Chemistry Atomic Structure

Give two reasons why it is necessary to ionise the isotopes of chromium

before they can be analysed in a TOF mass spectrometer.

• ions create a current when it hits the detector

• ions get accelerated by the eletric field

Describe how the molecules are ionised using electronspray ionisation

• sample is dissolved in water

• injected by needle at high voltage

• each molecule gains a proton

formulas for ionisation

electron impact ionisation:

X(g) → X+(g) + e-

electronspray ionisation:

X(g) + H+ → XH+ (g)

trend period 2 ionisation energy

• ionisation energy increases

• more protons/ increased nuclear charge

• so stronger attraction betwene nucleus and outer electron

• similar shieldin

g

period 2 exceptions:

Beryllium → Boron

• Boron lower

• 2p electron higher in energy than 2s

Nitrogen → Oxygen

• Oxygen lower

• 2 electrons in 2p need to pair

• paring causes repulsion

Group 2 Ionisation energy trend

• decrease in first ionisaiton energy

• ions get bigger

• weaker attraction of ion to lost electron

Describe how the molecules are ionised using electron impact ionisation

• sample is bombarded with high energy electrons

• the sample molevcule loses an electron forming M-

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

• positve ions are accelerated by an electric field at constant kinetic energy

• ions with mz ratio of 104 have same KE and move faster than ions with mz ratio 118

• ions with mz ratio 104 will reach detector first

State how relative abundance of the ions is determined

• ions hit a detector and produce a current