Atomic structure (MASS SPECTROMOTERY) )

Question 1

Give the relative mass and charge of protons, electrons and neutrons

Answer 1

Relative mass | Charge
Proton: 1 + 1
Neutron: 1 0
Electron: 1/1840 -1

Question 2

Define nucleons

Answer 2

Total number of protons and neutrons in an atoms nucleus

Question 3

Define relative isotopic mass

Answer 3

average mass of an atom of an isotope relative to one twelfth of the mass of one atom of carbon -12

Question 4

Define relative atomic mass (Ar)

Answer 4

average mass of an atom of an element relative to one twelfth of the mass of one atom of carbon -12

Question 5

Uses of mass spectrometry

Answer 5

Used in space probes to identify elements and compounds on surface of planets
Used to calculate relative atomic mass
Carbon - 14 dating
Pharmaceutical industry
Drug testing

Question 6

Give the 4 stages of mass spectrometry

Answer 6

Ionisation
Acceleration
Deflection
Detection

Question 7

Describe stage one of mass spectrometry

Answer 7

Ionisation - Heated metal coil gives off electrons that are attracted to the positively charged plate above it. Gaseous sample is bombarded by electrons to form 1+ ions.

Question 8

Describe stage two of mass spectrometry

Answer 8

Accelerated using an electric field

Question 9

Describe stage 3 of mass spectrometry

Answer 9

Charged particles deflected by a magnetic field - with an electromagnet

Question 10

What does the size of deflection depend on

Answer 10

Mass of ion - lighter ions are deflected more than heavier ones
Charge on the ion- 2+ ions are deflected more than a +1 ion. This halves the m/z (mass/charge) value but the abundance stays the same.

Question 11

What does the m/z ratio show

Answer 11

The mass of the isotope

Question 12

Describe stage 4 of mass spectrometry

Answer 12

The beam of ions passing through the machine is detected electrically.

Question 13

What does the mass spectrum for monatomic elements show

Answer 13

Lines represent different isotopes in that element

Question 14

What is fragmentation

Answer 14

When a molecule becomes ionised to form a molecular ion (M+) which is unstable and can fragment into different pieces

Question 15

what does the mass spectrum for a molecule with different atoms show

Answer 15

It shows the different fragments produced when the molecular ion breaks up

Question 16

Describe the mass spectrum for chlorine.

Answer 16

In the ionisation chamber, an electron is knocked off the molecule to form a molecular ion - Cl2+. This can then form a chlorine atom and cl+ ions. As the atom has no charge it is not included in mass spectra. The cl+ atom will form lines at 35 and 37 in the ratio 3:1. However, you will also get lines for the unfragmented (cl2) +. This ion could either be from the isotopes:
chlorine 35 and chlorine 35
chlorine 35 and chlorine 37
chlorine 37 and chlorine 37
Therefore, you get lines in the ratio: 9:6:1

Question 17

Which peak is the molecular ion

Answer 17

M+ ion is the heaviest so it is furthest to the right

Question 18

Why must a vacuum be used in a mass spectrometer

Answer 18

no interference from atoms or molecules in air
- Molecular fragments could not exist otherwise