Atomic Structure

Question 1

Two lead isotopes were analysed in a TOF mass spectrometer.
Ions of both isotopes were accelerated with the same kinetic energy
207^Pb took 1.67 x 10^-5 s to travel through the mass spectrometer tube
Calculate the time taken for the 211^Pb ion to travel through the spectrometer

Answer 1

KE= 0.5 x m x v^2 so…
0.5 x (m of 207^Pb) x (d^2/t^2) = 0.5 x (m of 211^Pb) x (d^2/t^2)
can be simplified to…
(207 / 1.67 x 10^-5) = (211 / t^2)

Question 2

How is electrospray ionisation carried out?

Answer 2

Dissolved sample injected through a hypodermic needle at a high voltage, it gains a H+ ion

Question 3

Give an equation for electrospray ionisation

Answer 3

X(g) + H+ –> XH+(g)

Question 4

How is electron impact ionisation carried out?

Answer 4

Dissolved sample fired through high energy electrons, ‘knock off’ an electron from the atom

Question 5

Give an equation for electron impact ionisation

Answer 5

X(g) –> X+(g) + e-

Question 6

What equation can be used to find the mass of one atom?

Answer 6

mass of one atom = Ar / L (6.022 x 10^23)

Question 7

Explain why it is necessary to ionise molecules when measuring their mass in a TOF mass spectrometer

Answer 7

• need to be ions in order to interact with and be accelerated by an electric field
• Only ions will create a current when hitting the detector

Question 8

Chlorine exists as two isotopes 35Cl and 37Cl in the ratio 3:1
Give a statement describing the peaks that would appear in the mass spectrum of Cl2

Answer 8

Peaks at m/z = 70, 72 and 74 in the ratio 9:6:1

Question 9

What is the definition of relative atomic mass?

Answer 9

the average mass of all atoms in an element relative to 1/12 mass of a C atom

Question 10

Why do isotopes have similar chemical properties?

Answer 10

they all have the same electronic structure (configuration)

Question 11

State and explain the trend in the first ionisation energies of the elements in Group 2 from magnesium to barium

Answer 11

• Decrease
• Ions get bigger / more (energy) shells
• Weaker attraction of ion to lost electron

Question 12

State and explain which of the elements magnesium and aluminium has the lower first ionisation energy

Answer 12

• Al
• (Outer) electron in (3)p orbital
• Higher in energy / further from the nucleus so easier to remove

Question 13

What is the plum-pudding model?

Answer 13

the model of the atom suggesting that atoms were small spheres of positive matter, with negative charge/electrons distributed throughout

Question 14

What is the relative mass of an electron?

Answer 14

1/1840

Question 15

What is an isotope?

Answer 15

atoms of the same element with the same atomic number of P+ and e-, but a different number of neutrons

Question 16

How are ions accelerated in TOF mass spectroscopy?

Answer 16

accelerated to the same KE as they interact with an electric field

Question 17

How are ions detected?

Answer 17

flow onto a negatively charged detector/plate, causing a current. Higher abundance= greater current

Question 18

How can the Ar be calculated using mass spectra?

Answer 18

Ar = m/z x abundance (repeated if more than one peak) / total abundance

Question 19

Why do successive I.Es often require more energy?

Answer 19

as the electron is being removed from a positively charged ion, so electrostatic attraction is stronger

Question 20

Why is the 3rd I.E of Mg much higher than the 2nd I.E of Mg?

Answer 20

• electron removed is from the 2p sub-shell
• so the electron being removed is less shielded