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Atoms

Atomic structure: subatomic particles, proton (Z) and nucleon (A) numbers, and fast proton/neutron/electron counting in atoms and ions.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • state the relative charges and approximate relative masses of a proton, a neutron and an electron
  • describe, with the aid of diagrams, the structure of an atom as consisting of protons and neutrons (nucleons) in the nucleus and electrons arranged in shells (energy levels) (knowledge of s, p, d and f classification is not required; a copy of the Periodic Table will be available in Papers 1 and 2)
  • define proton (atomic) number and nucleon (mass) number
  • deduce the numbers of protons, neutrons and electrons in atoms and ions given proton and nucleon numbers.

A reliable particle count starts with proton number and nucleon number, then adjusts the electron count only if the particle is an ion.

1. Definition

An atom is the smallest particle of an element that retains the chemical properties of that element.

2. Key Ideas

  • An atom contains protons, neutrons, and electrons.
  • 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.

3. Detailed Explanations

Quick Recall (counting rules)
  • Z (proton number) = protons (and electrons for a neutral atom).
  • A (nucleon number) = protons + neutrons.
  • Neutrons = A - Z.
  • Ions: electrons change, protons do not.

A. Structure of an Atom

  • Nucleus: at the centre; contains protons and neutrons (together called nucleons).
  • Electron shells (energy levels): regions around the nucleus where electrons are found.
Structure of a carbon-12 atomA carbon-12 atom model: six protons and six neutrons form the nucleus; six electrons occupy shells. The drawing is schematic and not to scale.electron (−1)nucleusproton: charge +1relative mass 1neutron: charge 0relative mass 1electrons: negligiblerelative massSchematic model—not to scale. Electrons occupy shells around a tiny nucleus.
A carbon-12 atom model: six protons and six neutrons form the nucleus; six electrons occupy shells. The drawing is schematic and not to scale.

Properties of Subatomic Particles

ParticleRelative MassRelative ChargeLocation
Proton1+1Nucleus
Neutron10 (Neutral)Nucleus
Electron1/1840-1Electron Shells

B. Proton Number (Z) and Nucleon Number (A)

  • 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

C. Neutral Atoms vs Ions

In a neutral atom, number of electrons = number of protons, so total charge is zero.

In an ion, the atom has gained or lost electrons:

  • Positive ion (e.g., Mg²⁺): has lost electrons.
  • Negative ion (e.g., Cl⁻): has gained electrons.
Recall: isotopes

Isotopes have the same Z but different A (different number of neutrons). See Elements & Isotopes.

4. Common Mistakes

  • 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.

5. Exam Tips

The only two formulas you need

Protons = Z
Neutrons = A - Z

Fast electron counting for ions

Start with electrons = Z.
Then adjust for the charge: 2 + means subtract 2 electrons; 1- means add 1 electron.

6. Worked Examples

Modelled example 1

Counting Particles in a Neutral Atom

Core

Problem

An atom is represented by ²³₁₁Na. Determine the numbers of protons, neutrons and electrons.
Study the worked solution

  1. 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.

  2. 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.

  3. 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.

  4. State all particle counts

    Working

    11 protons, 12 neutrons and 11 electrons.

Guided practice 2

Counting Particles in a Positive Ion

About 5 min

Problem

A magnesium ion is ²⁴₁₂Mg²⁺. Calculate the numbers of protons and electrons.

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

  1. Count the protons

    Method

    Read the proton number.

    Reason

    Changing electron count does not change the element.

    Working

    Protons = Z = 12.

  2. 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

Error Analysis (Fix the Student)

Find and correct the mistake

Learner response

A student writes: “Na⁺ has 11 protons and 11 electrons.” Locate the first error and correct the statement.

Use the charge to test the electron count

Correct electron count

View solution step by step

  1. 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.

  2. 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.

  3. State the correction

    Working

    Na⁺ has 11 protons and 10 electrons.

Examiner practice 4

Counting Particles in a Negative Ion

3 marks

Examination question

A chloride ion is ³⁵₁₇Cl⁻. Calculate the numbers of protons, neutrons and electrons. [3 marks]

Write each count before self-marking

View solution step by step

  1. Count protons

    1 mark

    Method

    Use proton number Z.

    Reason

    The lower number identifies the number of protons.

    Working

    Protons = 17.

  2. Count neutrons

    1 mark

    Method

    Subtract proton number from nucleon number.

    Reason

    A counts all protons and neutrons.

    Working

    Neutrons = 35-17 = 18.

  3. Count electrons

    1 mark

    Method

    Add one electron to the neutral count.

    Reason

    The 1- charge shows that the atom gained one electron.

    Working

    Electrons = 17 + 1 = 18.

Challenge 5

Isotope Trap

Minimal support

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

Particle count that differs
Relationship

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

  1. 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.

  2. Name the relationship

    Working

    Atoms of the same element with different neutron numbers are isotopes.

7. Mind Stretchers

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.

8. Quiz

Quiz time

Ready to check your understanding? Try the practice links, then review anything you missed.

Practice next

Lesson quizCheck this lesson quickly with targeted MCQs.Structured practiceSwitch to exam-style written responses after the quiz.