Atomic Structure/Amount of Substance

Question 1

Relative mass of electron

Answer 1

0.0005 or 1/2000

Question 2

Mass number

Answer 2

total number of nucleons (neutrons and protons)

Question 3

Suggest points to consider when interpreting electron impact mass spectrum

Answer 3

• greatest m/z value (of relatively high abundance) is from molecular ion and its m/z value gives relative molecular mass
• exactly half m/z value could be 2+ ion, e.g. Cu+ (63 m/z) and Cu2+ (31.5 m/z)
• ions slightly greater than greatest m/z value are due to molecular ions with 2H or 13C

Question 4

Suggest points to consider when interpreting electrospray ionisation mass spectrum

Answer 4

• greatest m/z value (of relatively high abundance) is from molecular ion and its relative molecular mass is m/z minus 1
• always MINUS 1 from m/z to account for proton
• ions slightly greater than greatest m/z value are due to molecular ions with 2H or 13C

Question 5

Explain Cu and Cr electron configuration

Answer 5

copper - [Ar] 3d104s1
chromium - [Ar]3d54s1
- completely filled and half-filled subshells are more stable than partially filled ones
- electron from 4s orbital becomes excited and rises to 3d orbital

Question 6

Describe process of electron impact/electrospray ToF mass spectrometry

Answer 6

• sample vaporised and fired at by high speed electrons with electron gun
• removes electrons from each particle to form ions with a single positive charge
  OR
• sample dissolved in volatile solvent and injected through hypodermic needle attached to positive terminal of high voltage s]power supply
• particles gain proton to form ions with a single positive charge
• acceleration by electric field so particles have same kinetic energy but different velocity depending on mass
• positive ions travel through a hole in a negatively charged plate into flight tube where time taken to hit detector is measured
• positive ions gain electrons when they hit negatively charged detector plate
• size of current gives measure of abundance of ion hitting plate

Question 7

Give reasons why atoms are put into certain blocks

Answer 7

• last electron found in that subshell

- highest energy subshell

Question 8

Reason isotopes have the same chemical properties

Answer 8

same electron arrangement

Question 9

Atomic number

Answer 9

number of protons in an element

Question 10

Suggest why relative atomic mass calculated differs from the value in the Periodic Table

Answer 10

Periodic Table takes into account other isotopes/different amounts of isotopes

Question 11

Suggest what information can be obtained from a mass spectrometer

Answer 11

• m/z (relative isotopic mass)

- relative abundance of isotopes

Question 12

Factors affecting ionisation energy

Answer 12

• nuclear charge
• shielding
• atomic radius
• whether electron is removed from neutral or charged species

Question 13

What do electron arrows in orbitals represent

Answer 13

represent clockwise and anti-clockwise spin

Question 14

Suggest why electrons in 2p sublevels have a higher energy than in 2s sublevels

Answer 14

further from nucleus so so more shielding

Question 15

Explain why first ionisation energy of helium is very large

Answer 15

electron is not shielded from nucleus

Question 16

How to calculate relative formula mass from mass spectrometer

Answer 16

sum of m/z multiplied by relative abundance divided by sum of relative abundances

Question 17

Reasons particles must be ionised before being analysed in a mass spectrometer

Answer 17

• accelerated

- attracted to negative plate to be detected

Question 18

Term given to .xH20

Answer 18

waters of crystallisation

Question 19

Electron orbital

Answer 19

region around nucleus of an atom where electrons are most likely to be found

Question 20

Give mass spectrum for diatomic molecules with two isotopes and explain why

Answer 20

2A-2A (25%)
1A-1A (25%)
1A-2A or 2A-1A (so 50%)
molecule has two isotopes and is a diatomic molecule so there are three possible combinations of isotopes in molecule

Question 21

Suggest why there is a minimum energy for the electron gun used ionisation

Answer 21

so that only one electron is removed from atoms so ions with a single positive charge are formed

Question 22

Suggest a benefit of electrospray mass spectrometry

Answer 22

soft ionisation so larger molecules do not fragment as much

Question 23

Suggest which peak in mass spectrum is molecular ion peak

Answer 23

highest m/z value (but with relatively high abundance)

Question 24

Suggest how infrared mass spectrometry is used to differentiate between bonds in a molecule

Answer 24

• different bonds vibrate at different discrete frequencies
• infrared radiation absorbed by bonds excites them to a higher state of vibration
• so different discrete frequencies of infrared radiation are absorbed based on bonds present
• transmittance graph show peaks where infrared radiation is absorbed by bonds

Question 25

Fingerprint Region

Answer 25

• between 1500-400cm-1
• allows compounds with same functional groups to be distinguished
• must have exact match to spectrum of known compound

Question 26

Relative atomic mass

Answer 26

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

Question 27

Suggest uses of a mass spectrometer

Answer 27

• identify element
• find mass and abundance of each isotope in element to determine relative atomic mass
• find relative molecular mass of molecule

Question 28

Compare and contrast electron impact and electrospray ionisation ToF mass spectrometry

Answer 28

• ionisation by electron gun vs dissolving in volatile solvent to protonate particles
• elements and low Mr vs large molecules (e.g. proteins) and high Mr
• electrospray referred to as soft ionisation since less fragmentation
• Mr = greatest m/z vs greatest m/z minus 1
• particles accelerated by an electric field for both
• time measured for particles to travel through flight tube for both
• abundance of each ion determined by size of current for both

Question 29

Suggest what peaks below greatest m/z value are due to for electron impact ionisation of molecules

Answer 29

fragmentation of molecular ion

Question 30

Explain why electrons in an orbital have opposite spin

Answer 30

• electrons repel strongly

- opposite spin minimises repulsion

Question 31

Explain why 4s subshell is filled before 3d

Answer 31

lower energy level

Question 32

What does Avogadro’s constant tell us

Answer 32

number of particles in a mole

Question 33

State ideal gas equation and units for each variable

Answer 33

pV=nRT 
p - pressure in Pa
V - volume in m3
n - moles 
R - gas constant 
T - temperature in K (Celsius plus 273)

Question 34

Suggest what evidence there is from pattern in first ionisation energies across Period 3 to suggest 2 is maximum number of electrons in s-subshell

Answer 34

two elements before drop in ionisation energy

Question 35

Describe how to identify group or element from successive ionisation energies

Answer 35

• calculate difference between each ionisation energy
• any big jumps (above 1500-2000 kJmol-1) indicate electron is removed from a different energy level
• smaller jumps refer to subshells (around 1000)
• even smaller but still evident jumps are due to orbital pairing (unlikely to be obvious so ignore)

Question 36

How to use infrared spectroscopy to prove an aldehyde is definitely pentanal

Answer 36

• compare with known spectrum for pentanal

- fingerprint regions should match exactly

Question 37

What needs to be true for molecular ions to have the same precise molecular mass

Answer 37

must contain same numbers of each atom not just same relative molecular mass