Topic 3 - Atomic Structure

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  • Created by: Lauren
  • Created on: 20-12-12 13:46

Mass Spectroscopy

A mass spectrometer is used to measure the mass of positively charged particles. It is used to investigate the isotopic composition of elements and to investigate the structure of compounds.

  • The sample is vaporised
  • The vapour is ionised by fast moving electrons, which knock off electrons from the atoms in the sample
  • A negative potential accelerates the positive ions
  • The ions are deflected by a magnetic feild, which is steadily increased to only allow ions of a particular mass to charge ratio through at one time
  • The ions are detected and their relative amounts are calculated by the machine

All Intelligent Aardvarks Dribble Delightfully

Atomisation Ionisation Acceleration Delflection Detection

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Mass Spectrums

A mass spectrum shows us the masses of the ions detected and their relative abundance i.e - the isotopic composition of an element. From this, the relative atomic mass of the element can be calculated.

Mass spectrometry can also be used to measure the relative molecular mass of a compound.


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The uses of Mass Spectrometry

Mass spectrometry has a wide range of uses, some of which are:

  • Radioactive dating of archaeologuical specimens - the percentage abundance of radioactive isotopes is measured
  • Space research - Identification of molecules detected by planetary probes
  • Detection of illegal drugs - each molecule can be identified by its unique mass spectrum
  • The pharmaceutical industry - the fragmentation pattern and molecular ions can be used to test which compounds will be useful in the drugs industry
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Ionisation energies and the evidence for electron

Ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms, to form one mole of gaseous singly positively charged ions. It is an endothermic process.

The series of successive ionisation energies for an element provide evidence for the existence of electron (quantum) shells (or energy levels) around the nucleus.

For a group 1 element:

  • The first ionisation energy is the lowest, because an electron in an outer shell, further from the nucleus, is removed first
  • The very large increase between first and second ioisation energy arises because the second electron has to be removed from a shell that is closer to the nucleus
  • As you keep removing electrons, the ionisation energy keeps increasing since each time you remove an electron, the effective nuclear charge on the outer shell becomes greater
  • The next 'big jump' will occur for the next electron shell, and so on

You can work out which group an element is in from where the first 'big jump' is in its successive ioisation energies

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The evidence for electron sub-shells


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Ionisation energies and the evidence for electron

The first ionisation energies of successive elements provides evidence for electron sub-shells. For example, removing the outer electron for nitrogen (z=7) involves breaking into a half-full p sub-shell. This requires more energy than removing the outer electron from oxygen (z=8) which leaves a half full sub-shell.

Generally, ionisation energies increase across a period, such as from lithium to neon due to greater force of attraction between the nucleus and outer electrons. However, ioisation energy decreases slightly between beryllium and boron and between nitrogen and oxygen. This suggests that a given shell is divided into sub shells that can have slightly different energies.

The reasons for the different energy levels of sub shells within a a shell can be explained by the rules for shell filling and electron pairing.

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Electronic Configuration

The electronic configuration of an atom specifies the number of electrons in each electron shell or sub shell. The lowest energy shells are filled first - 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.

Electrons populate orbitals singly before pairing up. This exaplains why removing the outer electron for boron requires less energy than removing the outer electron from beryllium - the outer electron goes to a new p sub shell rather than filling the s sub shell. The p sub shell is at a higher energy level than the s subshell, so less energy is required to remove an electron from it.

The electronic configuration for aluminium is shown in the diagram:(

The electronic configuration is 1s2 2s2 2p6 3s2 3p1

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Periodic Properties

Going across a period, the ioisation energy generally increases. This is due to:

  • The outer electron being drawn closer to the increasingly positive nucleus

Overall there is a greater force of attraction bewtween the nucleus and the outer electrons, and it becomes more difficult to remove one.

Going down a group, the ionisation energy decreases. This is due to:

  • The outer shell electron is further from the nucleus due to the increasing number of electron shells
  • The electron sheilding by the increasing number of electron shells, further reduces the effective nuclear charge
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