Protons, Neutrons & Electrons (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

Revision Note

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Author

Ann H

Last updated

28 August 2024

Relative charge

Charge can be positive or negative
Protons, neutrons and electrons have different properties and different charges
- Protons have a positive charge
- Electrons have a negative charge
- Neutrons have no charge
Relative charge can be used to compare these properties
- Relative charge is the ratio of the charge of a particle compared to the fundamental charge
- Because relative charge is a ratio, it has no units
Fundamental charge is equal to the magnitude of the charge on a proton and an electron = 1.6 × 10⁻¹⁹ C
The relative charge of a proton is

$proton = \frac{1.6 \times 10^{- 19} C}{1.6 \times 10^{- 19} C} = + 1$

The relative charge of an electron is

$electron = \frac{- 1.6 \times 10^{- 19} C}{1.6 \times 10^{- 19} C} = - 1$

The relative charge of a neutron is

$neutron = \frac{0 C}{1.6 \times 10^{- 19} C} = 0$

Table of relative charges

Particle	Relative charge
Proton	+1
Neutron	0
Electron	−1

Examiner Tip

You don't need to know how to calculate relative charge, but you do need to understand what it is. You do also need to remember what the relative charges are for each subatomic particle.

Nuclear charge

Extended tier only

Nuclear charge is the relative charge of the nucleus
This is determined by the proton number of the atom
Relative charge is calculated by:

$nuclear charge = number of protons in the nucleus \times relative charge of a proton$

An element of carbon has a proton number of 6
- So the relative charge of carbon is 6 × (+1) = 6

Worked example

What is the relative charge of the chromium nucleus ${}_{24}^{52}{Cr} ?$

Answer:

Step 1: Determine the number of protons

The number of protons is the proton number
This is the Z number in the AZX notation

${}_{Z}^{A}X$

- This chromium nucleus has 24 protons

${}_{24}^{52}{Cr}$

Step 2: State the relative mass of 1 proton

1 proton has a relative charge of +1

Step 3: Multiply relative charge of 1 proton by the number of protons

This nucleus of chromium therefore has a relative charge of +24

Examiner Tip

Charge can be either positive (+) or negative (–). Usually, when a number is positive, we don't write the + sign, but when dealing with relative charge, we do.

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