3.1.1 Atomic structure

Question 1

What is mass number (A) and atomic (proton) number (Z)?

Answer 1

Mass number (A) - Number of protons + neutrons.
Atomic (proton) number (Z) - Number of protons.

Question 2

What is the relative charge and mass of a proton, neutron and electron?

Answer 2

Proton: charge +1, mass 1
Neutron: charge 0, mass 1
Electron: charge -1, mass 1/1840

Question 3

What are isotopes?

Answer 3

Atoms of the same element, with the same number of protons but a different number of neutrons.
So different mass numbers.

Question 4

Why do isotopes have similar chemical properties?

Answer 4

They have the same electronic configuration.

Question 5

What is mass spectrometry used for?

Answer 5

• Can be used to identify elements
• Can be used to determine relative molecular mass

Question 6

What are the stages of a TOF mass spectrometer?

Answer 6

1. Ionisation
2. Acceleration
3. Ion drift
4. Detection

Question 7

What are the 2 methods of ionisation in TOF mass spectrometry?

Answer 7

1. Electrospray
2. Electron impact

Question 8

What are the steps of electrospray ionisation?

Answer 8

1. Sample is dissolved in a volatile, polar solvent.
2. Injected through a fine needle at a high voltage.
3. Causes the molecule (M) to gain a proton (H+), forming MH+.

Question 9

What are the steps of electron impact ionisation?

Answer 9

1. Vaporised sample is injected at low pressure.
2. Electron gun fires high-energy electrons at the sample, knocking out an outer electron.
3. This forms positive ions with different charges (e.g. Ti(g) —> Ti+(g) + e-) OR (Br2(g) —> Br2+(g) + e-)

Question 10

What types of molecules are usually ionised by electron impact and electrospray ionisation, and why?

Answer 10

• Electron impact generally used for elements & compounds with a low relative molecular mass, as larger organic molecules tend to fragment under the high-energy conditions.
• Electrospray is suitable for larger organic molecules, such as proteins, because it is a gentler method that minimises fragmentation.

Question 11

What happens during acceleration in TOF mass spectrometry?

Answer 11

• Positive ions are accelerated by an electric field generated by a negatively charged plate.
• Each ion is given the same kinetic energy.

Question 12

What determines the velocity of each positive ion in TOF mass spectrometry?

Answer 12

• The mass of the ions determines their velocity.
• Lighter ions (lower m/z values) move faster (higher velocity).
• Heavier ions (higher m/z value) move slower (lower velocity).

Equation: KE = 1/2mv2

Question 13

What happens during ion drift in TOF mass spectrometry?

Answer 13

• Different ions have different flight times due to differing m/z values.
• Their flight times are recorded once they reach detector.

Question 14

What happens during detection in TOF mass spectrometry?

Answer 14

• Ions reach the detector and each ion gains an electron from it.
• This electron gain generates a current.
• The size of the current is proportional to the abundance of the isotope detected.

Question 15

What is relative atomic mass?

Answer 15

(Average/mean mass of 1 atom of an element) / (1/12 mass of 1 atom C-12)

Question 16

What is the formula for R.A.M?

Answer 16

R.A.M = ∑(isotopic mass x % abundance) / 100

Question 17

What is the isotopic composition for chlorine and bromine?

Answer 17

• The isotopic composition for Chlorine is 35Cl (75%), 37Cl (25%).
• The isotopic composition for Bromine is 79Br (50%), 81Br (50%).

Question 18

What is a molecular ion?

Answer 18

The highest m/z peak on the mass spectrum.

Question 19

What must you remember when identifying the molecular ion with electrospray ionisation?

Answer 19

You must subtract 1 from the m/z value to account for the H+ ion.

Question 20

How are electrons configured in an atom?

Answer 20

• Principle energy levels (shells) 1,2,3,4
• Divided into sub energy levels (sub-shells) s,p,d,f
• Divided into orbitals (each hold 2 electrons of opposite spin)

Question 21

How many electrons does each sub shell hold?

Answer 21

• s holds 2
• p holds 6
• d holds 10
• f holds 14

Question 22

In what order does an atom fill with electrons?

Answer 22

1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6

Question 23

What are the exceptions to the conventional electronic configuration?

Answer 23

• Copper (Cu) - Chromium (Cr)
  They only half fill the 4s orbital before the 3d orbital
• Copper 4s^1 3d^10
• Chromium 4s^1 3d^5

Question 24

What is first ionisation energy?

Answer 24

The enthalpy change when 1 mole of gaseous electrons is removed from 1 mole of gaseous atoms, forming 1 mole of gaseous 1+ ions.

Question 25

What is the first ionisation equation for magnesium?

Answer 25

Mg(g) —> Mg**+**(g) + e**-**

Question 26

What is second ionisation energy?

Answer 26

The enthalpy change when 1 mole of gaseous electrons is removed from 1 mole of gaseous 1+ ions, forming 1 mole of gaseous 2+ ions.

Question 27

What is the second ionisation equation for magnesium?

Answer 27

Mg**+**(g) —> Mg**2+**(g) + e**-**

Question 28

What are the factors affecting ionisation energy?

Answer 28

- Nuclear charge. - Atomic radius. - Shielding.

Question 29

Why are successive ionisation energies always larger than the first?

Answer 29

- After each electron is removed, the positive charge of the ion increases. - This results in a stronger attraction between the nucleus and the remaining electrons. - Therefore, more energy is required to remove each subsequent electron.

Question 30

What does a large jump in successive ionisation energies indicate?

Answer 30

Removal of electron from a higher principle energy level.

Question 31

Why does helium have the largest first ionisation energy?

Answer 31

- Outer electron is in the shell closest to the nucleus (smallest atomic radius). - No shielding from inner electron shells. - Helium has one more proton than hydrogen (greater nuclear charge). - Strongest attraction between the nucleus and outer electron.

Question 32

Why do first ionisation energies decrease down a group?

Answer 32

- Atomic radius increases (electrons occupy shells further from nucleus). - Increased shielding from inner electron shells. - Weaker attraction between the nucleus and outer electron. - Requires less energy to remove outer electron.

Question 33

Why is there a general increase in first ionisation energy across a period?

Answer 33

- Electrons in same energy level / shell so similar shielding. - More protons / increased nuclear charge. - Stronger attraction between the nucleus and outer electron. - Requires more energy to remove outer electron.

Question 34

Why is the first ionisation energy of Al lower than Mg?

Answer 34

- Al outer electron in 3p. - Mg outer electron in 3s. - 3p sub shell higher in energy than 3s. - 3p experiences additional shielding from 3s sub shell. - Weaker attraction between the nucleus and outer electron. - Requires less energy to remove outer electron from Al.

Question 35

Why is the first ionisation energy of S lower than P?

Answer 35

- S 4th electron in 3p sub shell pairs up with electron already present. - Pairing causes repulsion between the 2 negative electrons. - Weaker attraction between the nucleus and outer electron. - Requires less energy to remove outer electron from S.