Edexcel A-Level Chemistry AS Paper 1, June 2018: Question 2

15 marks · Medium difficulty · Open Response

Calculate the relative atomic mass of hydrogen, write equations for its first ionisation energy, and explain periodic trends and properties relating to its position in the Periodic Table.

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Question

Question text

2 This question is about hydrogen, the element with atomic number Z = 1.

(a) (i) Hydrogen has two stable isotopes, 1H and 2H. Complete the table to show the number

of subatomic particles present in the nuclei of these two isotopes of hydrogen.

(1)

Isotope Number of protons Number of neutrons

(ii) Use the data in the table to explain the term isotopes.

(2)

(b) The relative atomic mass of hydrogen in the Periodic Table is 1.0.

This is correct to two significant figures.

The table gives data for the relative isotopic mass and natural abundance of the

two stable isotopes of hydrogen.

Isotope Relative isotopic mass Percentage abundance

1H 1.007825 99.9885

2H 2.014101 0.0115

(i) Using the data in the table, give a reason why it can be estimated that the

relative atomic mass of hydrogen is greater than 1.0.

(1)

(ii) Calculate the relative atomic mass of hydrogen from these data, giving your

answer to four decimal places.

4 (2)

*P51459A0424*

Include state symbols.

(2)

(ii) The sequence of the first three elements in the Periodic Table is hydrogen,

helium and then lithium.

Explain why the first ionisation energy of hydrogen is less than that of helium,

but greater than that of lithium.

(4)

(d) Hydrogen can be placed in several different positions in periodic tables. One is

immediately above lithium in Group 1. Another is in the centre of the first row, as 5

shown in the Periodic Table on the back cover.*P51459A0524*

Criticise the position of hydrogen immediately above lithium by giving

one reason in favour and two against.

(3)

(Total for Question 2 = 15 marks)

Mark scheme

Show the mark scheme

Question

Acceptable Answer Additional Guidance Mark

Number

2(a)(i) (1H ) protons 1, neutrons 0 (1)

All four correct needed

(1H ) protons 1, neutrons 1

Question

Acceptable Answer Additional Guidance Mark

Number

2(a)(ii) An explanation that makes reference to the following points: (2)

(atoms that) have the same number of protons (1) Ignore any references to electrons

but a different number of neutrons (1)

Question

Acceptable Answer Additional Guidance Mark

Number

2(b)(i) An answer that makes reference to following: (1)

both isotopes have an isotopic mass of greater than 1 / Award mark if it is stated that the

1.0 / one (only) other isotope is 2H

there are no isotopes with an isotopic mass of less than Ignore calculation of value, even if

one incorrect.

Question

Acceptable Answer Additional Guidance Mark

Number

2(b)(ii) Example of calculation (2)

calculation to find Ar (1) Ar = (1.007825 x 99.9885) + (2.014101 x 0.0115)

value of Ar to 4 DP (1) (= 1.0079407) = 1.0079

Correct answer with no working scores (2)

Allow TE for M2 for incorrect transfer of data or for

one incorrect % abundance (e.g. 1.15%), provided

that the final Ar value is between 1 and 2

Ignore units even if incorrect

Question

Acceptable Answer Additional Guidance Mark

Number

2(c)(i) An answer that makes reference to the following: (2)

equation (1) + (-)

H(g) H (g) + e

state symbol, (g), on both H and H+ (1)

(-) +

H(g) - e H (g)

Ignore state symbol for electron

+ (-)

H2(g) H2 (g) + e scores only M2

(-) +

H2(g) - e H2 (g) scores only M2

+ (-)

H2(g) 2H (g) + 2e scores 0

+ (-)

X(g) X (g) + e scores only M2

Question

Acceptable Answer Additional Guidance Mark

Number

2(c)(ii) An explanation that makes reference to the following points: (4)

H < He:

He more protons than H / He greater nuclear charge Ignore references to shielding for H

than H (1) and He

in helium the outer electron is in the same shell as Ignore references to atomic radius or

hydrogen (1) electrons being closer to or the same

distance from the nucleus in helium

H > Li:

in lithium the outer electron is in a higher energy level / Allow lithium has more shells of

a new shell / further from the nucleus / in a 2s orbital electrons

(1)

(and) is shielded by inner electrons / 1s2 electrons (1) Allow (outer electron of) lithium has

more shielding than hydrogen / is

shielded

Question

Acceptable Answer Additional Guidance Mark

Number

2(d) An answer that makes reference to the following: (3)

(in favour)

electronic structure of hydrogen is s1 / 1s1 / has one Allow 1 electron in outer shell / has 1

electron in s orbital / form 1+ ions (1) valence electron

Do not award ‘last electron is in s

orbital’ unless it is clear there is only

one

Do not award just ‘single unpaired

electron’

(against) any two from

the rest of Group 1 are (alkali) metals / metallic Allow hydrogen is not a metal

(hydrogen is not) (1) Ignore hydrogen is a gas but Group 1

elements are solid

hydrogen does not react in the same way as / has Do not award just ‘different

different reactivity to the rest of Group 1 / has different properties’ or ‘different behaviour’

chemical properties (1) Allow hydrogen forms covalent bonds

as a chemically different property

Ignore trends in physical properties

forms a H‒ ion (1) Allow hydrogen can gain one electron

to form a stable ion / become stable /

fill its outer shell

(Total for Question 2 = 15 marks)

Question

Acceptable Answer Mark

Number

How to answer it

Hydrogen: Isotopes, Relative Atomic Mass, and Periodic Trends

What this question tests

This question assesses your foundational knowledge of atomic structure, definitions of isotopes, calculation of relative isotopic and atomic masses, writing ionisation energy equations with state symbols, explaining trends in first ionisation energies across periods, and critically evaluating the placement of hydrogen in the Periodic Table.

Question 2(a)

Subatomic Particles & Defining Isotopes

✅ Correct Answers

(i) Table:
¹H: Protons = 1, Neutrons = 0
²H: Protons = 1, Neutrons = 1
(ii) Isotopes: Atoms with the same number of protons but a different number of neutrons.

💡 Key Knowledge

Mass number minus atomic number equals the number of neutrons. Protium (¹H) is unique for having zero neutrons.
When defining isotopes, always refer to protons and neutrons. Never mention electrons here.

❌ Common Errors

Students often lose marks in part (ii) by mentioning electrons or varying electron arrangements instead of focusing strictly on nuclear composition.

Mark breakdown: (a)(i) 1 mark for all four cells correct. (a)(ii) 2 marks (1 for same protons, 1 for different neutrons).

Question 2(b)

Relative Atomic Mass Calculations

✅ Correct Answers

(i) Reason: Both isotopes have an isotopic mass greater than 1.0 (or no isotope has a mass less than 1.0).
(ii) Calculation: 1.0079 (to 4 decimal places).

📐 Step-by-Step Calculation

Formula: Aᵣ = Σ(isotopic mass × percentage abundance) / 100
Substitute values: ((1.007825 × 99.9885) + (2.014101 × 0.0115)) / 100
Calculate numerator: 100.77692 + 0.023162 = 100.800082
Divide by 100 & round: 1.0079407... rounds to 1.0079

🧠 Exam Technique

Pay close attention to required significant figures or decimal places. Part (ii) explicitly asks for four decimal places. Truncating too early will lose you the final accuracy mark.

Mark breakdown: (b)(i) 1 mark. (b)(ii) 2 marks (1 for working/expression, 1 for final evaluated answer to 4 d.p.).

Question 2(c)

First Ionisation Energy & Period Trends

✅ Correct Answers

(i) Equation: H(g) → H⁺(g) + e⁻ (or H(g) - e⁻ → H⁺(g) )
(ii) Trend Explanation:
H vs He: Helium has more protons / greater nuclear charge, while its outer electron is in the same shell.
H vs Li: Lithium's outer electron is in a higher energy level / new shell (further from the nucleus) and experiences more shielding from inner electrons.

❌ Common Errors

For ionisation energy equations, omitting state symbols (g) on both the atom and the ion results in the loss of a mark. Do not use diatomic molecules like H₂ for standard atom ionisation definitions.

🧠 Exam Technique

When explaining periodic trends across or down groups, structure your answer clearly by comparing nuclear charge, shielding, and distance from the nucleus step-by-step.

Mark breakdown: (c)(i) 2 marks (1 for species/equation, 1 for correct state symbols). (c)(ii) 4 marks (1 for each valid comparative point regarding charge, shell level, distance, and shielding).

Question 2(d)

Evaluating Hydrogen's Periodic Table Position

✅ Correct Answers

In favour (pick 1): Hydrogen has 1 electron in its outer s-orbital (s¹) / forms 1+ ions / electronic configuration resembles Group 1.
Against (pick 2): Hydrogen is not a metal (unlike the rest of Group 1) / has different chemical properties / forms covalent bonds / can form a hydride (H⁻) ion.

💡 Key Knowledge

Hydrogen sits comfortably in multiple groups conceptually because it exhibits unique properties: it acts like an alkali metal due to its single valence electron, but behaves more like a non-metal gas sharing electron pairs.

🧠 Exam Technique

Read the command words carefully! The question asks for one reason in favour and two against. Providing extra "in favour" points will not gain additional credit if the negative criteria are missed.

Mark breakdown: 3 marks total (1 mark for the supporting reason, 2 marks for two distinct opposing reasons).

Topics

Physical Chemistry · Inorganic Chemistry · Topic 1: Atomic Structure and the Periodic Table · Topic 4: Inorganic Chemistry and the Periodic Table

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.