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 it usually gives the lowest whole-number ratio of ions needed for charge balance, such as $NaCl$ or $CaCO_3$. You read a formula any time you need to identify a substance precisely, which is the first step before balancing equations or converting moles to mass.

A Quick Reference Of Common Formulas

If you came looking for common compounds and their formulas, start here.

How To Read Any Formula, Step By Step

A formula tells you composition, but the meaning depends on the kind of compound, so work through it in order.

1. Identify the compound type. Decide whether it is molecular or ionic, because formulas are interpreted differently. For molecular compounds, subscripts give the actual number of each atom in one molecule: $CO_2$ has one carbon and two oxygens per molecule. For ionic compounds, the formula is about charge balance: $NaCl$ pairs one $Na^+$ with one $Cl^-$, while $CaCl_2$ pairs one $Ca^{2+}$ with two $Cl^-$.
2. Read each subscript carefully. A subscript changes the count of the symbol before it, unless parentheses show that it applies to a whole group.
3. Check charge balance for ionic compounds. In an ionic formula the total positive and negative charge must cancel to zero.
4. Keep formulas and coefficients separate. A coefficient counts whole units of a compound; subscripts are part of the compound's identity. In $2H_2O$, the coefficient $2$ means two molecules, while the subscript $2$ means two hydrogen atoms per molecule.

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

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

Apply the steps. Calcium forms a $Ca^{2+}$ ion; hydroxide is the polyatomic ion $OH^-$. To make a neutral compound the charges must cancel, so one $Ca^{2+}$ ($+2$) needs two $OH^-$ ions (total $-2$):

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.

Where Each Step Goes Wrong

• Step 4, mixing up subscripts and coefficients. In $2H_2O$ the coefficient $2$ counts molecules; the subscript $2$ counts atoms per molecule.
• Step 1, reducing a molecular formula you should not. $H_2O_2$ is not simplified to $HO$; molecular formulas are part of the substance's identity.
• Step 3, forgetting to balance charge in ionic compounds. If the charges do not cancel, the formula is not complete.
• Step 2, ignoring parentheses around polyatomic ions. In $Ca(OH)_2$ or $Al_2(SO_4)_3$ the parentheses show the whole ion repeats; without them the formula is read incorrectly.

A fast self-check is to ask four questions: Is this molecular or ionic? Do the subscripts match the intended atom counts or ion ratios? If ionic, do the charges balance to zero? If a polyatomic ion repeats, did I use parentheses correctly? Those catch most beginner errors.

Practice Reading Formulas

Read $CaCl_2$, $Na_2CO_3$, and $C_6H_{12}O_6$ out loud in words. For each, say whether the formula describes a molecule or the lowest ion ratio: $C_6H_{12}O_6$ (glucose) is molecular, while $CaCl_2$ and $Na_2CO_3$ are ionic ratios. That comparison is a fast way to confirm the concept clicked.