Lesson 1 of 4
Particles, isotopes and electric fields
Connect mass and charge to particle counts and beam paths.
A-Level 9476 (2026-2027)
I can describe the three subatomic particles.Syllabus
Syllabus 9476, 1(a). Describe the three subatomic particles.
I can deduce particle-beam behaviour in an electric field.Syllabus
Syllabus 9476, 1(b). Deduce particle-beam behaviour in an electric field.
I can locate the mass and charges of an atom.Syllabus
Syllabus 9476, 1(c). Locate the mass and charges of an atom.
I can calculate particle counts in atoms and ions.Syllabus
Syllabus 9476, 1(d). Calculate particle counts in atoms and ions.
I can explain nuclear composition and isotopes.Syllabus
Syllabus 9476, 1(e). Explain nuclear composition and isotopes.
Almost all the mass is in the nucleus
Proton number identifies the element; electron number determines its charge.
An atom contains a tiny, positively charged nucleus surrounded by electrons. Protons and neutrons supply almost all its mass. Electrons occupy the much larger surrounding region and provide negative charge. A neutral atom has equal numbers of protons and electrons; neutral does not mean that it contains no charged particles.
| Particle | Relative charge | Relative mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | About 1/1840 | Orbitals outside the nucleus |
In AZX, the proton number Z is the number of protons and the nucleon number A is 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. Gaining or losing electrons does not change A or Z.
Worked exampleRead a charged isotope correctly
How many protons, neutrons and electrons are in 5626Fe3+ and 3717Cl-?
Try it first. Then show the worked stepsHide the worked steps
- Fe has 26 protons and 56 - 26 = 30 neutrons. Its 3+ charge means it has lost three electrons: 26 - 3 = 23.
- Cl has 17 protons and 37 - 17 = 20 neutrons. Its negative charge means one extra electron: 17 + 1 = 18.
Fe3+: 26 p, 30 n, 23 e. Cl-: 17 p, 20 n, 18 e. The charge changes the electron count, not the proton count.
Isotopes are atoms of the same element with the same proton number but different neutron numbers. Chlorine-35 and chlorine-37 both have 17 protons, but have 18 and 20 neutrons. Their neutral atoms have the same electron configuration, so their chemical properties are very similar; their masses differ. Do not confuse an isotope difference with an ionisation change.
Check your understanding
Two species each have 18 electrons. Must they be isotopes of one element?
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No. Ar, Cl- and Ca2+ all have 18 electrons but different proton numbers.
Species with the same number of electrons are isoelectronic. Isotopes must have the same proton number and different neutron numbers.
A beam bends according to charge and mass
Positive particles bend towards the negative plate; negative particles bend towards the positive plate.
Make a guess. You are not marked.
A beam of protons and a beam of electrons pass between two charged plates at the same speed. How do their paths compare?
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Electrons bend the other way, and much more sharply
Deflection depends on charge divided by mass. The charges are equal and opposite, but an electron is about 1840 times lighter, so it bends far more.
Three beams in the same electric field
A positive upper plate and negative lower plate deflect an electron upwards and a proton downwards. A neutron continues straight. The electron path bends more strongly. These are schematic paths for equal initial speed and the same field length.
The electric force is F = qE. A neutron has no net charge and is not deflected by the uniform electric field. A proton and electron have equal charge magnitudes, so the force magnitudes are equal but opposite. The electron has much less mass, giving much greater acceleration under that force.
For particles entering at the same speed, the time inside the field is the same. Deflection is then proportional to charge-to-mass ratio, |q|/m. If speeds or initial kinetic energies differ, that comparison alone is insufficient. Read the beam conditions before choosing the greatest deflection.
Worked exampleCompare ion beams
At equal initial speeds, which bends more: 4He2+ or 20Ne+?
Try it first. Then show the worked stepsHide the worked steps
- Use relative |q|/m values: He2+ gives 2/4 = 0.50; Ne+ gives 1/20 = 0.050.
- The helium ion has ten times the ratio, so its deflection is greater under the stated conditions.
- Both are positive, so both bend towards the negative plate.
He2+ bends more strongly; its sign determines direction and its charge-to-mass ratio determines the comparison in these conditions.