Dalton's atomic theory and atomic mass

An idea to explain two laws

John Dalton, an English teacher, asked why the two laws are true. In 1808 he gave an answer, the atomic theory. It was based on an old idea that matter is made of tiny particles.

Dalton's atomic theory

1. All matter is made of very small particles called atoms.
2. Atoms cannot be created or destroyed in a chemical reaction.
3. All the atoms of a given element are alike. They have the same mass and the same chemical properties.
4. Atoms of different elements have different masses and different chemical properties.
5. Atoms combine in small whole number ratios to form compounds.
6. In a given compound, the relative number and kind of atoms are constant.

How the theory explains the laws

• Conservation of mass. Atoms are not created or destroyed. So the number of atoms, and the total mass, stay the same.
• Constant proportions. In a compound, the atoms are always in the same ratio. Each kind of atom has its own mass, so the elements are in the same ratio by mass.

What science has learned since

Dalton's picture was a very good beginning. We now know that:

• an atom can be split into smaller particles (protons, neutrons and electrons), which we study in the next chapter;
• atoms of the same element can have slightly different masses (these are called isotopes).

But the main ideas still hold for chemical reactions.

How small is an atom?

Atoms are extremely small. An atom of hydrogen has a radius of about $0.1\ \text{nm}$, which is $10^{-10}\ \text{m}$. A line of five million hydrogen atoms would be only about $1\ \text{mm}$ long. We cannot see an atom even with a microscope that uses light, though special instruments can make pictures of them.

Symbols

Each element has a symbol, such as H, O, C, Na and Fe. In the past, Dalton used drawings. Now we use letters. A symbol stands for one atom of the element.

Atomic mass

An atom is so light that its mass in grams is a very small number. An atom of hydrogen has a mass of about $1.66 \times 10^{-24}\ \text{g}$. So we use a special unit, the atomic mass unit, with the symbol u.

One atomic mass unit is one twelfth of the mass of one atom of carbon-12.

The mass of an atom of an element in u is its atomic mass. Carbon-12 has an atomic mass of exactly $12\ \text{u}$. Some atomic masses are:

You should find that about $1.9 \times 10^{22}$ atoms of sulphur make a gram, and that hydrogen needs $32$ times as many.

Practise with new numbers

Try it in Python

This program lists the atomic masses, and finds the mass of one atom in grams.

Remember these

• Dalton's atomic theory: matter is made of atoms. Atoms are not created or destroyed in a reaction. They combine in small whole number ratios.
• An atom is extremely small, about $0.1\ \text{nm}$ across.
• $1\ \text{u}$ is one twelfth of the mass of a carbon-12 atom. Atomic mass is measured in u.

Check yourself

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

1. Which idea of Dalton's theory explains the law of conservation of mass?

2. How many times as heavy as a carbon atom is an atom of sulphur? Use $\text{C} = 12\ \text{u}$ and $\text{S} = 32\ \text{u}$.

3. What is the mass, in u, of three atoms of nitrogen together?

4. What part of Dalton's theory has had to change since 1808?