO₄).
If it contains only nonmetals, it is molecular (such as CO₂ or N₂O₄).
If it starts with hydrogen and includes one or more nonmetals, it is an acid (like HCl or H₂SO₄).

If the compound is ionic, determine whether the metal forms only one charge or more than one charge:

Metals with only one charge, such as Na⁺ or Ca²⁺, are named by writing the metal name followed by the nonmetal ending in -ide (for example, NaCl is sodium chloride).
Metals that can form multiple charges, usually transition metals like Fe²⁺/Fe³⁺ or Cu⁺/Cu²⁺, require a Roman numeral in the name to show the charge (FeS is iron(II) sulfide and FeCl₃ is iron(III) chloride).

If the compound is molecular, prefixes such as mono-, di-, tri-, and tetra- are used to indicate the number of atoms, and the second element always ends in -ide (for example, CO₂ is carbon dioxide and N₂O₄ is dinitrogen tetroxide).

If the compound is an acid and contains hydrogen and a nonmetal with no oxygen, it is a binary acid and is named using the pattern: hydro + root + -ic acid.

Formula Mass: The Mass of a Molecule or Formula Unit

Formula mass is the total mass of all the atoms in a chemical formula.

& 6.3 Counting Atoms by the Gram

Atoms are too small and too numerous to count individually.
A counting unit, the mole, is used as a more convenient way to count atoms.
1 mole = 6.022 × 10²³ things.
The mole has the unit mol and is sometimes represented by the symbol n.

The mass of 1 mol of atoms in grams is equal to the atomic mass of the atoms expressed in atomic mass units (amu).

Example:

The mass of a single copper atom is 63.55 amu.
1 mol of copper atoms has a mass of 63.55 g.
The molar mass of copper is 63.55 g/mol.

Definition of molar mass

Molar mass:

Mass of 1 mol of atoms of an element.
The value of an element’s molar mass in g/mol is numerically equal to the element’s atomic mass in amu.

Counting Molecules by the Gram

The concept of molar mass applies not only to atoms but also to molecules, ions, and formula units.

For atoms, molar mass is derived from the atomic mass.
For compounds, molar mass is derived from the formula mass.

Chemical Formulas as Conversion Factors

What if you need to determine quantities of a specific element within a compound? The chemical formula of a compound gives a relationship between the moles of atoms. For example, one mole of CH₄ has 4 moles of hydrogen.

Mass Percent Composition from a Chemical Formula

Chemical formulas can be used to calculate the mass percent composition of an element in a compound.