Atomic Structure Notes


Atoms are tiny particles which make up matter. They contain a nucleus which is surrounded by electrons. The nucleus itself contains smaller particles called protons and neutrons.

The nucleus, and protons and neutrons, are much more massive than electrons.

Protons and electrons have a very small electrical charge. Protons are positively charged; electrons are negatively charged.

Atoms of a certain element all have the same number of protons and electrons, e.g. all atoms of lithium have 3 protons and 3 electrons.




Atoms can gain or lose electrons so that they have unequal numbers of protons and electrons. These new particles are called ions.

They are indicated by a charge.

e.g         27Al3+  this ion has lost 3 electrons.


        18O2-  this ion has gained 2 electrons.



The number of neutrons can vary. Isotopes are atoms of the same element with different numbers of neutrons and different mass numbers.

e.g. chlorine, carbon, hydrogen

Isotopes of an element have identical chemical properties, but they can have different physical properties. Isotopes of the same element with more neutrons have:

  • higher density
  • higher melting and boiling points
  • slower rate of diffusion

Many isotopes are radioactive; these are called radioisotopes (radioactive isotopes) The nuclei of radioisotopes breaks down spontaneously emitting radiation – either α, β or γ. Their uses include nuclear power generation, sterilisation of medical equipment, finding cracks and stresses in metal, and the preservation of food. Some radioisotopes of certain elements are particularly useful.

Mass spectrometry

A mass spectrometer is a machine that can:

  • measure the relative masses of different isotopes
  • measure the relative abundances of the different isotopes in a sample of an element.

It separates positively charged ions according to their mass. A sample that is going to be tested in the mass spectrometer has to be in the gaseous state. If the sample is a liquid or a solid, a heater is used to vaporise some of it. There are five main stages in the operation of the mass spectrometer: vapourisation, ionisation, acceleration, deflection and detection.


A sample is heated at low pressure to turn it into a gas. It is then injected into the instrument.


An electron gun produces a stream of high-energy electrons from a heated metal filament. These high-energy electrons bombard the vaporised sample. When a high-energy electron hits an atom in the sample it knocks an electron out of the atom, forming an ion with a single positive charge:

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X(g) + e  X+(g) + 2e 


An electric field accelerates the positive ions into the rest of mass spectrometer. Negatively charged plates attract the positive ions from the ionised sample, and the ions are focussed through slits in the plates into a narrow beam.


The beam of ions is deflected around a bend by a magnetic field.  The lighter the ion, the more it will be deflected. Electromagnets are used so that the strength of the magnetic field can be varied.


When an ion hits the detector, a tiny electrical current ...

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