What Are Atoms Made Of?

What you will learn

• Name the particles in an atom and their electric charges.
• Explain the roles of the nucleus and of the electrons.
• Read an atom drawing correctly, including the parts that are out of scale.

Before you start

No specialist background is needed; technical terms are explained as they appear.

What is in the nucleus?

The nucleus is the core of every atom. It consists of electrically positive protons and electrically neutral neutrons. A force called the strong force holds these particles together. It is the strongest known fundamental force.

The physicist Ernest Rutherford discovered the nucleus in 1911. Not every nucleus is stable. Some are unstable and may undergo radioactive decay.

The word “nucleus” names only one part of an atom. If you draw just the cluster of particles at the centre and call it the whole atom, you have left out the electrons.

Where are the electrons and what do they do?

Electrons are negatively charged and surround the nucleus. The number of electrons usually matches the number of protons in the nucleus.

The chemical properties of a substance are determined by the negatively charged electrons around the nucleus. So understanding an atom does not stop at counting the particles in the nucleus.

The picture in this article uses a surrounding region to suggest where electrons are. The dots only help tell the parts apart. They are not photographs or paths of single electrons.

Why is an atom drawing not to scale?

The nucleus makes up much less than 0.01% of the atom’s volume. Yet it typically contains more than 99.9% of the atom’s mass. The nucleus is both very small and very heavy compared with the whole atom.

Drawn to scale, the nucleus would be too small to see. So drawings enlarge it to let you recognise the parts. Read the picture as a sorting diagram, not as a map of sizes.

Scale is hard to picture in the other direction too. A typical grain of sand contains more than 10 million trillion nuclei, which is 100 times more than the number of seconds since the beginning of the universe.

Real-world example

Suppose you explain atoms to a younger child with a drawing. Point to the cluster in the middle and say it is the nucleus, made of positive protons and neutral neutrons. Point to the surrounding region and say that is where the negative electrons are.

Then ask two questions. What determines chemical properties? Which part is heaviest? The answers are the electrons for the first and the nucleus for the second. If both are answered correctly, the drawing has done its job.

Common misunderstandings

The nucleus is not the whole atom. An atom is the nucleus plus the electrons around it.

Neutrons carry no charge. Protons are positive and electrons are negative.

An atom drawing does not show real sizes. It enlarges the nucleus so you can see it.

Frequently asked questions

What holds protons and neutrons together in the nucleus?

The strong force, the strongest known fundamental force, holds the particles of the nucleus together.

Why do electrons matter for chemistry?

The chemical properties of a substance are determined by the electrons around the nucleus.

How much of an atom’s mass is in the nucleus?

Typically more than 99.9%, although its volume is much less than 0.01% of the atom.

Summary

• The nucleus holds positive protons and neutral neutrons; negative electrons surround it.
• Electrons determine chemical properties, and the nucleus holds almost all the mass.
• Atom drawings enlarge the nucleus and are not to scale.