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Chemistry Recap – Atomic Structure and Periodic Trends | Vanilla (2026)

Atomic Structure and Periodic Trends

Subject: Chemistry

Vanilla (2026)

For use on Thursday, 24 September 2026

01

Classification, Isotopes and Periodic Table

Multiple-Choice Questions

Choose the best answer for each question.

  1. Three substances, P, Q and R, have the following properties:
    • P has a variable composition and can be separated into two liquids by fractional distillation.
    • Q has a fixed composition and can be decomposed into two elements by electrolysis.
    • R contains atoms with the same proton number but may contain atoms of different masses.
    Which classification is correct?
    Option P Q R
    A Compound Mixture Element
    B Mixture Compound Element
    C Mixture Element Compound
    D Element Compound Mixture
    1. Classification A
    2. Classification B
    3. Classification C
    4. Classification D
  2. An element consists of two isotopes with relative isotopic masses 63 and 65. The percentage abundance of the isotope with relative isotopic mass 63 is 69.2%. What is the relative atomic mass of the element?
    1. 63.31
    2. 63.62
    3. 64.00
    4. 64.38
  3. An element consists only of isotopes with relative isotopic masses 10 and 11. Its relative atomic mass is 10.8. What is the percentage abundance of the isotope with relative isotopic mass 11?
    1. 20%
    2. 40%
    3. 80%
    4. 90%
  4. Atoms P and Q have the following electron arrangements:
    P: 2,8,2
    Q: 2,8,7
    Which statement is correct?
    1. P and Q are in the same group.
    2. P is a Group II element, while Q is a Group VII element.
    3. P normally gains two electrons, while Q normally loses seven electrons.
    4. P is a non-metal, while Q is a metal.
  5. Which statement is not a correct general description of the Group I elements from lithium to caesium?
    1. The atomic radii generally increase.
    2. The reactions with water generally become more vigorous.
    3. The melting points generally decrease.
    4. The densities increase regularly with no exceptions.
  6. Magnesium shows little reaction with cold water, whereas calcium reacts readily with cold water. Which conclusion is best supported by this observation?
    1. Reactivity generally decreases down Group II.
    2. Reactivity generally increases down Group II.
    3. All Group II metals react equally rapidly with water.
    4. Magnesium is below calcium in Group II.
  7. Three halogens, P, Q and R, are chlorine, bromine and iodine, but not necessarily in that order.
    • P displaces bromide ions and iodide ions from their solutions.
    • Q displaces iodide ions but does not displace chloride ions.
    • R does not displace chloride ions or bromide ions.
    Which identification is correct?
    Option P Q R
    A Chlorine Bromine Iodine
    B Bromine Chlorine Iodine
    C Iodine Bromine Chlorine
    D Chlorine Iodine Bromine
    1. Identification A
    2. Identification B
    3. Identification C
    4. Identification D
  8. Which set of trends is generally observed down Group VII from chlorine to iodine?
    Option Colour Melting and boiling points Reactivity
    A Becomes lighter Decrease Increases
    B Becomes darker Increase Decreases
    C Becomes darker Decrease Increases
    D Remains unchanged Increase Remains unchanged
    1. Set A
    2. Set B
    3. Set C
    4. Set D
  9. Argon has a higher boiling point than neon, but both elements are very unreactive. Which statement gives the best explanation?
    1. Argon has stronger attractions between its atoms, while both elements have complete outer electron shells.
    2. Argon has stronger covalent bonds, while neon atoms contain no electrons.
    3. Argon is more reactive because it has more occupied electron shells.
    4. Neon has a higher mass density, so it has a lower boiling point.
  10. Element T is in Period 3 and Group VII. It consists of two isotopes with relative isotopic masses 35 and 37. Its relative atomic mass is 35.5. Which statement about T is correct?
    1. The abundance ratio of isotope 35 to isotope 37 is 1:3, and each atom has seven occupied electron shells.
    2. The abundance ratio of isotope 35 to isotope 37 is 3:1, and each atom has seven outer-shell electrons.
    3. The two isotopes have different proton numbers but identical numbers of neutrons.
    4. T is a metal that normally loses seven electrons during chemical reactions.
10 marks
02

Core Concepts and Periodic Trends

True or False

Write “True” or “False” for each statement. If a statement is false, rewrite it correctly.

  1. A compound may have physical and chemical properties that are very different from those of the elements from which it is formed.
  2. The components of a mixture are chemically combined in a fixed ratio.
  3. A pure compound can be separated into its constituent elements by physical methods such as filtration or distillation.
  4. Isotopes of the same element have the same number of protons but different numbers of neutrons.
  5. Isotopes of the same element normally have similar chemical properties because their atoms have the same electron arrangement.
  6. The relative atomic mass of an element must be a whole number because the relative isotopic masses are whole numbers.
  7. Across a period, elements are arranged in order of increasing proton number, while elements in the same group have related chemical properties.
  8. Potassium is less reactive than sodium because potassium has more occupied electron shells.
  9. A more reactive halogen can displace a less reactive halogen from a solution containing the less reactive halogen’s halide ions.
  10. The boiling points of the Group 0 elements generally increase down the group, while their chemical reactivity remains very low.
10 marks
03

Isotopes, Relative Atomic Mass and Classification

Structured Question

Element M occurs naturally as three isotopes. The isotopic composition of M is shown below.

Isotope Relative isotopic mass Percentage abundance
M-24 24 78.6%
M-25 25 10.1%
M-26 26 11.3%

An atom of M has the electron arrangement 2,8,2.

  1. State what is meant by an isotope. [2 marks]
  2. Explain why the three isotopes of M have almost identical chemical properties. [2 marks]
  3. Calculate the relative atomic mass of M. Give your answer to two decimal places. [3 marks]
  4. Using the electron arrangement of M:
    1. identify element M;
    2. state its group; and
    3. state its period.
    [3 marks]
  5. A sample contains powdered M mixed with powdered sulfur. The proportion of M to sulfur differs between samples. Classify the sample as an element, compound or mixture. Explain your answer. [2 marks]
  6. When the mixture is heated strongly, M reacts with sulfur to form a single pure substance. Explain two differences between the product formed and the original mixture. [4 marks]
  7. A student states:
    “The relative atomic mass of M should be exactly 24 because M-24 is the most abundant isotope.”
    Explain why the student’s conclusion is incorrect. [2 marks]
18 marks
04

Groups I, II, VII and 0

Structured Question

Part I — Group I and Group II

Three Group I metals, A, B and C, are lithium, sodium and potassium, but not necessarily in that order. Their reactions with cold water are described below.

Metal Observation
A Floats, moves slowly and gives gentle effervescence
B Melts into a ball and moves rapidly across the water surface
C Reacts very vigorously and may ignite
  1. Identify A, B and C. [3 marks]
  2. State the trend in reactivity down Group I. [1 mark]
  3. Explain the trend in reactivity down Group I in terms of:
    • the number of occupied electron shells;
    • shielding of the outermost electron; and
    • the attraction between the nucleus and the outermost electron.
    [3 marks]
  4. Magnesium reacts very slowly with cold water, while calcium reacts readily with cold water.
    1. State the trend in reactivity down Group II.
    2. Explain this trend using electronic structure.
    [3 marks]
  5. Sodium and magnesium are both in Period 3, but sodium generally loses an electron more readily than magnesium. Explain this difference. [2 marks]

Part II — Group VII

Three halogens, P, Q and R, are chlorine, bromine and iodine, but not necessarily in that order. The results of displacement experiments are shown below. A tick means that a displacement reaction occurs.

Halogen added Solution containing chloride ions Solution containing bromide ions Solution containing iodide ions
P — ✓ ✓
Q ✗ — ✓
R ✗ ✗ —
  1. Arrange P, Q and R in decreasing order of reactivity. [1 mark]
  2. Identify P, Q and R. [3 marks]
  3. Explain why P can displace iodide ions from a solution, but R cannot displace chloride ions from a solution. [2 marks]
  4. Explain why the reactivity of the halogens decreases down Group VII. [3 marks]

Part III — Group 0

The boiling points of four Group 0 elements are shown below.

Element Boiling point / °C
Neon −246
Argon −186
Krypton −153
Xenon −108
  1. Describe the trend in boiling point down Group 0. [1 mark]
  2. Explain this boiling-point trend in terms of atomic size, number of electrons and the strength of attractions between atoms. [2 marks]
  3. Despite the boiling-point trend, all four elements are chemically very unreactive. Explain why. [2 marks]
26 marks

Question 1 — Multiple-Choice Questions

  1. B — P is a mixture, Q is a compound and R is an element.

    A mixture can have a variable composition and can be separated physically. A compound has a fixed composition and can be decomposed chemically. All atoms of an element have the same proton number.

  2. B — 63.62.
    Relative atomic mass
    = (63 × 69.2 + 65 × 30.8) ÷ 100
    = 63.616 ≈ 63.62
  3. C — 80%.
    Let the abundance of isotope 11 be x.
    10(1 − x) + 11x = 10.8
    x = 0.8 = 80%
  4. B — P is in Group II and Q is in Group VII.

    P has two outer-shell electrons, while Q has seven outer-shell electrons.

  5. D — The densities do not increase regularly with no exceptions.

    There are variations in the density trend. For example, potassium is less dense than sodium.

  6. B — Reactivity generally increases down Group II.
  7. A — P is chlorine, Q is bromine and R is iodine.

    Chlorine is the most reactive of the three, followed by bromine and then iodine.

  8. B — The colour becomes darker, the melting and boiling points increase, and reactivity decreases.
  9. A — Argon has stronger attractions between its atoms, while both elements have complete outer electron shells.
  10. B — The abundance ratio is 3:1, and each atom has seven outer-shell electrons.
    (35 × 3 + 37 × 1) ÷ 4
    = 142 ÷ 4
    = 35.5
Key reminder: Relative atomic mass is a weighted average. It depends on both the relative isotopic masses and their relative abundances.

Question 2 — True or False

  1. True. The properties of a compound can be very different from those of its constituent elements.
  2. False. The components of a mixture are not chemically combined and may be present in different proportions.
  3. False. A compound must be decomposed by chemical methods. Physical methods such as filtration and distillation are used to separate mixtures.
  4. True. Isotopes have the same proton number but different numbers of neutrons.
  5. True. Chemical properties depend mainly on electron arrangement, especially the outer-shell electrons.
  6. False. Relative atomic mass is a weighted average of the relative isotopic masses and is often not a whole number.
  7. True. Proton number increases across a period, and elements in the same group have related chemical properties.
  8. False. Potassium is more reactive than sodium because its outermost electron is farther from the nucleus and experiences greater shielding.
  9. True. A more reactive halogen can displace a less reactive halogen from a solution containing its halide ions.
  10. True. Attractions between Group 0 atoms become stronger down the group, but the atoms remain very unreactive because they have complete outer electron shells.

Question 3 — Isotopes, Relative Atomic Mass and Classification

  1. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
  2. The three isotopes have the same proton number and therefore the same electron arrangement. Chemical properties depend mainly on electron arrangement, particularly the outer-shell electrons.
  3. Relative atomic mass of M
    = [(24 × 78.6) + (25 × 10.1) + (26 × 11.3)] ÷ 100
    = (1886.4 + 252.5 + 293.8) ÷ 100
    = 24.327
    = 24.33
    • Element: Magnesium
    • Group: Group II
    • Period: Period 3
    Magnesium has two outer-shell electrons and three occupied electron shells.
  5. The sample is a mixture.

    Magnesium and sulfur are not chemically combined in the sample, and the two substances can be present in different proportions.

  6. Any two of the following explained differences:
    • The compound has a fixed composition, whereas the mixture can have a variable composition.
    • The elements in the compound are chemically combined, whereas the substances in the mixture are not chemically combined.
    • The compound has properties different from those of magnesium and sulfur.
    • The compound cannot be separated into magnesium and sulfur by physical methods, whereas the mixture can be separated physically.
  7. The relative atomic mass is the weighted average of all the naturally occurring isotopes. Although M-24 is the most abundant isotope, the presence of M-25 and M-26 raises the average above 24.
Classification reminder: An element contains only one type of atom. A compound contains elements chemically combined in a fixed ratio. A mixture contains substances that are not chemically combined and may be present in different proportions.

Question 4 — Groups I, II, VII and 0

    • A: Lithium
    • B: Sodium
    • C: Potassium
  2. Reactivity increases down Group I.
  3. Down Group I:
    • the number of occupied electron shells increases;
    • the outermost electron becomes farther from the nucleus;
    • shielding by inner-shell electrons increases;
    • the attraction between the nucleus and the outermost electron becomes weaker; and
    • the outermost electron is therefore lost more easily.
  4. Reactivity increases down Group II.

    Down the group, the two outer-shell electrons become farther from the nucleus and experience greater shielding. The attraction between the nucleus and these electrons becomes weaker, so the electrons are lost more easily.

  5. Sodium and magnesium have their outermost electrons in the same occupied shell, but magnesium has a greater nuclear charge. With similar shielding, magnesium’s outer-shell electrons experience a stronger attraction to the nucleus. Sodium therefore loses its outermost electron more readily.
  6. P > Q > R
    • P: Chlorine
    • Q: Bromine
    • R: Iodine
  8. Chlorine has a stronger tendency to gain electrons than iodine. Chlorine can therefore remove electrons from iodide ions. Iodine has a weaker tendency to gain electrons and cannot remove electrons from chloride ions.
  9. Down Group VII:
    • the number of occupied electron shells increases;
    • atomic radius and shielding increase;
    • the attraction between the nucleus and an incoming electron becomes weaker; and
    • the tendency to gain an electron therefore decreases.
  10. The boiling point increases down Group 0.
  11. Down Group 0, the atoms become larger and contain more electrons. The attractions between the atoms become stronger, so more energy is required to separate the atoms during boiling.
  12. Group 0 atoms have complete outer electron shells. They have very little tendency to gain, lose or share electrons and are therefore chemically very unreactive.
Periodic trend summary: Group I and Group II metals generally become more reactive down their groups. Group VII halogens become less reactive down the group. Group 0 boiling points increase down the group, but the elements remain very unreactive.

Chemistry Reference Summary

Relative atomic mass
= Σ(relative isotopic mass × percentage abundance) ÷ 100
Element: A substance containing only one type of atom.
Compound: A substance containing two or more elements chemically combined in a fixed ratio.
Mixture: Two or more substances that are not chemically combined and may be present in different proportions.
Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons.
Group number: For the groups covered in this recap, the group number is related to the number of outer-shell electrons. Group 0 elements have complete outer electron shells.
Period number: The period number gives the number of occupied electron shells in an atom.
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