atomic structure

Question 1

The sample of chromium is analysed in a time of flight (TOF) mass spectrometer.
(d) Give two reasons why it is necessary to ionise the isotopes of chromium
before they can be analysed in a TOF mass spectrometer.

Answer 1

(Ions will interact with and) be accelerated (by an electric field)
Allow (ions) accelerated to a negative plate
Do not allow magnetic field
1
M2 Ions create a current when hitting the detector OR ions create a
current in the detector/electron multiplier

Question 2

The first ionisation energies of the elements in Period 2 change as the atomic
number increases.
Explain the pattern in the first ionisation energies of the elements from lithium to
neon.

Answer 2

Stage 1: General Trend (Li → Ne)
1a. 1st IE increases
1b. More protons/increased nuclear charge
1c. Electrons in same energy level / shell
1d. No extra/similar shielding
1e. Stronger attraction between nucleus and outer e OR outer e closer to
nucleus (ignore radius decreases)

Stage 2: Deviation Be → B
2a. B lower than Be
2b. Outer electron in (2)p
2c. higher in energy than (2)s
If Al vs Mg then do not award 2a or 2b

Stage 3: Deviation N → O
3a. O lower than N
3b. 2 electrons in (2)p need to pair
3c. pairing causes repulsion (do not award if it is clear reference to
repulsion is in s orbital)

Question 3

This question is about atomic structure.
In the nineteenth century JJ Thomson discovered the electron. He suggested
that negative electrons were found throughout an atom like ‘plums in a pudding
of positive charge’.
The diagram shows an atom of element R using the ‘plum pudding’ model.
An atom of R contains seven electrons.
(a) State two differences between the ‘plum pudding’ model and the model of
atomic structure used today.

Answer 3

Statement about the nucleus:
(Central) nucleus contains protons and neutrons.
Allow “protons and neutrons are in the centre of the
atom”
1
Statement about electrons
Electrons are now arranged in energy levels/shells/orbitals

Question 4

State which of the elements magnesium and aluminium has the lower first
ionisation energy.
Explain your answer

Answer 4

Al
Mg then CE = 0

1
(Outer) electron in (3)p sublevel / orbital
Not just level or shell
1
Higher in energy / further from the nucleus
so easier to remove OWTTE

Question 5

A TOF mass spectrometer can be used to determine the relative molecular
mass of molecular substances.
Explain why it is necessary to ionise molecules when measuring their mass
in a TOF mass spectrometer.

Question 6

Q28.
A sample of ethanedioic acid was treated with an excess of an unknown alcohol
in the presence of a strong acid catalyst. The products of the reaction were
separated and analysed in a time of flight (TOF) mass spectrometer. Two peaks
were observed at m / z = 104 and 118.
(a) Identify the species responsible for the two peaks.

Outline how the TOF mass spectrometer is able to separate these two
species to give two peaks

Answer 6

[CH3OCOCOOH]+
Allow names
1
[CH3OCOCOOCH3]
+

Positive ions are accelerated by an electric field
1
To a constant kinetic energy
1
The positive ions with m / z of 104 have the same kinetic energy as
those with m / z of 118 and move faster
1
Therefore, ions with m / z of 104 arrive at the detector firs

Question 7

electrospray ionisation

Answer 7

(Sample is) dissolved (in a volatile solvent)
Allow named solvent (eg water/methanol)
1
(Injected through) needle/nozzle/capillary at high voltage/positively
charged
Ignore pressure
1
Each molecule/particle gains a proton/H+

Question 8

What is the relative molecular mass of P?
Tick (✓) one one box.

mass spectrometer only one current 556

Answer 8

We’ve been told for our exam spec to minus 1 from the m/z value so maybe 555