Particles in an atom Revision Notes for AQA GCSE Chemistry Combined Science

Particles in an atom

What are subatomic particles?

All atoms are made up of three tiny particles called subatomic particles. Understanding these fundamental building blocks is essential for learning about atomic structure and how elements behave.

These particles are:

Protons - positively charged particles
Neutrons - particles with no charge (neutral)
Electrons - negatively charged particles

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These three subatomic particles determine all the properties of an element, including its identity, mass, and chemical behaviour.

You can work out exactly how many of each particle an atom contains by using two important numbers.

Atomic number

The atomic number tells you how many protons are in an atom. This number is really important because:

Every element has its own unique atomic number
No two different elements can have the same atomic number
The atomic number identifies which element you're looking at

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The atomic number is like an element's fingerprint - it's unique to that element and never changes. This is what makes hydrogen different from helium, or sodium different from chlorine.

For example, if an atom has 11 protons, it must be sodium because sodium's atomic number is 11.

Number of electrons

Atoms normally have no overall electrical charge. This happens because:

The number of electrons always equals the number of protons
Positive charges (protons) cancel out negative charges (electrons)
This makes the atom electrically neutral

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This electrical neutrality is why we don't get electric shocks from touching most everyday objects - the atoms that make them up have balanced charges.

So if an atom has 11 protons, it will also have 11 electrons.

Mass number

The mass number tells you the total number of particles in the atom's centre (called the nucleus).

To find the mass number, you add together:

$\text{Number of protons} + \text{Number of neutrons} = \text{Mass number}$

This means:

$\text{Number of neutrons} = \text{Mass number} - \text{Atomic number}$

How to work out particle numbers

Here's how to find each type of particle:

Protons = Atomic number
Electrons = Same as number of protons (in neutral atoms)
Neutrons = Mass number - Atomic number

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Worked Example: Finding Particles in Sodium

A sodium atom has atomic number 11 and mass number 23

Step 1: Find the number of protons Protons = Atomic number = 11

Step 2: Find the number of electrons
Electrons = Number of protons = 11

Step 3: Find the number of neutrons Neutrons = Mass number - Atomic number = 23 - 11 = 12

Answer: Sodium has 11 protons, 11 electrons, and 12 neutrons

Atomic diagrams

Scientists draw atoms as simple diagrams to show where the particles are located within the atomic structure. These visual representations help us understand the arrangement of subatomic particles:

Protons and neutrons sit together in the centre (nucleus)
Electrons move around the outside in circles called shells
The diagram shows the atomic number and mass number clearly

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Atomic diagrams are simplified representations that help us visualise particle locations. In reality, electrons don't move in perfect circles but in complex three-dimensional regions called orbitals.

In these diagrams:

Crosses (×) often represent electrons
Solid circles represent neutrons
The nucleus contains both protons and neutrons

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Key Points to Remember:

Atoms contain three subatomic particles: protons, neutrons and electrons
Atomic number = number of protons (and electrons in neutral atoms)
Mass number = protons + neutrons combined
Neutrons = mass number - atomic number
Atoms have no overall charge because protons and electrons balance out

Particles in an atom (AQA GCSE Chemistry Combined Science): Revision Notes