Protons, Neutrons & Electrons
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Lesson overview
Computing subatomic particle counts from atomic number and mass number.
Go to lesson quiz →Exam guidance · WAEC WASSCE Paper 1 + Paper 2 Lens
Pacing
Objective speed on Paper 1 + structured step-by-step method marks on Paper 2 Theory/Practical.
How WAEC marks this
Examiners award method marks (M1) for stating the formula/definition and accuracy marks (A1) ONLY when SI units are written.
Watch out for
Writing bare numbers without working or omitting SI units loses up to 50% of Paper 2 marks even when the final value is right.
Lesson content
Overview — Protons, Neutrons & Electrons
Computing subatomic particle counts from atomic number and mass number. This lesson is at the beginner level — by the end, you'll be able to recognise this question type, apply a reliable method, and avoid the traps that cost marks.
Key Concept
Every atom is made of protons (positive charge) and neutrons (no charge) in the nucleus, and electrons (negative charge) orbiting around it. The atomic number (Z) tells you the number of protons; the mass number (A) tells you protons + neutrons combined.
Why It's Tested This Way
Atomic structure is the entry point to all of chemistry — WAEC/JAMB test it first because every later topic (bonding, the periodic table, reactions) assumes you can read atomic number and mass number fluently.
Method
To find protons: read the atomic number directly. To find neutrons: subtract the atomic number from the mass number (A − Z). To find electrons in a neutral atom: it equals the number of protons.
Worked Example
Worked example: An atom with atomic number 11 and mass number 23 has 11 protons, 11 electrons, and 23 − 11 = 12 neutrons.
Second Worked Example
Second worked example: An atom with atomic number 8 and mass number 16 has 8 protons, 8 electrons, and 16 − 8 = 8 neutrons.
Common Traps
Common trap: mixing up atomic number and mass number — the SMALLER number is always the atomic number/proton count; the LARGER number is the mass number.
Lesson Summary
Remember: (1) identify what is being asked, (2) apply the WAEC strategy, (3) check against the traps. Complete the 10-question quiz below to unlock the next lesson — you need at least 60% to pass.
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