Atomic structure and particle counts
Describe the nucleus and electron shells, compare subatomic particles, and count protons, neutrons and electrons in atoms and single-atom ions.
On this page
A reliable particle count starts with proton number and nucleon number, then adjusts the electron count only if the particle is an ion.
Understand the key terms
An atom is a particle of an element with a nucleus surrounded by electrons. Its number of protons identifies the element. A neutral atom has equal numbers of protons and electrons.
Key ideas
- Atoms contain protons and electrons, and most also contain neutrons. The common hydrogen-1 atom has no neutron.
- The nucleus contains protons and neutrons and has almost all the mass.
- Proton number (Z) = number of protons = defines the element.
- Nucleon number (A) = protons + neutrons.
- Neutral atom: protons = electrons. Ion: electron number changes, proton number does not.
Build the explanation
- Z, the proton number, counts protons.
- A, the nucleon number, counts protons + neutrons.
- Neutrons = A-Z.
- A neutral atom has Z electrons. Formation of a single-atom ion changes electrons, not the nucleus.
Inside an atom
- Nucleus: at the centre; contains protons and neutrons (together called nucleons).
- Electron shells (energy levels): electrons occupy energy levels around the nucleus. Shell diagrams represent these arrangements; the circles are not literal tracks followed by electrons.
Swipe or scroll sideways to inspect the complete overview.
The nucleus occupies only a tiny part of the atom’s volume, but it contains almost all its mass. Each proton or neutron is roughly 1840 times as massive as an electron. The diagram enlarges the nucleus so you can see its particles; it is not to scale.
Protons make the nucleus positively charged. Negatively charged electrons are attracted to it. In a neutral carbon atom, six proton charges balance six electron charges; the six neutrons contribute mass but no electric charge.
Compare relative masses and charges
| Particle | Relative Mass | Relative Charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 (uncharged) | Nucleus |
| Electron | about 1/1840 | -1 | Electron shells |
Use proton and nucleon numbers
- Proton number (Z) = number of protons in the nucleus (also called the atomic number).
- Nucleon number (A) = number of protons + number of neutrons (also called the mass number).
From A and Z:
- Protons = Z
- Neutrons = A - Z
Compare neutral atoms and ions
In a neutral atom, number of electrons = number of protons, so total charge is zero.
A single-atom ion forms when an atom gains or loses electrons. The nucleus and element identity stay the same:
- Positive ion (e.g., Mg²⁺): has lost electrons.
- Negative ion (e.g., Cl⁻): has gained electrons.
Isotopes have the same Z but different A (different number of neutrons). See Elements & Isotopes.
Misconceptions to check
- Writing neutrons as A + Z (wrong). It is A - Z.
- Saying “an ion has different number of protons” (wrong). Proton number never changes when forming ions.
- Forgetting that a negative charge means electrons gained (not lost).
- Mixing up A and Z when reading nuclide notation A_(Z)X.
- Thinking electrons are in the nucleus. Electrons are in shells around the nucleus.
Check your reasoning
Protons = Z; neutrons = A-Z. Do not use a decimal relative atomic mass from the Periodic Table as a nucleon number: A describes one isotope and is a whole-number count.
Start with electrons = Z. Then adjust for the charge: 2 + means subtract 2 electrons; 1- means add 1 electron.
Worked examples
Modelled example 1
Counting Particles in a Neutral Atom
Problem
Study the worked solution
Read proton and nucleon numbers
Method
Identify Z = 11 and A = 23 from the nuclide notation.
Reason
The lower number is proton number and the upper number is total nucleon number.
Working
Z = 11, A = 23.Count protons and neutrons
Method
Use protons = Z and neutrons = A-Z.Reason
Nucleon number counts protons plus neutrons.Working
Protons = 11; neutrons = 23-11 = 12.Use electrical neutrality
Method
Set the electron count equal to the proton count.Reason
No ionic charge is shown, so the atom is neutral.Working
Electrons = 11.State all particle counts
Working
11 protons, 12 neutrons and 11 electrons.
Guided practice 2
Counting Particles in a Positive Ion
Problem
Keep protons fixed and adjust electrons
Hints
Hint 1: fix the element identity
Ion formation does not change the nucleus, so protons remain equal to Z.
Hint 2: interpret the positive charge
A 2 + charge means two electrons have been lost.
View solution step by step
Count the protons
Method
Read the proton number.Reason
Changing electron count does not change the element.
Working
Protons = Z = 12.Adjust the electron count
Method
Subtract two electrons from the neutral count.Reason
The 2 + charge shows that two electrons have been lost.
Working
Electrons = 12-2 = 10.
Common misconception 3
Check a sodium-ion electron count
Learner response
Use the charge to test the electron count
View solution step by step
Locate the first error
Method
Reject using the neutral electron count for Na⁺.Reason
Equal proton and electron counts would give a neutral sodium atom, not a positive ion.Working
The stated 11 protons and 11 electrons have net charge zero.Apply the ion charge
Method
Keep 11 protons and subtract one electron.Reason
Formation of Na⁺ removes one negatively charged electron while leaving the nucleus unchanged.Working
Electrons = 11-1 = 10.State the correction
Working
Na⁺ has 11 protons and 10 electrons.
Examiner practice 4
Counting Particles in a Negative Ion
Examination question
Write each count before self-marking
View solution step by step
Count protons
1 markMethod
Use proton number Z.Reason
The lower number identifies the number of protons.Working
Protons = 17.Count neutrons
1 markMethod
Subtract proton number from nucleon number.Reason
A counts all protons and neutrons.Working
Neutrons = 35-17 = 18.Count electrons
1 markMethod
Add one electron to the neutral count.Reason
The 1- charge shows that the atom gained one electron.
Working
Electrons = 17 + 1 = 18.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark proton, neutron and electron counts separately.
Challenge 5
Infer which particle count changes
Relationship transfer
Two atoms have the same proton number but different nucleon numbers. Deduce which particle count must differ and name the relationship.
Work backwards from A and Z
Hints
Hint 1: hold proton number fixed
The common Z means both atoms are the same element.
Hint 2: rearrange the nucleon relationship
Use neutrons = A-Z.
View solution step by step
Infer the changing particle count
Method
Use neutrons = A-Z.Reason
Z is fixed while A differs, so only the neutron contribution to nucleon number can differ.
Working
Same Z and different A → different neutron numbers.
Name the relationship
Working
Atoms of the same element with different neutron numbers are isotopes.
Try these independently
Mind stretcher 1: Unknown AExtension
Question: An atom has 13 protons and 14 neutrons. Write its nuclide notation in the form A_(Z)X.
Show Answer
Z = 13 so the element is aluminium, Al.
A = 13 + 14 = 27.
Final: ²⁷₁₃Al.
Mind stretcher 2: Mixed Data InterpretationExtension
Question: An ion has 18 electrons and a 2 + charge. (a) How many protons does it have? (b) Which element is it?
Show Answer
A 2 + ion has 2 fewer electrons than protons.
(a) Protons = electrons + 2 = 18 + 2 = 20
(b) Z = 20 is calcium, Ca.
Final: 20 protons; calcium.
Practise and check
See what you know across this topic, then go back to anything you got wrong.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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