Particles in an atom Revision Notes for AQA GCSE Chemistry

Particles in an atom

What are subatomic particles?

All atoms are made up of three tiny particles called subatomic particles. These are:

Protons - positively charged particles found in the nucleus
Neutrons - neutral particles (no charge) found in the nucleus
Electrons - negatively charged particles that orbit around the nucleus

Understanding these particles helps us work out the structure of any atom.

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The nucleus is incredibly small compared to the overall size of the atom, but it contains almost all of the atom's mass because protons and neutrons are much heavier than electrons.

Atomic number

The atomic number tells us how many protons an atom contains. This number is very important because:

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

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

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Remember: The atomic number is like an element's fingerprint - it never changes and always tells us exactly which element we're dealing with.

Number of electrons

In a normal atom, the number of electrons always equals the number of protons. This is because:

Atoms have no overall electrical charge
Protons are positive and electrons are negative
These opposite charges balance each other out perfectly

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

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This balance of charges is what makes atoms electrically neutral. When atoms gain or lose electrons, they become charged particles called ions.

Mass number

The mass number shows the total number of heavy particles in the nucleus. To find it, we add:

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

This means we can work out the number of neutrons by using:

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

Worked example with sodium

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Worked Example: Finding Particles in a Sodium Atom

Let's look at a sodium atom:

Atomic number = 11
Mass number = 23

From this information we can work out:

Step 1: Find the number of protons

Protons: 11 (this is the atomic number)

Step 2: Find the number of electrons

Electrons: 11 (same as the number of protons)

Step 3: Calculate the number of neutrons

Neutrons: 23 - 11 = 12 (mass number minus atomic number)

Atomic diagrams

We can draw atoms using simple diagrams that show the structure and arrangement of particles:

Nucleus in the centre (containing protons and neutrons)
Electrons arranged in circles around the nucleus
Protons often shown as white circles
Neutrons often shown as black circles
Electrons shown as crosses (x) on the outer rings

These diagrams make it easy to count each type of particle and understand atomic structure.

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Atomic diagrams are not drawn to scale - if they were, the electrons would be so far from the nucleus that the diagram would be enormous! The diagrams help us visualise the arrangement of particles.

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Key Points to Remember:

Protons, neutrons and electrons are the three subatomic particles in atoms
Atomic number = number of protons and identifies the element
In neutral atoms, electrons = protons so the charges balance out
Mass number = protons + neutrons giving the total heavy particles
Number of neutrons = mass number - atomic number is a useful calculation to remember

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