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Question 1 -- The Arrangement of the Periodic Table
Total: 12 marks
A student at a school in Bengaluru is given a copy of the Periodic Table with all the symbols and names removed, so that only the positions of the elements remain. She is asked to work out as much as she can about individual elements from their positions alone.
(a)[4]
Element A has a proton number of 15.
(i) Write the electronic configuration of an atom of A. [1]
(ii) State the group and the period of the Periodic Table in which A is found, and explain how you decided on each. [2]
(iii) State whether A is a metal or a non-metal. [1]
Model Answer -- 1(a)
(i) 2,8,5 — a neutral atom has the same number of electrons as protons, and the shells fill 2 then 8 then the rest [1]
(ii) Group V, because there are five electrons in the outer shell and for a main group the group number equals the number of outer-shell electrons [1]
(ii) Period 3, because three electron shells are occupied and the period number equals the number of occupied shells [1]
(iii) A is a non-metal; it lies on the right-hand side of the stepped line, and the element is phosphorus [1]
⚠ If you missed marks here: The commonest slip is swapping the two numbers over and writing Group III, Period 5. Fix the rule firmly: group number = outer-shell electrons, period number = number of occupied shells. A second frequent loss is giving the group or the period without the reason, when the question asks you to explain how you decided.
(b)[4]
Four elements have the following electronic configurations.
W 2,8,2 X 2,7 Y 2,8,8 Z 2,8,8,1
(i) Which two of these elements would react together most vigorously? Give the formula of the compound formed. [2]
(ii) Which element is a noble gas? Explain your answer. [2]
Model Answer -- 1(b)
(i) X and Z; Z has a single outer electron so it is the most reactive metal shown, and X needs only one electron so it is the most reactive non-metal shown [1]
(i) The formula is ZX, because one electron is transferred from each Z atom to each X atom, giving Z⁺ and X⁻ ions in a 1 : 1 ratio [1]
(ii) Y is the noble gas [1]
(ii) Y has a complete outer shell of eight electrons, so it has no tendency to lose, gain or share electrons and is unreactive [1]
⚠ If you missed marks here: Many candidates pick W as the metal because it comes first in the list, but reactivity in Group I beats Group II and the element with four shells is lower down the group than the one with three. In part (ii) a bare answer of "it has a full outer shell" earns the second mark only if you say what that means for reactivity.
(c)[4]
Element Q lies in Group II of Period 4.
(i) State the charge on an ion of Q and explain how the ion is formed. [2]
(ii) Give the formula of the oxide of Q. [1]
(iii) State whether this oxide is acidic or basic, and give one reaction that supports your answer. [1]
Model Answer -- 1(c)
(i) The charge is 2⁺ [1]
(i) A Group II atom has two outer-shell electrons and loses both, leaving two more protons than electrons and a complete shell beneath [1]
(ii) QO — the 2⁺ ion balances the 2⁻ charge of the oxide ion in a 1 : 1 ratio [1]
(iii) Basic; it neutralises dilute acid to give a salt and water, for example QO + 2HCl → QCl₂ + H₂O, and if it dissolves the solution has a pH above 7 [1]
⚠ If you missed marks here: Writing the oxide as Q₂O or QO₂ shows that the charges have not been balanced — always cross the charges over and then cancel. Saying the oxide is basic "because Q is a metal" is acceptable reasoning but the question asks for a reaction, so quote the neutralisation of an acid.
Question 2 -- Group I: The Alkali Metals
Total: 12 marks
A teacher in Manchester demonstrates the reaction of the alkali metals with water. Small pieces of lithium, sodium and potassium are added in turn to a large trough of cold water containing a few drops of universal indicator.
(a)[4]
(i) Write the word equation and the balanced symbol equation for the reaction of sodium with water. [2]
(ii) Describe two observations that would be made when the sodium is added. [2]
Model Answer -- 2(a)
(i) sodium + water → sodium hydroxide + hydrogen [1]
(i) 2Na + 2H₂O → 2NaOH + H₂ — two atoms are needed because each supplies one electron and two are required per hydrogen molecule [1]
(ii) The metal floats on the surface because its density is less than that of water, and it melts into a shiny ball because the reaction is strongly exothermic [1]
(ii) It fizzes and skates rapidly over the surface as hydrogen is released, gradually getting smaller, and the indicator turns purple as alkaline sodium hydroxide forms [1]
⚠ If you missed marks here: A very common error is writing sodium oxide as the product; the oxide is formed only when the metal burns in air. In the symbol equation, forgetting the two in front of the water leaves the hydrogen atoms unbalanced. For observations, "it reacts" is not an observation — describe what your eyes actually see.
(b)[5]
The reaction with water becomes more vigorous as Group I is descended.
(i) Explain this trend in terms of the structure of the atoms. [3]
(ii) Rubidium lies immediately below potassium. Predict what would be seen if a small piece of rubidium were added to cold water, and name the solution formed. [2]
Model Answer -- 2(b)
(i) Going down the group each element has one more occupied electron shell, so the atomic radius increases and the outer electron is further from the nucleus [1]
(i) There are also more complete inner shells, which shield or screen the outer electron from the attraction of the positive nucleus [1]
(i) The net attraction on the outer electron is therefore weaker, so it is lost more easily and the metal reacts more readily [1]
(ii) The reaction would be violent: the metal would melt at once, shoot across the surface and the hydrogen would ignite, possibly with a small explosion and a red-violet flame [1]
(ii) The solution formed is rubidium hydroxide, RbOH, which is strongly alkaline with a pH of about 14 [1]
⚠ If you missed marks here: Answers that stop at "the atom is bigger" score one mark at most. The examiner wants the full chain: more shells, so greater distance and more shielding, so weaker attraction, so the outer electron is lost more easily. Writing that the metal "gains an electron more easily" describes Group VII and reverses the chemistry completely.
(c)[3]
(i) State two physical properties of the Group I metals that are unusual for metals. [2]
(ii) Explain why sodium is stored under oil. [1]
Model Answer -- 2(c)
(i) They have very low densities — lithium, sodium and potassium all float on water — and they are soft enough to be cut with a knife [1]
(i) They have low melting points for metals, and these fall down the group from 181 °C for lithium to 29 °C for caesium [1]
(ii) Sodium reacts rapidly with oxygen and with water vapour in the air, so the layer of oil seals the surface and keeps both away from the metal [1]
⚠ If you missed marks here: "Good conductor" and "shiny" are properties of metals in general, so they do not answer a question about what is unusual. In part (ii), saying the oil "stops it exploding" is too vague; name oxygen and water vapour as the substances being kept out.
Question 3 -- Group VII: Properties and Trends
Total: 12 marks
Sealed samples of chlorine, bromine and iodine are set out on a bench in a laboratory in Singapore so that students can compare their appearance without opening the containers.
(a)[4]
(i) Give the colour and the physical state at room temperature of chlorine, of bromine and of iodine. [3]
(ii) State what is meant by saying that these elements are diatomic, and give the formula of one of them. [1]
Model Answer -- 3(a)
(i) Chlorine is a pale yellow-green gas [1]
(i) Bromine is a red-brown liquid that gives off a red-brown vapour [1]
(i) Iodine is a shiny grey-black solid that sublimes to a purple vapour on warming [1]
(ii) Diatomic means that each molecule contains two atoms joined by a single covalent bond, so that both atoms complete their outer shells; for example Cl₂ (or Br₂ or I₂) [1]
⚠ If you missed marks here: Colour and state must both be given for each element to earn the mark — "chlorine is green" leaves out the state. Describing bromine as brown alone is usually accepted, but describing iodine as purple is not: purple is the colour of its vapour, and the solid is grey-black.
(b)[4]
(i) Describe and explain the trend in melting point down Group VII. [2]
(ii) Describe and explain the trend in reactivity down Group VII. [2]
Model Answer -- 3(b)
(i) Melting point increases down the group, which is why the elements change from gas to liquid to solid [1]
(i) The molecules become larger down the group, so the attractive forces between neighbouring molecules are stronger and more energy is needed to separate them [1]
(ii) Reactivity decreases down the group, so chlorine is more reactive than bromine, which is more reactive than iodine [1]
(ii) A halogen reacts by gaining one electron; down the group the outer shell is further from the nucleus and is shielded by more inner shells, so an incoming electron is attracted less strongly [1]
⚠ If you missed marks here: A frequent error in (i) is to say that the covalent bonds get stronger; melting a molecular solid separates whole molecules and leaves the bonds inside them untouched. In (ii), do not simply copy the Group I explanation — here the electron is gained, not lost, so the same change in structure produces the opposite trend.
(c)[4]
Astatine, At, lies below iodine in Group VII.
(i) Predict the colour and the physical state of astatine at room temperature. [2]
(ii) Predict how the reactivity of astatine compares with that of iodine, and describe what you would expect to see if iodine solution were added to aqueous sodium astatide, NaAt. [2]
Model Answer -- 3(c)
(i) The colours darken down the group, so astatine would be black or almost black [1]
(i) Melting point rises down the group, so astatine would be a solid at room temperature, melting somewhere above iodine's 114 °C [1]
(ii) Reactivity falls down the group, so astatine would be less reactive than iodine and the least reactive halogen of all [1]
(ii) Iodine is above astatine, so it would displace astatine from the astatide solution; the colourless solution would darken as free astatine was released [1]
⚠ If you missed marks here: Predictions must follow the trend you have just described — predicting a gas, or predicting that astatine is more reactive than iodine, contradicts the pattern and scores nothing. In (ii), remember that the more reactive halogen is the displacing one, and here that is iodine.
Question 4 -- Halogen Displacement Reactions
Total: 12 marks
A student in Nairobi adds a few drops of each halogen solution to each of three colourless potassium halide solutions and records whether the mixture changes colour. Some of her results are missing.
added to →
KCl(aq)
KBr(aq)
KI(aq)
chlorine water
no change
?
?
bromine water
?
no change
turns dark brown
iodine solution
no change
no change
no change
(a)[3]
Complete the three missing results in the table, describing in each case what would be seen. [3]
Model Answer -- 4(a)
Chlorine water with potassium bromide: the solution turns orange or red-brown as bromine is displaced [1]
Chlorine water with potassium iodide: the solution turns dark brown as iodine is displaced, and a black solid may settle out [1]
Bromine water with potassium chloride: no change, because bromine is below chlorine and cannot displace it [1]
⚠ If you missed marks here: The pattern is one-way: a halogen only displaces those below it. Candidates who fill in "reacts" for every empty cell lose the last mark, and candidates who write only "a reaction occurs" lose marks for not describing the colour that would actually be seen.
(b)[5]
Consider the reaction between chlorine and aqueous potassium bromide.
(i) Write the word equation and the balanced symbol equation. [2]
(ii) Write the ionic equation, including state symbols. [2]
(iii) Name the species that is oxidised in this reaction. [1]
(ii) The potassium ions are spectator ions and are correctly left out of the ionic equation [1]
(iii) The bromide ions are oxidised, because each one loses an electron to become part of a bromine molecule; chlorine is reduced [1]
⚠ If you missed marks here: Two formula units of the bromide are needed to balance the two chlorine atoms — forgetting this is the commonest equation error. In the ionic equation, marks are lost both for keeping the potassium ions in and for omitting the state symbols. Remember OIL RIG: oxidation is loss of electrons, so it is the negative ion that is oxidised.
(c)[4]
(i) Use the completed table of results to place chlorine, bromine and iodine in order of reactivity, starting with the most reactive, and state the general rule that the results illustrate. [2]
(ii) Explain, in terms of electrons, why a more reactive halogen displaces a less reactive one. [1]
(iii) Predict what would happen if fluorine could be added safely to each of the three halide solutions. [1]
Model Answer -- 4(c)
(i) Order of reactivity: chlorine, then bromine, then iodine [1]
(i) The rule is that a halogen displaces from solution any halogen that lies below it in Group VII, but never one above it [1]
(ii) The more reactive halogen attracts an extra electron more strongly, so it takes electrons from the ions of the less reactive halogen, becoming ions itself while the other halogen is set free [1]
(iii) Fluorine is above all three, so it would displace chlorine, bromine and iodine from every one of the solutions [1]
⚠ If you missed marks here: In (i) the order must be quoted the way round the question asks, and the rule must be stated as a rule, not just as a repeat of the results. In (ii), talking about "wanting" electrons is too vague for the mark — say that the electron is attracted more strongly.
Question 5 -- Transition Elements
Total: 10 marks
A trainee technician is asked to compare a sample of iron with a sample of sodium, and then to identify an unknown metal from the way its compounds behave.
(a)[4]
Give four ways in which the properties of iron, a transition element, differ from those of sodium, a Group I metal. [4]
Model Answer -- 5(a)
Iron has a much higher density (7.9 g/cm³) than sodium (0.97 g/cm³), and it is hard rather than soft [1]
Iron has a far higher melting point, 1538 °C compared with 98 °C for sodium [1]
Iron forms ions with more than one oxidation number, Fe²⁺ and Fe³⁺, whereas sodium forms only Na⁺ [1]
Iron forms coloured compounds and can act as a catalyst, for example in the Haber process, whereas sodium compounds are white or colourless and sodium is not used as a catalyst; iron is also far less reactive with water [1]
⚠ If you missed marks here: Each difference must be a genuine comparison — writing "iron is dense" without saying that sodium is not scores nothing. Do not offer properties shared by both, such as conducting electricity or being shiny, as these are common to all metals.
(b)[3]
Transition elements and their compounds are widely used as catalysts.
(i) State what a catalyst does. [1]
(ii) Name one transition element or compound used as a catalyst and state the process it is used in. [1]
(iii) Give one economic reason why industry uses catalysts. [1]
Model Answer -- 5(b)
(i) A catalyst increases the rate of a chemical reaction but is chemically unchanged in mass and composition at the end of it [1]
(ii) Iron in the Haber process for making ammonia; nickel in the hydrogenation of vegetable oils; manganese(IV) oxide in the decomposition of hydrogen peroxide — any one [1]
(iii) A faster reaction at a lower temperature means less fuel is burned and more product is made per day, so costs fall; the catalyst can also be recovered and reused many times [1]
⚠ If you missed marks here: A definition that says only "it speeds up a reaction" is incomplete; the words "chemically unchanged at the end" are needed for the mark. Saying a catalyst "increases the yield" is wrong — it changes only how quickly equilibrium or completion is reached.
(c)[3]
An unknown metal M dissolves slowly in dilute acid to give a pale green solution. On standing in air the solution slowly turns orange-brown. A separate sample of M shows no reaction at all with cold water.
Explain, using all three observations, why M is a transition element rather than a Group I metal. [3]
Model Answer -- 5(c)
The solutions are coloured, and coloured compounds are characteristic of transition elements, whereas Group I compounds are white solids giving colourless solutions [1]
Two different colours are produced from the same metal, showing that M forms ions with more than one oxidation number; a Group I metal forms only a 1⁺ ion [1]
M does not react with cold water, whereas every Group I metal reacts vigorously with cold water, so M must be far less reactive [1]
⚠ If you missed marks here: The question says "using all three observations", so an answer that only mentions colour cannot score more than one mark. Each observation must be linked to the property it demonstrates and contrasted with what a Group I metal would do.
Question 6 -- The Noble Gases
Total: 12 marks
The noble gases make up Group VIII, the last column of the Periodic Table. They were the last group of elements to be discovered, largely because they take part in almost no chemical reactions.
(a)[4]
(i) Write the electronic configuration of a helium atom and of an argon atom. [2]
(ii) Use these configurations to explain why the noble gases are so unreactive. [2]
Model Answer -- 6(a)
(i) Helium is 2 [1]
(i) Argon is 2,8,8 [1]
(ii) In each case the outer shell is complete — two electrons fill the first shell and eight fill the second and third [1]
(ii) With a full outer shell there is no tendency to lose, gain or share electrons, and since all chemical bonding involves one of these three processes the atoms take part in almost no reactions [1]
⚠ If you missed marks here: Writing helium as 2,8 is a common slip — the first shell holds only two electrons in total. In (ii), the answer "because it is a noble gas" is circular; the mark is for the link between a complete outer shell and the absence of any tendency to transfer or share electrons.
(b)[4]
For each of the following uses, name the noble gas involved and explain why it is suitable.
(i) Filling weather balloons. [1]
(ii) Filling filament lamps. [1]
(iii) Making brightly coloured advertising signs. [1]
(iv) Shielding a hot metal weld from the air. [1]
Model Answer -- 6(b)
(i) Helium, because it has a very low density so the balloon rises, and unlike hydrogen it cannot burn or explode [1]
(ii) Argon, because it is unreactive so the white-hot filament cannot oxidise and burn away, which makes the lamp last far longer [1]
(iii) Neon, because it glows a bright red-orange when a high voltage is applied across it, and being unreactive the tube is stable for years [1]
(iv) Argon (or helium), because it forms an inert blanket that keeps oxygen and nitrogen away from the hot metal, preventing oxidation and the formation of brittle nitrides [1]
⚠ If you missed marks here: Each mark needs both the gas and the reason, so naming helium without saying why loses the mark. A frequent error is to say that neon "burns" to give the colour — noble gases never burn; the glow comes from electrical excitation, not combustion.
(c)[4]
The boiling points of four noble gases are helium −269 °C, neon −246 °C, argon −186 °C and krypton −152 °C.
(i) State what is meant by saying that a noble gas is monatomic, and explain why this is so. [2]
(ii) Describe the trend in boiling point shown by the data and explain it. [1]
(iii) Predict the boiling point of xenon, the next element down the group. [1]
Model Answer -- 6(c)
(i) Monatomic means that the element exists as separate single atoms rather than as molecules [1]
(i) Because the outer shell is already full, an atom gains nothing by bonding even to another atom of the same element, so no molecules form [1]
(ii) The boiling point rises steadily down the group, because the atoms become larger and the attractive forces between them become stronger, so more energy is needed to separate them [1]
(iii) Xenon should boil above krypton, at roughly −110 °C; any value between about −80 °C and −130 °C follows the trend correctly (the true value is −108 °C) [1]
⚠ If you missed marks here: Negative numbers catch many candidates out: −152 °C is higher than −186 °C, so the boiling points are rising down the group even though the figures look as if they are falling. A prediction below krypton's value therefore reverses the trend and scores nothing.
Question 7 -- Interpreting Periodic Table Data
Total: 10 marks
The table gives data for five elements, labelled A to E. The letters are not the chemical symbols of the elements.
element
proton number
melting point / °C
density / g cm⁻³
conducts electricity?
pH of oxide in water
A
11
98
0.97
yes
13
B
17
−101
gas
no
2
C
26
1538
7.87
yes
insoluble
D
18
−189
gas
no
no oxide formed
E
19
63
0.86
yes
14
(a)[4]
Using the data only, state which group or region of the Periodic Table each of A, B, C and D belongs to, and give one piece of evidence from the table for each. [4]
Model Answer -- 7(a)
A is a Group I metal: it conducts, its density is below 1 g cm⁻³ so it would float on water, its melting point is very low for a metal, and its oxide gives a strongly alkaline solution [1]
B is a Group VII non-metal: it is a gas that does not conduct and its oxide dissolves to give an acidic solution of pH 2 [1]
C is a transition element: it conducts, has a very high density and a very high melting point, and its oxide is insoluble rather than strongly acidic or alkaline [1]
D is a Group VIII noble gas: it is a gas with an extremely low melting point that does not conduct and forms no oxide at all [1]
⚠ If you missed marks here: Each mark needs the group and at least one supporting figure from the table; a bare list of groups scores very little. The clearest single clue for the noble gas is the fact that it forms no oxide, since that is the only element in the table with no chemistry at all.
(b)[3]
Elements A and E are in the same group. Predict which of them reacts more vigorously with cold water, and explain your prediction in terms of atomic structure. [3]
Model Answer -- 7(b)
E reacts more vigorously; its proton number of 19 places it one period lower in the group than A, whose proton number is 11 [1]
E therefore has one more occupied electron shell, so its outer electron is further from the nucleus and is shielded by more inner shells [1]
The attraction on that outer electron is weaker, so it is lost more easily and E reacts faster; this is supported by the higher pH of its oxide solution and its lower melting point [1]
⚠ If you missed marks here: The prediction alone is worth only one mark; the other two come from the explanation. Note that a higher proton number does not by itself make an element more reactive — it is the extra shell and the extra shielding that matter.
(c)[3]
A sixth element, F, has proton number 35 and is in the same group as B.
(i) Predict the colour and physical state of F at room temperature. [1]
(ii) Predict how the reactivity of F compares with that of B. [1]
(iii) State what would be seen when F is added to a solution of the potassium salt of B, and to a solution of potassium iodide. [1]
Model Answer -- 7(c)
(i) F is bromine: a red-brown liquid at room temperature, since colours darken and melting points rise below B, which is chlorine [1]
(ii) F is less reactive than B, because reactivity decreases down Group VII as the outer shell moves further from the nucleus and is more shielded [1]
(iii) With potassium chloride there would be no change, because F is below B in the group; with potassium iodide the solution would turn dark brown as iodine was displaced [1]
⚠ If you missed marks here: You must first work out that B is chlorine, from proton number 17, before the comparison in (ii) can be made. In (iii) both halves are needed for the single mark, so an answer that predicts a reaction in both tubes cannot score.
Self-Assessment
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A* : 56+
A : 48-55
B : 40-47
C : 32-39
D : 24-31
E : 16-23
U : <16
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