atomic structure past paper questions

Question 1

write the equation for the first ionisation energy for Rb

Answer 1

Rb(g) - Rb(g)+ + e-

always remember the gas state symbol as ionisation occurs when atoms are in a gaseous state

Question 2

Kr has an isotope with the relative atomic mass of 42 explain why

Answer 2

the 84 isotope has 2 electrons knocked off

sometimes two electrons may be removed from a particle forming a 2+ ion
e.g. 24 Mg2+ with a 2+ charge would have a m/z of 121

Question 3

state one change in the operation of the mass spectrometer that will change the path of the ion

Answer 3

changing the electromagnet/ electric field

Question 4

explain how the detector in a mass spectrometer enables the abundance to be measured

Answer 4

ions hit the detector and cause a current

the current is proportional to the abundance (of the isotope_

Question 5

compound Q is analysed in time of slight mass spectrometer. A sample compound Q in solution is injected into the mass spectrometer at high pressure
A high voltage is the applied to remove electrons from atoms of the sample

Give the name of this process

Answer 5

electrospray IONISATION

Question 6

explain how a time of flight mass spectrometer separates charged particles with different mass/charge ratios

Answer 6

the ions are accelerated to CONSTANT kinetic energy (1 mark) by an ELECTRIC FIELD (1 mark)

lighter ions are accelerated more than heavier ones ( 1 mark)

so ions with different masses take different amounts of time to travel through the mass spectrometer and reach the detector

Question 7

use your understanding of the electronic structure of aluminium, silicon and phosphorus to explain your answer to question 3.4

Answer 7

aluminum, silicon, and phosphorus all have their outer electron in the same 3p subshell (1 mark)
so the shielding effect and the distance between the nucleus and the outer electron is the same/ very similar for all three elements

but the nuclear charge of silicon is higher than aluminum but lower than phosphors.
so the energy needed to remove the outer electron/ first ionisation energy of silicon should be greater than aluminium but lower than phossporus

Question 8

give the full eloectron configuration of copper and explain why it deviates from the normal rules of electron configuration

Answer 8

1s2 2s2 2p6 3s2 3p6 3d10 4s1

copper donates one of its 4s electrons to the 3d sub shell to make a more stable 3d sub shell

remember for transition metals, the 3d sub shell fills up first then the 4s orbital fills up

( look at the textbook for reference)

Question 9

there is a large jump in the trend of the successive ionisation energies of element X between the removal of the eighth and ninth electrons

Use your understanding of the factors that affect ionisation energy to explain why this is the case

(element X being neon)

Answer 9

the eighth electron is being removed from the second electron shell but the ninth electron is being removed from the first electron shell

The ninth electron is much closer to the nucleus than the eighth and experiences less shielding so it takes more energy to remove it

Question 10

explain electronspray

Answer 10

sample is dissolved
injected through a needle at high voltage which is positively charged
each particle gains a proton

Question 11

explain how first and successive ionisation energies in Period 3 (Na–Ar) and in Group 2 (Be–Ba) give evidence for electron configuration in sub-shells and in shells

Answer 11

this is giving reference on how some sub- shells have paired electrons and some do not

Question 12

what is the relative atomic mass

Answer 12

average mass of 1 atom/ 1/2 of the MASS of C-12 ATOM

Question 13

In addition to the major peaks at m/z = 63 and 65, much smaller peaks at m/z = 31.5
and 32.5 are also present in the mass spectrum. Identify the ion responsible for the
peak at m/z = 31.5 in the mass spectrum. Explain why your chosen ion has this m/z
value and suggest one reason why this peak is very small

Answer 13

Identity of the ION: 63Cu 2+

Explanation for m/z value:
63/2 = 31.5

the reason why this peak is very small:
more energy is needed to remove the 2nd electrons

Question 14

how are the ions detected

Answer 14

POSITIVE ions collide with detector

causing the current to flow

Question 15

why must the atoms be ionised

Answer 15

ONLY IONS can be accelerated by in electric field

Question 16

what are some top tips for relative atomic mass

Answer 16

make sure you add up the bottom number when working out the relative atomic mass

always to 1.d.p

Question 17

One of the isotopes of Ge, given in the table in part (c), has an ion that forms a small
peak in the mass spectrum which is indistinguishable from a peak produced by 36S+
ions.
Identify the ion and explain why

Answer 17

• identity of the ion: 72 Ge 2+

Same m/z as 36S+

Question 18

what is the definition of isotope

Answer 18

atoms with the same number of protons but different number of neutrons

Question 19

what is the definition of relative isotopic abundance

Answer 19

the percentage of different types of atom

present in the natural isotopic mixture

Question 20

how are the ions distinguished by

Answer 20

the different flight times

Question 21

describe acceleration

Answer 21

Positive ions are accelerated by an electric field

to a constant kinetic energy

Question 22

why do we use electrospray for larger molecules

Answer 22

electrospray ionisation is used for larger organic molecules

This is because the “softer” conditions of this technique mean that FRAGMENTATION DOES NOT OCCUR

Question 23

what happens when a molecule is put through with an electron impact ionisation instead of electrospray e.g. C4H10

Answer 23

it will often break up and give a series of peaks caused by the fragements

The peak with the larges m/z, however will be die to the complete molecule and will ve equal to the relative molecular mass,Mr, of the molecule

This peak is called the parent ion or molecular ion

Question 24

what do we have to do if a molecule is put through a mass spectrometer with electrospray ionization

Answer 24

As fragmentation will not occur

There will be one peak that is equal to the mass of the MH+ ion
It will therefore be necessary to subtract 1 to get the Mr of the molecule. This is because with electrospray ionisation the molecular ion gains a proton

so if there was a peak that was 521.1 dor the MH+, the relative molecular mass of the molecule is 520.1

Question 25

what is ionic drift

Answer 25

the stages of the mass spectrometer is

1. vaporisation
   the sample must be vaporised so that it can travel through the TOF
2. ionisation
3. acceleration

4.ION DRIFT
particles travel through (the flight tube) with a constant speed and kinetic energy. They drift through and particles with a lower mass/charge ratio will travel faster

5. ions are detected as electrical current and is made when particles hit the plate. Particles with lower m/z reach the detector first as they travel faster

Question 26

stock definition for the relative atomic mass

Answer 26

the mass of an atom of an element measured on a scale which the mass of an atom of 12C is exactly 12