Topic 1 of 3
Nucleus and charge
Read particle counts and field evidence.
A-Level 8873, revised syllabus (2026-2027)
Count particles before counting electrons
Atomic number identifies the element; electron loss changes charge, not the nucleus.
An atom has a very small, positively charged nucleus containing protons and neutrons. Almost all its mass is concentrated there. Electrons occupy the surrounding space, which accounts for almost all the volume. A neutral atom has equal numbers of protons and electrons; an ion does not.
| Particle | Relative charge | Relative mass | Where it is found |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | About 1/1836 | Outside the nucleus |
The proton number, Z, is the number of protons and defines the element. The nucleon number, A, counts protons plus neutrons. Therefore neutrons = A - Z. For an ion with signed charge q, electrons = Z - q: subtract a positive charge, but add the magnitude of a negative charge. Ion formation transfers electrons; it does not change A or Z.
Worked example
Read an isotope and its ion
An aluminium-27 ion has Z = 13 and charge 3+. How many of each particle are present?
- Protons = 13, because the element is aluminium.
- Neutrons = 27 - 13 = 14.
- A neutral atom has 13 electrons. Losing three gives 10 electrons.
- Charge check: 13 positive charges and 10 negative charges give +3.
The ion contains 13 protons, 14 neutrons and 10 electrons. It is still aluminium-27.
Isotopes have the same proton number but different neutron numbers. Chlorine-35 and chlorine-37 both have 17 protons, but have 18 and 20 neutrons respectively. Neutral atoms of both have 17 electrons, so their electron arrangements and chemical behaviour are very similar. Their masses differ; isotopes are not different ions.
Check your understandingCompare 23Na and 23Na+ (Z = 11). Are they isotopes of one another?Think it through, then reveal the answer
Use a field to identify the charge
Direction identifies the sign; the amount of bending also depends on the beam conditions.
Deflection of particle beams
Electrons bend towards the positive upper plate, protons towards the negative lower plate, and neutrons continue straight. At the same initial speed an electron bends much more because its mass is much smaller.
Opposite charges attract. A proton beam deflects towards the negative plate; an electron beam deflects towards the positive plate. Neutrons have no charge and are not deflected by the electric field. In the same field, force depends on charge and acceleration depends on force divided by mass.