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DSE Chemistry Recap – Periodic Trends and Group Properties | Vanilla (2026)

Periodic Trends and Group Properties

Subject: Chemistry

Vanilla (2026)

For use on Friday, 18 September 2026

01

Periods, Groups and Periodic Trends

True or False

State whether each statement is true or false. If a statement is false, rewrite it correctly.

  1. Elements in the same period have the same number of occupied electron shells.
  2. Elements in the same group have the same number of outermost-shell electrons and therefore have similar chemical properties.
  3. Across a period from left to right, atomic radius generally increases because the number of electrons increases.
  4. Across Period 3 from sodium to chlorine, metallic character generally decreases.
  5. From sodium to silicon in Period 3, the melting point generally increases.
  6. Group I metals become more reactive down the group because their outermost-shell electron is lost more easily.
  7. Potassium has a smaller atomic radius and is less reactive than lithium.
  8. In the same period, a Group I metal is generally more reactive than a Group II metal because the Group I metal only needs to lose one outermost-shell electron to obtain a stable electron arrangement.
  9. Calcium is less reactive than magnesium because calcium has a larger atomic radius.
  10. The relative atomic mass of an element is always a whole number because atoms contain whole numbers of protons and neutrons.
10 marks
02

Periods and Periodic Trends

Properties Across Period 3

The table gives information about four Period 3 elements. The letters W, X, Y and Z are not their chemical symbols.

Element Proton number Electronic arrangement Melting point / °C Boiling point / °C
W 11 2,8,1 98 883
X 12 2,8,2 650 1091
Y 13 2,8,3 660 2470
Z 14 2,8,4 1414 3265
  1. Explain why W, X, Y and Z are all placed in Period 3.
  2. State the group of:
    1. W;
    2. X.
  3. Arrange W, X, Y and Z in order of increasing atomic radius.
  4. Explain the change in atomic radius from W to Z.
  5. Describe the general change in metallic character from W to Z.
  6. Refer to the melting-point and boiling-point data in the table.
    1. Describe the general change in melting point from W to Z.
    2. Describe the change in boiling point from W to Z.
    3. Calculate the difference between the melting point and boiling point of Y.
  7. Which of W, X, Y and Z requires the highest temperature to boil? Explain your answer using the information in the table.
14 marks
03

Group I and Group II Metals

Comparing the Reactivity of Metals

Lithium, sodium and potassium are Group I metals. Magnesium and calcium are Group II metals.

  1. Arrange lithium, sodium and potassium in order of increasing reactivity with water.
  2. State two observations when sodium reacts with cold water.
  3. Write a balanced chemical equation for the reaction between sodium and water. Include state symbols.
  4. Explain why the reactivity of Group I metals increases down the group.
  5. Sodium and magnesium are both Period 3 metals.
    1. State which metal reacts more vigorously with cold water.
    2. Explain the difference in their reactivity in terms of their electronic arrangements.
  6. A student makes the following statement:
    “All Group II metals are unreactive because they must lose two electrons.”
    Explain why this statement is incorrect.
  7. Magnesium and calcium are both Group II metals.
    1. Which metal is more reactive?
    2. Explain your answer.
15 marks
04

Group VII and Relative Atomic Mass

Halogens and Isotopic Abundance

The table shows information about three Group VII halogens.

Halogen Electronic arrangement Physical state at room temperature
Chlorine 2,8,7 Gas
Bromine 2,8,18,7 Liquid
Iodine 2,8,18,18,7 Solid
  1. Explain why chlorine, bromine and iodine have similar chemical properties.
  2. Predict the change in reactivity from chlorine to bromine to iodine.
  3. Explain your answer to part (b).
  4. A more reactive halogen can displace a less reactive halogen from a solution of its compound.
    For example, chlorine is more reactive than bromine. When chlorine water is added to aqueous potassium bromide, chlorine molecules gain electrons and form chloride ions. Bromide ions lose electrons and form bromine molecules.
    The two half equations are:
    Cl2 + 2e− → 2Cl−

    2Br− → Br2 + 2e−
    Use the information above and your answer to part (b) to answer the following questions.
    1. Predict whether chlorine reacts with aqueous potassium bromide.
    2. Predict whether chlorine reacts with aqueous potassium iodide.
    3. Combine the two half equations to write the overall ionic equation for the reaction between chlorine and bromide ions.
    4. Explain why chlorine can displace bromine and iodine from their compounds.
  5. Naturally occurring chlorine contains two isotopes.
    Isotope Relative isotopic mass Relative abundance
    Chlorine-35 35.0 75.0%
    Chlorine-37 37.0 25.0%
    Calculate the relative atomic mass of chlorine. Show your working.
  6. A sample of element Q contains two isotopes, Q-63 and Q-65. Its relative atomic mass is 63.6. Calculate the percentage abundance of Q-65. Show your working.
14 marks

Question 1

  1. True. Elements in the same period have the same number of occupied electron shells.
  2. True. Elements in the same group have the same number of outermost-shell electrons and usually have similar chemical properties.
  3. False. Across a period from left to right, atomic radius generally decreases because nuclear charge increases while the number of occupied electron shells remains unchanged.
  4. True. Metallic character generally decreases across Period 3 from sodium to chlorine.
  5. True. The melting point generally increases from sodium to silicon in Period 3.
  6. True. Down Group I, the outermost-shell electron becomes farther from the nucleus and is lost more easily. Therefore, reactivity increases.
  7. False. Potassium has a larger atomic radius and is more reactive than lithium.
  8. True. A Group I metal needs to lose one outermost-shell electron, while a Group II metal needs to lose two outermost-shell electrons.
  9. False. Calcium is more reactive than magnesium. Calcium has a larger atomic radius, so its outermost-shell electrons are farther from the nucleus and are lost more easily.
  10. False. Relative atomic mass is the weighted average of the relative isotopic masses of the naturally occurring isotopes of an element. Therefore, it is not necessarily a whole number.

Question 2

  1. W, X, Y and Z each have three occupied electron shells. Therefore, they are all placed in Period 3.
    1. W has one outermost-shell electron. W is in Group I.
    2. X has two outermost-shell electrons. X is in Group II.
  3. The order of increasing atomic radius is:
    Z < Y < X < W
  4. From W to Z, the number of protons and the nuclear charge increase. The electrons are added to the same main electron shell, so the number of occupied electron shells remains unchanged. The attraction between the nucleus and the outermost-shell electrons becomes stronger. Therefore, the atomic radius decreases from W to Z.
  5. Metallic character decreases from W to Z. The atoms become less likely to lose their outermost-shell electrons across the period.
    1. The melting point generally increases from W to Z.
    2. The boiling point increases from W to Z.
    3. For Y:
      Difference = 2470 − 660
      Difference = 1810°C
      The difference is 1810°C.
  7. Z has the highest boiling point in the table.
    Boiling point of Z = 3265°C
    Therefore, Z requires the highest temperature to boil.
Key idea: The period number gives the number of occupied electron shells. For the groups covered here, the group number gives the number of outermost-shell electrons.

Question 3

  1. The order of increasing reactivity is:
    lithium < sodium < potassium
  2. Any two of the following:
    • Sodium floats on the water.
    • Sodium moves rapidly on the water surface.
    • Effervescence occurs.
    • Sodium melts into a silvery ball.
    • The sodium gradually disappears.
    • Sodium may burn with a yellow flame.
    • An alkaline solution is formed.
  3. The balanced chemical equation is:
    2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
  4. Down Group I, the number of occupied electron shells increases and the atomic radius increases. The outermost-shell electron becomes farther from the nucleus. The attraction between the nucleus and the outermost-shell electron becomes weaker, so the electron is lost more easily. Therefore, the reactivity of Group I metals increases down the group.
    1. Sodium reacts more vigorously with cold water.
    2. Sodium has the electronic arrangement 2,8,1. Magnesium has the electronic arrangement 2,8,2. Sodium only needs to lose one electron to obtain a stable electron arrangement, while magnesium needs to lose two electrons. Therefore, sodium is more reactive than magnesium.
  6. The statement is incorrect because Group II metals can react by losing two outermost-shell electrons to form ions with a charge of 2+. The reactivity of Group II metals also increases down the group because their outermost-shell electrons become farther from the nucleus and are lost more easily. Group II metals are reactive, although they are generally less reactive than Group I metals in the same period.
    1. Calcium is more reactive than magnesium.
    2. Calcium has more occupied electron shells and a larger atomic radius than magnesium. Its outermost-shell electrons are farther from the nucleus and are lost more easily. Therefore, calcium is more reactive.
Key idea: The reactivity of both Group I and Group II metals increases down the group because their outermost-shell electrons are lost more easily.

Question 4

  1. Chlorine, bromine and iodine each have seven outermost-shell electrons. They tend to gain one electron during chemical reactions. Therefore, they have similar chemical properties.
  2. The order of decreasing reactivity is:
    chlorine > bromine > iodine
    Reactivity decreases down Group VII.
  3. Down Group VII, the number of occupied electron shells increases and the atomic radius increases. The outermost electron shell becomes farther from the nucleus. The attraction between the nucleus and an incoming electron becomes weaker, so the atom gains an electron less readily. Therefore, reactivity decreases down Group VII.
    1. Chlorine reacts with aqueous potassium bromide.
    2. Chlorine reacts with aqueous potassium iodide.
    3. The electrons in the two half equations cancel when the equations are added together.
      Cl2(aq) + 2Br−(aq) → 2Cl−(aq) + Br2(aq)
    4. Chlorine is more reactive than bromine and iodine. Chlorine molecules gain electrons more readily than bromine and iodine molecules. Therefore, chlorine can displace bromine and iodine from solutions of their compounds.
  5. Relative atomic mass is calculated using the weighted average:
    Ar(Cl) = [(35.0 × 75.0) + (37.0 × 25.0)] / 100

    = (2625 + 925) / 100

    = 35.5
    Relative atomic mass of chlorine = 35.5.
  6. Let the percentage abundance of Q-65 be x%. The percentage abundance of Q-63 is therefore (100 − x)%.
    [65x + 63(100 − x)] / 100 = 63.6

    65x + 6300 − 63x = 6360

    2x = 60

    x = 30
    Percentage abundance of Q-65 = 30%.
Key idea: Relative atomic mass is a weighted average. An isotope with a greater abundance makes a greater contribution to the relative atomic mass.
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