What is a chemical formula?
A chemical formula化学式 uses element symbols and numbers to show what a substance is made of. Every pure substance has a definite formula: water is and carbon dioxide is . Formulas aren’t made up. They come from experiments that measure what a substance contains.
Four things a formula tells you
Using as an example:
| What it tells you | For | |
|---|---|---|
| Substance | Which substance it is | Water |
| Substance | Which elements the substance is made of | Water is made of hydrogen and oxygen |
| Particles | One molecule of the substance | One water molecule |
| Particles | Which atoms make up one molecule, and how many | One water molecule contains 2 hydrogen atoms and 1 oxygen atom |
Formulas for three kinds of substances
- Substances made of molecules, such as , , and : the formula shows what one molecule contains.
- Substances made directly of atoms: metals (such as and ), noble gases (such as ), and a few solid nonmetals (such as ). The formula is just the element symbol.
- Substances made of ions, such as sodium chloride, . There are no molecules, so the formula gives the ratio of atoms: 1 sodium to 1 chlorine.
Pick a formula below, and notice what changes when a coefficient is added in front:
About the substance
- It stands for the substance water.
- Water is made of the elements hydrogen and oxygen.
About the particles
- It stands for one molecule of water.
- One molecule of water contains 2 hydrogen atoms and 1 oxygen atom.
About the substance
- It stands for the substance carbon dioxide.
- Carbon dioxide is made of the elements carbon and oxygen.
About the particles
- It stands for one molecule of carbon dioxide.
- One molecule of carbon dioxide contains 1 carbon atom and 2 oxygen atoms.
About the substance
- It stands for the substance oxygen.
- Oxygen is made of the element oxygen.
About the particles
- It stands for one molecule of oxygen.
- One molecule of oxygen contains 2 oxygen atoms.
About the substance
- It stands for the substance sodium chloride.
- Sodium chloride is made of the elements sodium and chlorine.
About the particles
- Sodium chloride is made of ions, so there are no sodium chloride molecules.
- The ratio of sodium to chlorine atoms is 1 : 1.
About the substance
- It stands for the element iron.
- It stands for the substance iron.
About the particles
- It stands for one iron atom.
About the substance
- It stands for the element hydrogen.
About the particles
- It stands for one hydrogen atom.
About the substance
—
About the particles
- It stands for 2 hydrogen atoms.
With a coefficient in front, it only describes particles.
About the substance
—
About the particles
- It stands for 2 molecules of water.
With a coefficient in front, it only describes particles.
Where a number sits changes what it means
| Position | Example | Meaning |
|---|---|---|
| In front (coefficient) | , | Number of particles: 2 hydrogen atoms, 2 water molecules |
| Lower right (subscript) | the 2 in | Number of atoms of an element in one molecule |
| Upper right | the 2 in | Charge on one ion |
| Directly above | the +2 in | Valence of the element (next lesson) |
Note: once a formula has a coefficient in front, it only describes particles. means 2 water molecules, not “2 waters.”
Example 1: Carbon dioxide
What four things does tell you?
- The substance. It stands for the substance carbon dioxide.
- Its elements. Carbon dioxide is made of the elements carbon and oxygen.
- One molecule. It stands for one carbon dioxide molecule.
- What’s in the molecule. One carbon dioxide molecule contains 1 carbon atom and 2 oxygen atoms.
Example 2: Four look-alike symbols
What do , , , and each mean?
- : the element nitrogen, and also one nitrogen atom.
- : with a coefficient, it only means 2 nitrogen atoms.
- : the substance nitrogen gas, made of the element nitrogen; also one nitrogen molecule, which contains 2 nitrogen atoms.
- : with a coefficient, it only means 2 nitrogen molecules.
- The 2 in
It's in front of the symbol, so it means 2 hydrogen atoms.
- The 2 in
It's a subscript, so one oxygen molecule contains 2 oxygen atoms.
- The 2 in
It's at the upper right, so one magnesium ion carries 2 units of positive charge.
- The 2 in
It's directly above the symbol, so magnesium has a valence of +2 in magnesium oxide.
- The 2 in
It's in front of the formula, so it means 2 carbon monoxide molecules.
- The 2 in
It's the subscript on H, so one sulfuric acid molecule contains 2 hydrogen atoms.
- The upper-right 2 in
One sulfate ion carries 2 units of negative charge.
Common mistakes
Mistake 1: Mixing the substance level with the particle level. “Water is made of 2 hydrogen atoms and 1 oxygen atom” is wrong. Describe a substance with elements (water is made of hydrogen and oxygen) and a molecule with atoms (one water molecule contains 2 hydrogen atoms and 1 oxygen atom).
Mistake 2: “ means 2 hydrogens.” With a coefficient it only describes particles: 2 hydrogen molecules.
Mistake 3: “A molecule of sodium chloride.” Sodium chloride is made of ions, so it has no molecules at all.
Quick check
Which statement about is correct?
A mixes the substance with individual atoms. C is wrong because carbon dioxide is a pure substance, not a mixture. D is wrong because molecules are made of atoms, and a carbon dioxide molecule contains no oxygen molecule.
Which symbol only describes particles, not a substance?
A coefficient in front means a number of particles. only means 2 nitrogen atoms.
Summary
- A chemical formula uses element symbols and numbers to show what a substance is made of.
- For the substance, it tells you which substance it is and which elements it contains. For particles, it tells you about one molecule and which atoms it contains.
- A number in front counts particles, a subscript counts atoms in one molecule, an upper-right number is an ion’s charge, and a number directly above is a valence.
- With a coefficient, a formula only describes particles. Substances made of ions have no molecules.
On to AP (for later)
AP topic 1.3 uses formulas to describe the elemental composition of substances and distinguishes molecular formulas from empirical (simplest) formulas. For ionic compounds, AP says “formula unit” instead of “molecule.”