Chemical Formulas of Common Compounds

Chemical formulas show which elements are in a compound and how many atoms or ions are involved. For a molecular compound, the formula gives the number of each atom in one molecule, such as $H_2O$ or $CO_2$. For an ionic compound, the formula usually gives the lowest whole-number ratio of ions needed for charge balance, such as $NaCl$ or $CaCO_3$.

If you searched for common compounds and their formulas, start with this quick reference.

Common Compounds And Their Chemical Formulas

Compound	Formula	Notes
Water	$H_2O$	Two hydrogen atoms and one oxygen atom per molecule
Carbon dioxide	$CO_2$	One carbon and two oxygen atoms per molecule
Ammonia	$NH_3$	Common molecular compound made of nitrogen and hydrogen
Methane	$CH_4$	Simplest hydrocarbon
Hydrogen peroxide	$H_2O_2$	Not the same substance as water
Sodium chloride	$NaCl$	Ionic compound in a 1:1 ion ratio
Calcium carbonate	$CaCO_3$	Common in limestone, chalk, and shells
Sodium bicarbonate	$NaHCO_3$	Also called baking soda
Sodium hydroxide	$NaOH$	Common strong base
Calcium hydroxide	$Ca(OH)_2$	Parentheses show two hydroxide ions
Hydrochloric acid	$HCl$	Common acid; often written $HCl(aq)$ in water
Sulfuric acid	$H_2SO_4$	Two acidic hydrogens attached to sulfate

What A Chemical Formula Means

A formula tells you composition, but the meaning depends on the kind of compound.

For molecular compounds, subscripts give the actual number of each atom in one molecule. In $CO_2$, every molecule has one carbon atom and two oxygen atoms.

For ionic compounds, the formula is mainly about charge balance. In $NaCl$, one $Na^+$ balances one $Cl^-$. In $CaCl_2$, one $Ca^{2+}$ needs two $Cl^-$ ions, so the formula has a 1:2 ratio.

That difference matters. A molecular formula is not usually reduced, because changing it changes the substance. $H_2O_2$ is hydrogen peroxide, not water. An ionic formula, by contrast, is normally written in the smallest ratio that still balances charge.

Worked Example: Why Calcium Hydroxide Is $Ca(OH)_2$

Calcium forms a $Ca^{2+}$ ion. Hydroxide is the polyatomic ion $OH^-$.

To make a neutral compound, the total positive and negative charge must cancel. One calcium ion has a charge of $+2$, so it needs two hydroxide ions with total charge $-2$.

$$Ca^{2+} + 2OH^- \rightarrow Ca(OH)_2$$

The parentheses matter. Writing $CaOH_2$ would say something different, because the subscript 2 would apply only to hydrogen. The correct formula, $Ca(OH)_2$, shows two complete hydroxide groups.

Common Mistakes

Mixing Up Subscripts And Coefficients

In $2H_2O$, the coefficient 2 means two water molecules. The subscript 2 means each water molecule contains two hydrogen atoms. Those are different jobs.

Reducing A Molecular Formula When You Should Not

$H_2O_2$ should not be simplified to $HO$. Molecular formulas are part of the identity of the substance.

Forgetting To Balance Charge In Ionic Compounds

For ionic compounds, the formula must be electrically neutral. If the charges do not cancel, the formula is not complete.

Ignoring Parentheses Around Polyatomic Ions

In compounds such as $Ca(OH)_2$ or $Al_2(SO_4)_3$, the parentheses show that the whole ion repeats. Without them, the formula is read incorrectly.

When Chemical Formulas Are Used

Chemical formulas show up any time you need to identify a substance precisely. They are used in reaction equations, stoichiometry, molar-mass calculations, lab labels, safety documents, and ingredient lists in technical contexts.

They are also the bridge between a compound's name and its quantitative chemistry. If you cannot read the formula correctly, later steps such as balancing equations or converting moles to mass become harder than they need to be.

A Simple Way To Check A Formula

Ask four quick questions:

1. Is this molecular or ionic?
2. Do the subscripts match the intended atom counts or ion ratios?
3. If it is ionic, do the charges balance to zero?
4. If a polyatomic ion repeats, did I use parentheses correctly?

Those checks catch most beginner errors fast.

Try A Similar Problem

Try reading $CaCl_2$, $Na_2CO_3$, and $C_6H_{12}O_6$ out loud in words. For each one, say whether the formula describes a molecule or the lowest ion ratio. That comparison is a fast way to check whether the concept really clicked.