The structure of the atom Revision Notes for AQA GCSE Physics

The structure of the atom

What atoms are made of

All atoms are built from three tiny particles. These particles have different properties that make them special.

Protons have a positive electrical charge (+1) and a mass of 1. Neutrons have no electrical charge (0) and a mass of 1. Electrons have a negative electrical charge (-1) and almost no mass (0).

infoNote

These three fundamental particles work together to form every atom in the universe. Understanding their properties is essential for grasping how atoms behave and interact.

How atoms are arranged

Atoms have a specific structure that follows the same pattern everywhere. At the centre of every atom sits the nucleus. This nucleus contains all the protons and neutrons packed tightly together.

The electrons don't stay in the nucleus. Instead, they move around the nucleus in paths called orbits or energy levels. Think of it like planets orbiting the sun, but much more complex.

infoNote

The solar system analogy helps visualise atomic structure, but remember that electrons don't follow fixed circular paths like planets do. Their movement is much more complex and follows quantum mechanical principles.

Key facts about the nucleus

The nucleus is the control centre of the atom. Here are the most important things to remember:

chatImportant

Critical Nuclear Properties:

The nucleus always has a positive charge because it contains protons
More than 99% of the atom's mass comes from the nucleus
The number of protons in the nucleus must equal the number of electrons orbiting around it
Electrons always have a negative charge and orbit the nucleus
Atoms are neutral overall because positive protons cancel out negative electrons
Different atoms of the same element can have different numbers of neutrons

Size of atoms and nuclei

Atoms are incredibly tiny. The whole atom has a diameter of about $10^{-10}$ metres. That's 0.0000000001 metres!

The nucleus is even smaller. It has a diameter of about $10^{-15}$ metres. This means the nucleus is about 100,000 times smaller than the whole atom.

infoNote

Most diagrams in textbooks don't show the real size difference. If they did, you wouldn't be able to see the nucleus at all because it would be far too small to represent accurately on the page.

How electrons move between energy levels

Electrons can jump between different energy levels around the nucleus. This happens when they absorb or release energy.

When an electron absorbs electromagnetic radiation (like light), it gains energy and moves to a higher energy level further from the nucleus.

When an electron emits electromagnetic radiation, it loses energy and drops to a lower energy level closer to the nucleus.

chatImportant

This movement of electrons between energy levels explains how atoms can absorb and give off light. This principle is fundamental to understanding how we can identify different elements through spectroscopy.

Working with atomic scales

Here's how to calculate how many nuclei would fit across an atom:

lightbulbExample

Worked Example: Calculating Nuclear Scale

Given:

Atom diameter: $4.8 \times 10^{-10}$ m
Nucleus diameter: $1.2 \times 10^{-15}$ m

Calculation: $\frac{4.8 \times 10^{-10}}{1.2 \times 10^{-15}} = 4 \times 10^5 = 400,000 \text{ nuclei}$

Method: Divide the numbers at the front, then subtract the powers. $\frac{4.8}{1.2} \times 10^{(-10-(-15))} = 4 \times 10^5$

Tip for standard form division: Divide the numbers at the front, then subtract the powers.

bookmarkSummary

Key Points to Remember:

Atoms contain protons (positive, mass 1), neutrons (neutral, mass 1), and electrons (negative, mass 0)
The nucleus is tiny but contains nearly all the atom's mass
Atoms are neutral because they have equal numbers of protons and electrons
Electrons orbit the nucleus and can move between energy levels
Atoms are incredibly small, and nuclei are even smaller

The structure of the atom (AQA GCSE Physics): Revision Notes