2.1.2

# Specific Charge, Protons & Neutron Numbers

Test yourself

## Specific Charge

Charged particles can be described in terms of their specific charge.

### Specific charge

• Specific charge is calculated by dividing the charge of a particle (in coulombs) by its mass (in kilograms):
• Specific charge (C/kg) = charge (C) ÷ mass (kg)

### Example: sodium ion

• A sodium ion has a mass of 23 proton masses (mp).
• This is 3.85 × 10-26kg.
• It also has a charge of +1 electron charges (e).
• This means its charge is 1.60 × 10-19C.
• So:
• Specific charge = (1.60 × 10-19C) ÷ (3.85 × 10-26kg)
• Specific charge = 4.16 × 106C/kg

## Proton and Nucleon Numbers

Proton numbers and nucleon numbers tell us information about atoms.

### Proton number (Z)

• The number of protons in a nucleus tells us what element an atom is.
• For example, hydrogen atoms always have 1 proton.

### Nucleon number (A)

• The nucleon number is the total number of protons and neutrons in a nucleus.
• Atoms of the same element can have different numbers of neutrons.
• For example, hydrogen can have 0, 1 or 2 neutrons.