Isotopes
Start here
Lesson overview
Applying atomic and mass number to identify an atom's composition.
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 — Isotopes
Applying atomic and mass number to identify an atom's composition. 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
Isotopes are atoms of the same element (same number of protons) but with different numbers of neutrons, giving them different mass numbers.
Why It's Tested This Way
Isotopes matter beyond the exam — carbon dating and nuclear medicine both rely on isotopes of the same element behaving chemically identically while differing in mass, which is exactly what WAEC/JAMB test your understanding of.
Method
To identify if two atoms are isotopes of each other, check that they have the SAME atomic number but DIFFERENT mass numbers.
Worked Example
Worked example: Carbon-12 and Carbon-14 both have 6 protons (atomic number 6), but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons — they are isotopes of carbon.
Second Worked Example
Second worked example: Chlorine-35 and chlorine-37 both have 17 protons (atomic number 17), but 18 and 20 neutrons respectively — they are isotopes of chlorine.
Common Traps
Common trap: assuming isotopes are different elements — they are the same element with identical chemical properties, since chemical behaviour depends on electrons/protons, not neutrons.
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.
Mastery Check · Lesson Quiz · pass 60%
Sign in to take the quiz and track your progress.
Create a free account to take quizzes, earn checkpoints and track your scores — no card needed.
Create your free account