Lesson 1 of 3
Nucleus and charge
Read particle counts and field evidence.
A-Level 8873, revised syllabus (2026-2027)
I can identify the subatomic particles.Syllabus
Syllabus 8873, 1(a). Identify the subatomic particles.
I can predict beam deflection.Syllabus
Syllabus 8873, 1(b). Predict beam deflection.
I can locate atomic mass and charge.Syllabus
Syllabus 8873, 1(c). Locate atomic mass and charge.
I can count particles in atoms and ions.Syllabus
Syllabus 8873, 1(d). Count particles in atoms and ions.
I can interpret nuclear notation and isotopes.Syllabus
Syllabus 8873, 1(e). Interpret nuclear notation and isotopes.
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 exampleRead an isotope and its ion
An aluminium-27 ion has Z = 13 and charge 3+. How many of each particle are present?
Try it first. Then show the worked stepsHide the worked steps
- 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 understanding
Compare 23Na and 23Na+ (Z = 11). Are they isotopes of each other?
Show the answer
No. Both have 11 protons and 12 neutrons; only the electron count differs.
Isotopes have the same proton number but different neutron numbers. These have the same nucleus, so they are the same isotope. The atom has 11 electrons and the ion has 10.
Use a field to identify the charge
Direction identifies the sign; the amount of bending also depends on the beam conditions.
Make a guess. You are not marked.
A beam of neutrons passes between two charged plates. What happens to its path?
Show the answer
It goes straight through, as neutrons have no charge
Neutrons have no charge, so the electric field exerts no force on them.
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.
Check your understanding
A beam of protons, electrons and neutrons passes between charged plates. What happens if the plate polarities are reversed?
Show the answer
Protons and electrons bend the other way; neutrons still go straight.
Reversing the field reverses the force on each charged particle. Neutrons have no charge, so the field has no effect on them.