Exam specific answers (atomic structure)

Question 1

state and explain the trend in the first ionisation energies of the elements in Group 2 from magnesium to barium. (3)

Answer 1

decreases, because the number of energy shells increases, so the ion has weaker attraction to the lost electron

Question 2

state the element in Period 3 that has the highest first ionisation energy. Explain your answer. (3)

Answer 2

argon, because it has the highest nuclear charge but the same amount of shielding as the other elements

Question 3

explain why the second ionisation energy of boron is higher than the first ionisation energy of boron. (1)

Answer 3

the electron being removed is closer to the nucleus

Question 4

predict the element in Period 3 that has the highest second ionisation energy and why. (2)

Answer 4

sodium, because the electron is removed from the 2p orbital

Question 5

explain why the ionisation energy of every element is endothermic. (1)

Answer 5

heat or energy is needed to overcome the attraction between the negative electron and positive nucleus

Question 6

explain why the value of the first ionisation energy of sulfur is less than the value of the first ionisation energy of phosphorus. (2)

Answer 6

sulphur has paired electrons in a 3p orbital and they repel each other

Question 7

explain how ions are detected and relative abundance is measured in a TOF mass spectrometer. (2)

Answer 7

ions hit a negative plate and gain an electron
the relative abundance is proportional to the size of the current

Question 8

state whether magnesium or aluminium has the lower first ionisation energy and why. (3)

Answer 8

Al, because its outer electron is in a 3p sub-level which is higher in energy and therefore easier to remove

Question 9

outline how the TOF mass spectrometer is able to separate. two species to give two peaks. (4)

Answer 9

positive ions are accelerated by an electric field
to a constant kinetic energy
positive ions with lower m/z have same kinetic energy as those with higher m/z and move faster
therefore ions with lower m/z reach detector first

Question 10

describe how ions are formed in a TOF mass spectrometer. (2)

Answer 10

in terms of electron impact:
sample is bombarded with high energy electrons
sample molecule loses one electron forming M^+

Question 11

explain why it is necessary to ionise molecules when measuring their mass in a TOF mass spectrometer. (2)

Answer 11

only ions, not molecules, will interact with and be accelerated by an electric field
only ions will create a current when hitting the detector

Question 12

deduce which of Na^+ and Mg^2+ is the smaller ion and explain your answer. (2)

Answer 12

Mg^2+ - they both contain the same number of electrons but the magnesium ion has more protons

Question 13

describe how a molecule is ionised using electrospray ionisation? (3)

Answer 13

sample is dissolved in a volatile solvent
sample is injected through needle at a high voltage
each molecule gains a proton

Question 14

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 14

1st IE increases
increased nuclear charge
electrons in same energy level
similar shielding
stronger attraction between nucleus and outer electron
B lower than Be
outer electron is in 2p
higher in energy than 2s
O lower than N
O has paired electrons in 2p orbital
paired electrons repel