This set covers particle counts, ions, isotopes, electron arrangements and classifying substances from atoms, elements and isotopes. Questions go from easy to harder. Write your working first, then open the answer and compare.
Use a periodic table for names and proton numbers. All data in this set is original or invented for practice.
Section A: counting particles
Q1 (easy). An atom is written 40/20 Ca. State the number of protons, neutrons and electrons.
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Protons = 20. Electrons = 20 (neutral). Neutrons = 40 − 20 = 20. Check: 20 + 20 = 40.
20 protons, 20 neutrons, 20 electrons.
Q2 (easy). A neutral atom has 9 protons and 10 neutrons. Give its nucleon number and name the element.
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Nucleon number = 9 + 10 = 19. Nine protons is fluorine. It has 9 electrons.
19/9 F
Q3 (medium). State the protons, neutrons and electrons in 39/19 K⁺.
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Protons = 19. Neutrons = 39 − 19 = 20. Electrons = 19 − 1 = 18. Check charge: +19 − 18 = +1.
19 protons, 20 neutrons, 18 electrons.
Q4 (medium). State the protons, neutrons and electrons in 31/15 P³⁻.
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Protons = 15. Neutrons = 31 − 15 = 16. Electrons = 15 + 3 = 18. Check charge: +15 − 18 = −3.
15 protons, 16 neutrons, 18 electrons.
Q5 (medium). A particle has 13 protons, 14 neutrons and 10 electrons. Write its full symbol with charge.
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Nucleon number = 13 + 14 = 27. Thirteen protons is aluminium. Charge = 13 − 10 = +3.
27/13 Al³⁺
Section B: isotopes
Q6 (medium). Four particles are written 35/17 P, 37/17 Q, 37/18 R and 39/19 S. (The letters are not real symbols.) Which two are isotopes of the same element? Give one similarity and one difference.
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P and Q both have proton number 17. Their nucleon numbers differ (35 and 37), so neutrons are 18 and 20.
P and Q. Similarity: same number of protons and same chemical behaviour. Difference: two more neutrons in Q. R has 18 protons, so it is a different element even though its nucleon number matches Q.
Q7 (harder). An element has two isotopes, 63 and 65. Using the invented teaching figures of 70% for 63 and 30% for 65, calculate the relative atomic mass.
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(63 × 0.70) + (65 × 0.30) = 44.1 + 19.5 = 63.6.
63.6. It is closer to 63 because that isotope is more abundant.
Section C: electron arrangements and patterns
Q8 (medium). Write the electron arrangement of Mg (12 electrons), and then of the ion 24/12 Mg²⁺.
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Mg: 2 + 8 = 10, remaining 2, so 2,8,2. Mg²⁺ has lost 2 electrons, leaving 10 electrons: 2,8. The protons stay at 12 and the neutrons at 24 − 12 = 12. The ion has the same arrangement as neon.
Q9 (harder). An atom has the arrangement 2,8,7. How many protons does it have, which element is it, and which group and period is it in?
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Electrons = 2 + 8 + 7 = 17, so 17 protons. Seventeen protons is chlorine. Seven outer electrons gives Group VII. Three shells gives Period 3.
Q10 (harder). Fluorine is 2,7 and chlorine is 2,8,7. Explain why they react in a similar way.
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Both have seven electrons in the outer shell, so both are in Group VII. Reactions involve the outer electrons, and both need one more electron to fill the outer shell, so both tend to form 1− ions. Chlorine’s outer shell is one shell further out, so reactivity is similar but not identical.
Section D: element, compound or mixture
Q11 (medium). Classify each as an element, compound or mixture: H₂, CH₄, Ar, NaCl, air, O₃.
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H₂: one kind of atom, element. CH₄: carbon and hydrogen bonded in a fixed ratio, compound. Ar: element. NaCl: compound. Air: several substances together in no fixed ratio, mixture. O₃: only oxygen atoms, element.
If you got these wrong
| What went wrong | Go back to |
|---|---|
| Used the nucleon number as protons or neutrons (Q1, Q2, Q5) | Read proton and nucleon numbers |
| Changed protons in an ion, or added the charge the wrong way (Q3, Q4, Q5) | Determine particles in a simple ion |
| Said isotopes are different elements, or took a simple average (Q6, Q7) | Compare isotopes without changing element identity |
| Overfilled a shell, mixed up group and period, or gave a vague explanation (Q8 to Q10) | Use an electron arrangement to explain a pattern |
| Counted atoms instead of kinds of atom (Q11) | Distinguish an element from a compound sample |
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