Atomic Structure + Mass Spec

Question 1

What are the four stages of TOF mass spectrometry?

Answer 1

Ionisation, acceleration, drift, detection, analysis

Question 2

Name the 2 methods of ionisation

Answer 2

Electro spray and electron impact

Question 3

Describe electro spray ionisation

Answer 3

• sample is dissolved in volatile solvent
• injected into chamber through a hypodermic needle
• needle is connected to high voltage power supply
• as sample is injected the atoms each gain a proton (H+)

Question 4

Describe electron impact ionisation

Answer 4

• sample is vaporised
• high energy electrons are fired at sample from an electron gun
  -> this electron gun is a hot wire filament which emits electrons as a current runs through it
• as the sample is hit, an electron is knocked off each particles forming 1+ ions

Question 5

Name and describe stage 2 of TOF mass spec

Answer 5

acceleration
- 1+ ions are accelerating using an electric field (a -vely charged plate)
- they are all accelerated to have the same kinetic energy
- since they all have the same kE their velocity will be dependent on their mass
-> lighter = quicker, heavier = slower

Question 6

Name and describe stage 3

Answer 6

(ion) drift
- 1+ ions then pass through a hole in the -ve plate and enter the flight tube
- the TOF of each ion depends on their velocity

Question 7

Name and describe stage 4

Answer 7

detection
- 1+ ions collide with the -ve detector plate
- as they hit the detector plate they gain an electron, this causes a current to be produced
- the size of this current is proportional to the abundance of the ions hitting the plate
- the detector is connected to a computer which produces a mass spectrum

Question 8

Give the full electron configuration of Mg

Answer 8

1s ² 2s ² 2p ⁶ 3s ²

Question 9

Give the shorthand electron configuration of Cr

Answer 9

[Ar] 4s ¹ 4d ⁵

Question 10

Define ionisation energy

Answer 10

The amount of energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of 1+ ions

Question 11

What are the four factors that affect the size of the 1st I.E

Answer 11

• atomic radius
• nuclear charge
• shielding
• (spin pair) repulsion

Question 12

General trend of I.E across a period + why

Answer 12

Increase; nuclear charge increases (this decreases atomic radius) but shielding + number of shells stays roughly the same ∴ energy required increases

Question 13

Exceptions to the trend (eg of period 2 but applies to 3 also)

Answer 13

There is a dip in 1st I.E between beryllium and boron (boron is slightly lower) as boron’s outer electron is in the 2p sub shell which is at a higher energy level than beryllium’s 2s electron
- entered a higher energy level -> slight increase in atomic radius + shielding
Dip in I.E between nitrogen and oxygen, as oxygens outer electron is paired - nitrogen’s outer electron is 2p ³ whereas oxygens is 2p ⁴, meaning it has spin pair repulsion. This repulsion between the two electrons makes it easier for the outer electron to be removed

Question 14

General trend of I.E down a group + why

Answer 14

Decrease; nuclear charge increases however number of shells increases meaning atomic radius increases , this results in increased shielding and it becomes easier to remove the outer electron