Atomic structure — KCSE Chemistry

KCSE Chemistry · 66 practice questions · 4 syllabus objectives

25 easy18 medium23 hard

What You'll Learn

Key learning outcomes for this topic, aligned to the KNEC KCSE syllabus.

Describe the structure of the atom in terms of protons, neutrons and electrons with their relative masses and charges

Define atomic number, mass number, isotopes and relative atomic mass and perform calculations using these concepts

Write the electronic configuration of elements with atomic numbers 1–20

Atomic structure

Sample Questions

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1
easySHORT ANSWER3 marks

Identify the electronic configuration of the element with atomic number 12 and state its group in the periodic table. (3 marks)

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Part (a) — 2 marks
Electronic configuration: 1s² 2s² 2p⁶ 3s² (2 mks)
Part (b) — 1 mark
Group: 2 (alkaline earth metals) (1 mk)
2
easySHORT ANSWER3 marks

An element X has atomic number 15 and mass number 24. (a) State the number of protons, neutrons and electrons in a neutral atom of X. (b) Write the electronic configuration of X.

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Part (a) — 2 marks
Protons = 15; Electrons = 15 (1 mk)
Neutrons = mass number − atomic number = 24 − 15 = 12 (1 mk)
Part (b) — 1 mark
Electronic configuration: 2.8.1 (1 mk)
3
easySHORT ANSWER3 marks

An element X has atomic number 6 and mass number 12. (a) State the number of protons, neutrons and electrons in a neutral atom of X. (b) Write the electronic configuration of X.

View Marking Scheme
Part (a) — 2 marks
Protons = 6; Electrons = 6 (1 mk)
Neutrons = mass number − atomic number = 12 − 6 = 12 (1 mk)
Part (b) — 1 mark
Electronic configuration: 2.8.5 (1 mk)
4

An element X has atomic number 20 and mass number 40. (a) State the number of protons, neutrons and electrons in a neutral atom of X. (b) Write the electronic configuration of X.

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