Atomic structure, ions, Isotopes and formulae UNIT 2

Question 1

What is meant by the term isotopes? [1]

Answer 1

OCR DEFINITION:
isotopes as atoms of the same element with
different numbers of neutrons and different
masses
MARK SCHEMES:
ATOMS(S) of an element
AND
with different numbers of neutrons (and with different
masses) ✔

EXAMINERS REPORT:
This question was well answered. The one
common error made was to omit any
reference to ‘atoms’ and so answers in
terms of the same element having different
number of neutrons received no credit.
Candidates should be advised to avoid
unnecessary references to isotopes having
the same number of electrons.

Question 2

A twenty pence coin contains copper and nickel.
Copper and nickel each exist as a mixture of isotopes.
State the similarities and differences between the atomic structure of isotopes of the same element [2]

Answer 2

Similarities:
(Same) number of protons AND electrons ✓
Differences: (Different) number of neutrons ✓

Examiner’s Comments
Most candidates identified that different
isotopes had the same number of protons
but then omitted electrons. The different
number of neutrons was usually seen
although sometimes atomic mass was
shown instead

Question 3

Explain what is meant by the term relative atomic mass of an element [2]

Answer 3

(Weighted) mean/average mass of an atom ✓
compared with 1/12th mass of carbon-12
OR compared with mass of carbon-12 which is 12 ✓

Examiner’s Comments
Most candidates were given at least one
mark but lost the second mark due to
omitting the word “atom”, or “mean” or “one-twelfth”

Question 4

A sample of strontium has a relative atomic mass of 87.73.
The sample consists of:

• 82.9% Sr-88
• 6.9% Sr-87
• one other isotope.

Determine the other isotope of strontium in the sample.[2]

Answer 4

Use of isotope data
Use of 87 × 6.9 AND 88 × 82.9 AND 10.2 anywhere
✓
Calculation of isotopic mass
(100x87.73)-(87x6.9)-(88x82.9)
Ans/10.2= 86 or 86.03

Examiner’s Comments
Algebra was used very well here and
allowed most candidates to obtain at least
one mark, with the majority obtaining 2
marks

Question 5

This question is about atomic structure and the compounds of calcium, nitrogen and oxygen.
Most elements contain different isotopes.
State two differences between isotopes of the same element. [1]

Answer 5

TWO correct responses from ✓

• Different numbers of neutrons
• Different (atomic) masses/mass numbers
• Different physical properties
  Physical required

Misconception
When discussing the mass of individual
isotopes, ‘mass’ or ‘mass number’ should
be used. The relative atomic mass is the
weighted average mass of all of the
isotopes of an element, and is consequently
the incorrect term to use in this context.

Question 6

A sample of silicon is analysed by mass spectrometry.
The mass spectrum shows peaks with the relative abundances below.
* 28Si 92.23%
* 29Si 4.68%
* 30Si 3.09%
Calculate the relative atomic mass of silicon in the sample.
Give your answer to two decimal places. [2]

Answer 6

FIRST CHECK ANSWER ON THE ANSWER LINE
IF answer = 28.11 (to 2 DP) award 2 marks
OR 28.1086 OR 28.109 ✓
= 28.11 (to 2 DP) ✓

Examiner’s Comments
Almost all candidates followed a well-learnt
procedure to complete the calculation.
Despite being in the rubric to the question,
some candidates did not give an answer to
two decimal places. Others made a
rounding error in reducing 28.1086 to two
decimal places, with 28.10 and 28.12 being
common errors

Question 7

One coin has a mass of 5.00 g and contains 84.0% of copper, by mass.
Calculate the number of copper atoms in one coin.
Give your answer in standard form and to three significant figures.[2]

Answer 7

FIRST CHECK ANSWER ON THE ANSWER LINE
If answer = 3.97 × 1022 (from 63.62) award 2 marks
If answer = 3.98 × 1022 (from 63.5) award 2 marks
____________________________
Using 63.62: correct Ar of Cu from 21(b)(i)
See bottom of answer zone
Cu atoms = 0.0660 × 6.02 × 1023 = 3.97 × 23 1022 ✓
Must be calculated in standard form AND to 3 SF
OR
Using 63.5: Ar of Cu from periodic table
Cu atoms = 0.0661 × 6.02 × 1023 = 3.98 × 1022 ✓
Must be calculated in standard form AND to 3 SF

Examiner’s Comments
This part was generally well answered with
most candidates processing the data
correctly. Candidates sometimes failed to
consider 84% or rounded incorrectly in
places.
Answer = 3.97 × 10(22) atoms

Question 8

Bromine has two isotopes, Br–79 and Br–81. The relative atomic mass of bromine is 79.9.
Calculate the percentage of Br–79 atoms in a sample of bromine. [2]

Answer 8

55% ✔

Examiner’s Comments
Although some very good algebraic
attempts were seen in this variant of an Ar
calculation, it was clear from the working
shown that even when the right answer was
given, some candidates had not got to this
answer in a systematic way but often in a
very muddled and confused manner.

Question 9

Different isotopes of antimony have the same chemical properties.
Explain why. [1]

Answer 9

same number of electrons in outer shell
OR
same electron configuration OR electron structure ✔

Examiner’s Comments
The key reason why isotopes show similar chemical properties (ie because they have
an identical numbers of electrons in the
outer shell) was not always understood.
Weaker candidates struggled and gave
answers referring to the number of protons
remaining the same. Even slightly improved
answers referring to the total number of
electrons remaining the same did not deliver
the required level of detail.

Question 10

Define the term relative atomic mass.[3]

Answer 10

The (weighted) mean mass of an atom (of an
element)
OR
The (weighted) average mass of an atom (of an
element) ✔
compared with 1 / 12th (the mass) ✔
of (one atom of) carbon-12 ✔

Examiner’s Comments
This familiar recall question was well
answered by all candidates. In the past
there have been problems with weaker
candidates omitting reference to average or
mean mass, or muddling comparisons by
referring to a single atom of the element and
then a mole of carbon-12. On this occasion,
however, such errors were rare and the
answers seen were extremely strong.

Question 11

A sample of antimony, Ar = 121.8, was analysed and was found to consist of 60% 121Sb
and one other isotope.
Determine the mass number of the other isotope in the sample of antimony.[1]

Answer 11

123 ✔

Question 12

Zinc carbonate, ZnCO3, reacts with dilute hydrochloric acid.
A student reacts a sample of ZnCO3 with an excess of dilute hydrochloric acid in a test-tube.
i. Describe what the student would see during this reaction.[1]

ii. Write the equation for the reaction between ZnCO3 and dilute hydrochloric acid.[1]

Answer 12

i) i. Effervescence OR fizzing OR bubbling OR gas
produced
AND
The solid OR zinc carbonate would dissolve OR
disappear ✓

Examiner’s Comments
Most candidates realised that effervescence and
dissolving would be seen.

ii. ZnCO(3) + 2HCl ◊ZnCl(2) + CO(2) + H(2)O ✓

Examiner’s Comments
Nearly all candidates were able to write the
equation successfully – including those who had
omitted effervescence in (i).

Question 13

Compounds of calcium have many uses.

i. Identify a compound of calcium that could be used to convert a soil pH from 5.8 to 7.5.
[1]

ii. Calcium phosphide, Ca3P2, is an ionic compound used in rat poison.
Calcium phosphide can be prepared by reacting calcium metal with phosphorus, P4.
Write the equation for the reaction of calcium with phosphorus to form calcium
phosphide.
[1]

iii. Draw a ‘dot-and-cross’ diagram to show the bonding in calcium phosphide, Ca3P2.
Show outer electrons only.

Answer 13

i.Ca(OH)2 OR Calcium hydroxide
OR CaO OR Calcium oxide ✓

Examiner’s Comments
The unusual equation involving P4 molecules
was answered well. Weaker candidates
assumed that phosphorus was monatomic and
consequentially lost credit.

ii 6Ca + P4 ◊ 2Ca3P2 ✓

Examiner’s Comments
This potentially difficult dot-and-cross diagram of
the ions present was done well by candidates.

iii Ca with 8 (or no) electrons AND phosphide ion with
dot-and-cross outermost octet ✓
Three Ca ions AND two phosphide ions with correct
charges ✓

Question 14

Gallium, atomic number 31, is in Period 4 of the Periodic Table. Gallium is a Group 3 element.
Predict the formula of a gallium ion.[1]

Answer 14

Ga3+ ✓

Question 15

This question is about compounds of Group 3 elements.
Aluminium will combine directly with fluorine.
Write the equation for the reaction between aluminium and fluorine[1]

Answer 15

2Al + 3F2 → 2AlF3 ✔

Examiner’s Comments
Although the formula of AlF3 was not given, this
question was well answered. When the mark
was not awarded it was rarely because of errors
in the formula for AlF3, but more in the ratio of
the reactants or in fluorine not being given as
diatomic. Occasionally, the symbol for fluorine
was given as Fl.

Question 16

A molecule of an alkane has 24 carbon atoms.
State the empirical formulae of this alkane [1]

Answer 16

C12H25 ✔

Examiner’s Comments
The majority of candidates were able to deduce
the correct empirical formula of the alkane.

Question 17

A salt used as a fertiliser has the empirical formula H4N2O3.
Suggest the formulae of the ions present in this salt. [2]

Answer 17

NH4+ ✔ NO3
- ✔

Examiner’s Comments
This question proved more difficult than
intended. The specification indicates the
polyatomic ions which the candidates are
expected to know the formulae of and it was little
surprising that many candidates could not make
the leap from the empirical formula given to the
possible ions present. Weak answers came from
candidates who took their lead from the empirical formula H4N2O3 to suggest that the
ions were H+ and N2O−. Among stronger
candidates it was more common to identify NO3
−
than NH4
+, although it remained rare to see both
correct ions given.

Question 18

A chemist carries out reactions of barium and barium nitride, Ba3N2.

Reaction 1 Barium is reacted with water.
Reaction 2 Barium nitride is reacted with water, forming an alkaline solution and an alkaline
gas.
Reaction 3 Barium is reacted with an excess of oxygen at 500°C, forming barium peroxide,
BaO2.
i. Write equations for Reaction 1 and Reaction 2.
Ignore state symbols. [3]

ii. Predict the structure and bonding of Ba3N2.[1]

iii. BaO2 formed in Reaction 3 contains barium and peroxide ions.
The peroxide ion has the structure [O−O]2−.
Suggest a ‘dot-and-cross’ diagram for BaO2.
Show outer shell electrons only[1]

Answer 18

i. Reaction 1: Ba + 2H2O → Ba(OH)2 + H2 ✔
Reaction 2: Ba3N2 + 6H2O → 3Ba(OH)2 + 2NH3
Correct products ✔
Balancing ✔

Examiner’s Comments
Both equations were relatively challenging.
Reaction 1 was a direct question about reactions
of Group 2 elements. Reaction 2 demanded a
higher level of application based upon
information given. Many identified the alkaline
gas as NH3, but then incorrectly assumed that
the alkaline solution was BaO instead of
Ba(OH)2.
Weaker candidates suggested equations with
hypothetical species that could not have born
any relation to formulae that they might have
encountered before.

ii. Giant ionic (lattice) ✔

ALLOW ‘Giant lattice with ionic bonds’
ALLOW ‘Giant ionic bonds’
DO NOT ALLOW ‘atoms or molecules or
dipoles’

Examiner’s Comments

This question was relatively well answered,
although some candidates did negate the mark
by referring to molecules of Ba3N2 either directly
or by indirect reference to intermolecular forces.

iii. Examiner’s Comments
This question was designed to be difficult, but
many candidates rose to the challenge. Weaker
candidates simply drew a ‘dot-and-cross’
diagram for BaO2 in which they treated each
oxygen species as an oxide ion each having a
single negative charge. Many stronger
candidates did realise from the structure given in
the question that there was only a single bond
between the two oxygen atoms, as was clear
from their suggested diagram. Only the stronger
candidates managed to incorporate correctly the
electrons from barium, to arrive at a correct
version of the bonding of BaO2.

Question 19

Bromine and mercury react with many elements and compounds.
Predict the formula of the compound formed when bromine reacts with aluminium.[1]

Answer 19

AlBr3

Question 20

MOLECULAR IONS:

Ammonium
Hydroxide
Nitrate
Hydrogen carbonate
Carbonate
Sulfate
Dichromate
Phosphate

Answer 20

Ammonium NH4+
Hydroxide OH-
NitrateNO3-
Hydrogen carbonate HCO3-
Carbonate CO3(2)-
Sulfate SO2 (2)-
Dichromate Cr2O7(2)-
Phosphate PO4(3)-

Question 21

Relative isotopic mass (1)

Answer 21

Mass of the isotope compared to 1/12th the mass of a C12 atom (1)

Question 22

Weighted mean mass (1)

Answer 22

Mean mass taking into account the relative abundancies of the isotopes (1)