Atomic Structure Practice Questions: Protons, Neutrons and Electrons

These twenty-five questions cover the atomic structure skills that appear in almost every introductory chemistry exam: counting particles, handling isotopes and ions, calculating relative atomic mass, writing electron arrangements and explaining the models. Work through each part, then check the answers at the end. If a section trips you up, the linked articles explain the ideas in detail.

Useful reminders

• Protons = atomic number (Z)
• Neutrons = mass number (A) − atomic number (Z)
• Electrons (neutral atom) = Z; for an ion, subtract a positive charge or add a negative charge
• Shells for the first 20 elements fill 2, 8, 8, 2

See mass number explained and electron shells explained.

Part A: Counting particles in atoms

1. How many protons, neutrons and electrons are in ¹⁹₉F?
2. How many protons, neutrons and electrons are in ⁴⁰₂₀Ca?
3. How many neutrons are in an atom of phosphorus-31 (Z = 15)?
4. An atom has 13 protons and 14 neutrons. What is its mass number, and which element is it?
5. Complete the table:

Symbol	Protons	Neutrons	Electrons	Mass number
²³Na	?	?	?	23
?	17	18	17	?
³²S	?	?	?	?

Part B: Isotopes

6. What are isotopes?
7. Carbon-12 and carbon-14 are isotopes. State one way they’re the same and one way they’re different.
8. Why do isotopes of the same element have the same chemical properties?
9. Hydrogen has three isotopes: ¹H, ²H and ³H. How many neutrons does each have?

Part C: Ions

10. How many electrons are in a sodium ion, Na⁺ (Z = 11)?
11. How many electrons are in a sulfide ion, S²⁻ (Z = 16)?
12. An ion has 20 protons and 18 electrons. Identify it and give its charge.
13. Complete the table:

Particle	Protons	Neutrons	Electrons
²⁴Mg²⁺	?	?	?
¹⁶O²⁻	?	?	?
²⁷Al³⁺	?	?	?

Part D: Relative atomic mass

14. Chlorine consists of 75% chlorine-35 and 25% chlorine-37. Calculate its relative atomic mass.
15. Copper is 69% copper-63 and 31% copper-65. Calculate its relative atomic mass.
16. Explain why the relative atomic mass of chlorine isn’t a whole number.

See relative atomic mass calculations.

Part E: Electron arrangement

17. Write the electron arrangement for: (a) nitrogen (Z = 7); (b) magnesium (Z = 12); (c) argon (Z = 18); (d) potassium (Z = 19).
18. An element has the electron arrangement 2, 8, 6. Which group and period is it in? Name it.
19. How many electrons are in the outer shell of a chlorine atom, and what ion does chlorine form?
20. Why is neon unreactive?

Part F: Models and history

21. Describe the plum pudding model of the atom.
22. In the gold foil experiment, most alpha particles passed straight through the foil. What does this show?
23. A few alpha particles bounced back. What does this show?
24. Which particle did James Chadwick discover, and why was it hard to detect?
25. State one way the modern model of the atom differs from the Bohr model.

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Answers

Part A

1. 9 protons, 19 − 9 = 10 neutrons, 9 electrons.
2. 20 protons, 20 neutrons, 20 electrons.
3. 31 − 15 = 16 neutrons.
4. Mass number = 13 + 14 = 27; Z = 13 is aluminium.
5. ²³Na: 11 protons, 12 neutrons, 11 electrons. Second row: Z = 17 is chlorine, mass number 35 (³⁵Cl). ³²S: 16 protons, 16 neutrons, 16 electrons, mass number 32.

Part B

6. Atoms of the same element (same number of protons) with different numbers of neutrons, so different mass numbers.
7. Same: 6 protons and 6 electrons (same element, same chemistry). Different: carbon-14 has 8 neutrons, carbon-12 has 6; carbon-14 is radioactive.
8. Chemical properties depend on the number and arrangement of electrons, which is the same for all isotopes of an element.
9. ¹H: 0; ²H (deuterium): 1; ³H (tritium): 2.

See isotopes, ions, isomers and allotropes.

Part C

10. 11 − 1 = 10 electrons.
11. 16 + 2 = 18 electrons.
12. Z = 20 is calcium; 2 fewer electrons than protons: Ca²⁺.
13. ²⁴Mg²⁺: 12, 12, 10. ¹⁶O²⁻: 8, 8, 10. ²⁷Al³⁺: 13, 14, 10. (All three have 10 electrons, the same as neon. See isoelectronic species.)

Part D

14. (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 35.5
15. (63 × 69 + 65 × 31) ÷ 100 = (4347 + 2015) ÷ 100 = 63.6
16. It’s a weighted average of the masses of two isotopes, each with a whole-number mass number, present in different proportions.

Part E

17. (a) 2, 5; (b) 2, 8, 2; (c) 2, 8, 8; (d) 2, 8, 8, 1.
18. Three shells → period 3; six outer electrons → group 16; sulfur.
19. 7 outer electrons; chlorine gains one electron to form Cl⁻ (2, 8, 8).
20. Its outer shell is full (2, 8), so it has no tendency to gain, lose or share electrons. See noble gases.

Part F

21. A sphere of positive charge with negative electrons embedded throughout it, like plums in a pudding. See Thomson’s plum pudding model.
22. Most of the atom is empty space.
23. The positive charge and most of the mass are concentrated in a tiny, dense nucleus, which strongly repels alpha particles that come close. See Rutherford’s gold foil experiment.
24. The neutron; it has no charge, so it isn’t deflected by electric or magnetic fields and doesn’t ionise matter directly. See Chadwick and the neutron.
25. Any one of: electrons are described by orbitals (regions of probability) rather than fixed circular orbits; shells contain subshells; electrons behave as waves; the exact position and momentum of an electron can’t both be known. See the quantum mechanical model.

How did you do?

• 22–25: excellent. Move on to electron configuration practice questions.
• 15–21: good; revisit the sections where you dropped marks.
• Below 15: work through the atomic structure study guide, then try again.

Common mistakes to watch for

• Adding electrons to the mass number. Mass number counts only protons and neutrons.
• Changing the number of protons when forming ions. Only electrons change; the nucleus stays the same.
• Getting the direction wrong for negative ions. A 2− ion has two more electrons than protons, not two fewer.
• Using relative atomic mass instead of mass number when counting neutrons. You can’t have 18.45 neutrons.
• Writing units for relative atomic mass. It’s a ratio, so it has none.
• Describing isotopes as “different elements”. They’re the same element, with different numbers of neutrons.

A quick habit that prevents most of these errors: before writing any answer, jot down Z, A and the charge, then fill in protons first, neutrons second and electrons last.

Key takeaways

• Protons = Z, neutrons = A − Z, electrons = Z minus the charge.
• Isotopes differ only in neutrons, so they share chemical properties.
• Relative atomic mass is an abundance-weighted average.
• Shells fill 2, 8, 8 for the first 18 elements; outer electrons decide the group and chemistry.
• Know the key evidence behind each atomic model.