Molecules and atomicity

Atoms that stay together

Most atoms are not found alone. The oxygen that we breathe is not made of single atoms. Two atoms of oxygen are joined together as a pair. A group of atoms that are joined together and behave as one particle is a molecule.

A molecule is the smallest particle of an element or a compound that can exist by itself and that shows the properties of that substance.

Think of a small family that always travels together. One person alone is an atom. A family is a molecule. A water molecule is a family of three: one oxygen atom and two hydrogen atoms, and you never find one of them wandering alone in a glass of water.

Molecules of elements

The atoms in a molecule of an element are all of the same kind.

• Hydrogen: $\text{H}_2$, two atoms.
• Oxygen: $\text{O}_2$, two atoms.
• Nitrogen: $\text{N}_2$, two atoms.
• Chlorine: $\text{Cl}_2$, two atoms.
• Ozone: $\text{O}_3$, three atoms of oxygen.
• Phosphorus: $\text{P}_4$, four atoms.
• Sulphur: $\text{S}_8$, eight atoms.

Notice that oxygen has two forms: $\text{O}_2$ is the oxygen that we breathe, and $\text{O}_3$ is ozone, a gas in the upper air that protects us from harmful rays of the Sun. Both are the element oxygen, but they are different molecules and have different properties.

Some elements are made of single atoms. The noble gases, such as helium, He, and neon, Ne, are like this. Metals such as iron, Fe, and carbon, C, are written with a single symbol.

Molecules of compounds

The atoms in a molecule of a compound are of different kinds, joined in a fixed ratio.

• Water: $\text{H}_2\text{O}$. Two atoms of hydrogen and one of oxygen.
• Carbon dioxide: $\text{CO}_2$. One atom of carbon and two of oxygen.
• Ammonia: $\text{NH}_3$. One atom of nitrogen and three of hydrogen.
• Methane: $\text{CH}_4$. One atom of carbon and four of hydrogen.
• Hydrogen chloride: HCl. One atom of hydrogen and one of chlorine.
• Glucose: $\text{C}_6\text{H}_{12}\text{O}_6$. Six atoms of carbon, twelve of hydrogen and six of oxygen, which is $24$ atoms in all.

The ratio of the atoms in a compound is fixed. Water is always $\text{H}_2\text{O}$. If the ratio were different, it would be a different compound. Two hydrogen atoms and two oxygen atoms is $\text{H}_2\text{O}_2$, hydrogen peroxide, a different substance with different properties.

Atomicity

The number of atoms in one molecule is its atomicity.

Atomicity counts all the atoms, of every kind. So $\text{H}_2\text{O}$ has atomicity $3$ ($2 + 1$) and $\text{CH}_4$ has atomicity $5$ ($1 + 4$).

How to read a formula

In a formula, the small number below and after a symbol tells how many atoms of that element are in the molecule. If there is no number, the number is $1$.

For $\text{H}_2\text{SO}_4$ (sulphuric acid): $2$ atoms of hydrogen, $1$ atom of sulphur and $4$ atoms of oxygen, which is $7$ atoms in all.

A number before a formula tells how many molecules. So $2\,\text{H}_2\text{O}$ means two molecules of water, with $4$ atoms of hydrogen and $2$ atoms of oxygen.

Brackets work like multiplication. In $\text{Ca(OH)}_2$ (calcium hydroxide), the small $2$ after the bracket multiplies everything inside. So there is $1$ atom of calcium, $2$ atoms of oxygen and $2$ atoms of hydrogen, which is $5$ atoms in all.

A worked example. Count the atoms in $3\,\text{Al}_2(\text{SO}_4)_3$.

• In one unit of $\text{Al}_2(\text{SO}_4)_3$: aluminium $2$, sulphur $3$, oxygen $4 \times 3 = 12$. That is $17$ atoms.
• For $3$ units: aluminium $6$, sulphur $9$, oxygen $36$, which is $51$ atoms.

You should find that the small number after the bracket multiplies both the sulphur and the oxygen, but not the aluminium.

Practise with new numbers

Try it in Python

The program finds the atomicity of four molecules from their formulas.

Common mistakes

• Reading the small number as part of the name. The $2$ in $\text{O}_2$ is the number of atoms, not a second element.
• Mixing up a small number and a big number. $\text{O}_2$ is one molecule with two atoms. $2\,\text{O}$ would be two separate atoms. $2\,\text{O}_2$ is two molecules, with four atoms.
• Forgetting the number $1$. In $\text{CO}_2$, the carbon has no number, so it is $1$.
• Forgetting that a bracket multiplies everything inside.
• Saying that ozone is a compound. It is an element: all its atoms are oxygen.

Remember these

• A molecule is a group of atoms that are joined together and act as one particle.
• A molecule of an element has atoms of one kind. A molecule of a compound has different kinds, in a fixed ratio.
• Atomicity is the number of atoms in a molecule. $\text{O}_2$ is diatomic, and $\text{O}_3$ is triatomic.
• A small number after a symbol: atoms in one molecule. A number before a formula: the number of molecules. A bracket multiplies what is inside.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How many atoms of each element are there in one molecule of acetic acid, $\text{C}_2\text{H}_4\text{O}_2$?

2. What is the atomicity of ozone, $\text{O}_3$, and is ozone an element or a compound?

3. How many hydrogen atoms are there in $4\,\text{NH}_3$?

4. How many atoms of each kind are there in $\text{Mg(OH)}_2$?