Relative values compare particle mass and charge
Definition
Relative mass
The mass of a particle compared with the mass of a proton, which is assigned a value of 111.
Definition
Relative electric charge
The charge of a particle compared with the magnitude of a proton's charge, which is assigned a value of +1+1+1.
- A proton has relative mass 111 and a neutron has relative mass 111.
- An electron and a positron each have relative mass about 11840\dfrac{1}{1840}18401, so their contribution to atomic mass is very small.
- A proton has relative charge +1+1+1, a neutron has 000, an electron has −1-1−1 and a positron has +1+1+1.
- Relative mass and relative charge are ratios, so they have no units.
Each sub-atomic particle has a distinct combination
- Proton: relative mass 111, relative charge +1+1+1 and located in the nucleus.
- Neutron: relative mass 111, relative charge 000 and located in the nucleus.
- Electron: relative mass about 11840\dfrac{1}{1840}18401, relative charge −1-1−1 and found outside the nucleus.
- Positron: relative mass about 11840\dfrac{1}{1840}18401 and relative charge +1+1+1.
- A positron is the electron's antiparticle, so it has the same mass as an electron but the opposite charge.
Common Mistake
- Do not confuse a positron with a proton; both have charge +1+1+1, but a positron has about 11840\dfrac{1}{1840}18401 of the proton's mass.
- Do not give relative mass or relative charge a unit because each value is a comparison.
Equal proton and electron numbers make an atom neutral
Definition
Neutral atom
An atom with no overall electric charge because it contains equal numbers of protons and electrons.
- Each proton contributes +1+1+1 and each electron contributes −1-1−1, so equal numbers give a total charge of 000.
- Neutrons do not affect overall charge because each neutron has relative charge 000.
- The total relative charge can be found from Q=Np−NeQ=N_{p}-N_{e}Q=Np−Ne, where NpN_pNp and NeN_eNe are the numbers of protons and electrons.
Example
Calculating overall charge
- An atom with eleven protons and eleven electrons has Q=11−11=0Q=11-11=0Q=11−11=0, so it is neutral.
- If it loses one electron, Q=11−10=+1Q=11-10=+1Q=11−10=+1, so it becomes a positive ion.
Exam technique
Recalling particle properties
- Give both the relative mass and the relative charge when a question asks for particle properties.
- Use the exact particle name required; Edexcel distinguishes a proton, positron, electron and neutron.
Self review
- What are the relative mass and charge of a proton?
- What are the relative mass and charge of a neutron?
- How do an electron and a positron differ?
- Why is an atom neutral?
- Why do neutrons not affect an atom's overall charge?
