P1: 1 Atomic structure

Question 1

What is an isotope?

Answer 1

Atom of the same element with the same number of protons but different number of neutrons.

Question 2

What happens in stage 1 of a TOF mass spectrometer?

Answer 2

Ionisation:

Electron impact - used for elements and low Mr compounds.
- high energy electrons are fired at the sample from an electron gun.
- this knocks off one electron from each atom to form 1+ ions.

Electrospray ionisation - used for high Mr compounds.
- sample is dissolved in a volatile solvent and injected through a fine hypodermic needle as a fine spray into a vacuum in the ionisation chamber.
- a very high voltage is applied to the end of the needle so it’s positively charged.
- the particles gain a proton and become ions as a fine mist.
- the solvent evaporates, leaving 1+ ions

Question 3

What happens in stage 2 of a TOF mass spectrometer?

Answer 3

Acceleration:

Ions are accelerated using an electric field so that all ions have the same kinetic energy.

Question 4

What happens in stage 3 of a TOF mass spectrometer?

Answer 4

Ion drift: separation of charged ions.

Ions enter the flight tube.
Ions with different masses have different times of flights.
Lighter ions travel faster and take less time to reach the detector.

Question 5

What happens in stage 4 of a TOF mass spectrometer?

Answer 5

Detection:

The detector is a negatively charged plate.
A current is produced when the ions hit the plate since electrons are transferred from the plate to the ions.
Current produced is proportional to the number of ions hitting the plate.

Data analysis:

The mass spectrum shows the abundance of each mass that hit the detector.
In an electrospray ionisation mass spectrum, the main peak is usually at Mr + 1 so the Mr is 1 less than the molecular ion peak.

Question 6

What does a mass spectrometer do?

Answer 6

Gives accurate information about relative isotopic mass and relative abundance of isotopes.

Question 7

What can mass spectrometry be used for?

Answer 7

To identify elements and to determine relative molecular mass.

Question 8

What is first ionisation energy?

Answer 8

The energy required to remove a mole of electrons from a mole of atoms in the gaseous state.

Question 9

What is ionisation energy measured in?

Answer 9

KJ mol^-1

Question 10

What is the equation for first ionisation energies?

Answer 10

X (g) -> X+ (g) + e-

Question 11

What is the equation for successive ionisation energies?

Answer 11

X+ (g) -> X2+ (g) + e-
X2+ (g) -> X3+ (g) + e-
Etc.