AQA Chemistry A Level - Atomic Structure

Relative charge and relative mass of proton, neutron, electron

What does an atom consist of?

A nucleus containing protons and neutrons, surrounded by electrons

Mass number

The sum of the number of neutrons and protons in an atomic nucleus (A)

Atomic number

The number of protons in the nucleus of an atom (Z)

Mass spectrometer gives accurate information about...

...relative isotopic mass and the relative abundance of isotopes

mass spectrometry can be used to...

...identify elements and determine relative molecular mass

Isotope

Atoms with the same proton number but a different neutron number, caused by instability in an atom

Electron impact ionisation/electron gun ionisation

The sample is bombarded by high energy electrons. Sample molecule loses an electron forming M+
High energy electrons from an electron gun are fired at the sample
This knocks off one electron from each particle forming a 1+ ion

Electrospray ionisation

Sample is dissolved in a polar volatile solvent and injected through a fine hypodermic needed as a fine spray. The tip of the needle has a high voltage.
Particles are ionised by gaining a proton from the solvent

Acceleration area:

Negative plate used to give ions the same kinetic energy

Ion drift:

Ions are separated in a flight tube (region with no electric field) depending on their mass and charge

Ion detector

Records flight times, only positive ions are recorded - they pick up an electron, causing a current and a peak

Data analysis

Flight times are analysed and recorded as a mass spectrum

A sample of 65Cu+ was accelerated to have 1 x 10-19 kJ of kinetic energy. How long would it take to drift along a 80cm long tube?

Mass of 1 mole = 65g = 0.0065kg
0.065/6.022 x 1023 = 1.08 x 10-25kg = mass of one ion
t = 0.8√(1.08 x 10-25)/(2 x 1 x 10-19)
= 5.87 x 10-4 seconds

First ionisation energy

The energy required to remove one electron from a gaseous atom
e.g. Na(g) ---> Na+(g) + e-

Second ionisation equation

Na+(g) ---> Na2+(g) + e-

What happens to the ionisation energy down group one?

Value decreases due to increased shielding and greater distance from the nucleus - outer electron is easier to remove

What happens to the ionisation energy down group two?

Values are greater than group one, more nuclear charge = stronger pull on electron = more energy to remove it
Deceases down the group due to more energy shells, weaker attraction of ion to lost electron

How many electrons does group s hold?

2

How many electrons does group p hold?

6

How many electrons does group d hold?

10

What are the 2 exceptions?

Chromium and copper

What is the rule when ions are formed?

4s shell is lost before 3d

Why are only a small proportion of the alpha particles deflected through a large angle

The nucleus is very small

Why do some alpha particles get deflected

The nucleus is positive so repels the alpha particles

Why don't some alpha particles get deflected

The nucleus is empty space

Finding the molecular ion peak

The one with the largest m/z value

The full electron configuration of Fe

1s2 2s2 2p6 3s2 3p6 3d6 4s2

The full electron configuration of Ni

1s2 2s2 2p6 3s2 3p6 3d8 4s2

Write the formula of the compound formed when an atom with 17 electrons reacts with a group 2 metal in period 4

CaCl2

Write the formula of the compound that contains a 2+ ion and a 3- ion that have the same electron configuration as neon

Mg3N2

The first 5 successive ionisation energies of an element Z are 799, 2420, 4000, 25000 and 328000. Which group does this element belong to

Group 3

State and explain the general trend in ionisation energy across the period

Increases as decreased shielding and shorter distance from the nucleus, outer electron harder to remove as greater nuclear charge
More protons=more nuclear charge=smaller atomic radius

Explain why the first ionisation energy for oxygen deviates from the general trend

Oxygen has a pair of electrons in a 2p orbital that repel each other

Equation for first ionisation energy of calcium

Ca(g) --> Ca+(g) + e-

What element in period 3 has the highest melting point and why

Silicon - lots of energy needed to break covalent bonds

Blocks in periodic table

What element in period 3 has highest first ionisation energy and why

Argon - most nuclear charge, same number of shells

Why do isotopes have similar chemical reactions

They have the same number of electrons

Why is the second ionisation energy of boron higher than the first ionisation energy of boron

The electron being removed is closer to the nucleus so a stronger attraction

What might cause relative atomic mass of a sample to be different from in the periodic table

There may be other isotopes present/absent

Write an equation for the reaction when a tellurium ion hits the detector

Te+ + e- --> Te

How are ions formed in a time of flight mass spectrometer

Sample bombarded by high energy electrons, sample molecule loses and electron forming M+

Why is it necessary to ionise molecules when measuring their mass in TOF

Only ions can be accelerated and cause a current to flow when they hit the detector

What 2 measurements are recorded for each isotope in the mass spectrometer

m/z
abundance

Relative atomic mass

The weighted mean mass of an atom of an element relative to 1/12th of the mass of an atom of carbon-12

How is a current generated in a mass spectrometer

Electrons flow from detector to positive ion

Relative mass of an electron

1/1840

Equation for second ionisation energy of boron

B+(g) --> e- + B2+(g)

Give 2 reasons a sample must be ionised

- to deflect the beam
- to be accelerated to have the same KE

What can be adjusted in the mass spectrometer to enable ions formed by the different isotopes to be directed onto the detector

Electric field

Explain why atoms may have different mass numbers

Different number of neutrons

Why is the first ionisation energy is krypton greater than bromine

More protons as higher atomic number, but similar shielding

How are ions formed in TOF mass spectrometer

- Sample bombarded with high energy electrons
- Sample molecule loses and electron forming M+
- High energy electrons from the gun get fired at the sample which knocks off an electron from the particle to form an ion

Suggest why krypton has lower first ionisation energy than argon

More electrons so more orbitals = increased shielding so less attraction to nucleus and easier to remove

Name the 3 processes that occur in a mass spectrometer before the vaporised isotopes can be detected

Ionisation - high energy electrons fired at sample
Acceleration - electric field and negative plate gives ions same kinetic energy
Deflection - with magnetic field

How is ionisation achieved in a mass spectrometer

- electron gun fired high speed electrons
- knocks off e- from particle

Why is ionisation necessary

So ions can be accelerated by electric field, deflected by magnetic field and detected

Relative isotopic abundance

The relative abundance of each isotope present in a sample of an element

Give the symbol for the isotope which has the mass number of 34 and has 18 neutrons

Sulphur 34, 16

What is adjusted in a mass spectrometer to direct the ions with different mz values onto the detector

Magnetic field on the negative accelerator

What happens to particles immediately after they are ionised

Accelerated

What factors determine how much a particle is deflected in a magnetic field

- mass
- charge
- speed

Time of flight equation

KE = Joules
D = Metres