Evolution of Atomic Understanding: Thomson to Mass Spectrometry

The electron was the first particle to be discovered by J J Thomson, who was looking into passing an electric current through gases. At the positive end of the tube a glow was visible. This glow could be deflected by both electric and magnetic fields, suggesting that there were rays of negative particles travelling from the cathode to the anode. These particles are obviously electrons. Thomson went on to suggest that atoms were like plum puddings with negative electrons embedded in a sphere of positive charge like the currants in a plum pudding.

This was disproved by the Geiger-Marsden experiment, which involved measuring the deflection of alpha particles that were directed at a thin gold foil. The pudding model suggests that these particles would pass through the foil almost undeflected. However Geiger and Marsden found that, although most of them did this, some bounced back.

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This gave a new picture of the atom with a small positively charged nucleus containing almost all the mass, surrounded by empty space through which the alpha particle had passed.

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Occasionally an alpha particle had passed so close to the nucleus that it was repelled backwards. The electrons must be found surrounding the nucleus to balance out the positive charge.

The Proton

Rutherford went on to prove that protons are the positive particles found in the nucleus of all atoms, and that their positive charge is the same magnitude as the negative charge on the electron. The number of protons is the proton number, which is, of course a fundamental property of the element.

The Neutron

This was discovered in 1932 and was found to be in the nucleus having the same mass as a proton but no charge. Protons and neutrons collectively are called neucleons, and the number of protons plus neutrons is called the neucleon number or the mass number.

Mass Charge position

The neutron must add mass to the atom. Atoms of the same element always have the same number of protons but may have different numbers of neutrons, giving rise to slightly different atoms called isotopes.

Relative Atomic Mass

These are atoms of the same element which have the same number of protons but differing numbers of neutrons e. g. Cl. And Cl. Most elements have isotopes which means that the mass of the element will depend on the relative amounts of isotopes present. This gives rise to the term Relative Atomic Mass which is similar to the mass number but includes the differing masses of the isotopes. Definition; The Relative Atomic Mass of an element Ar is the average mass of one atom relative to1/12th the mass of one atom of carbon-12.

The fact that it is an average means that it takes into account the differing numbers of isotopes. The relative isotopic mass will be for one isotope only. MASS SPECTROMETRY. In order to find out the Relative atomic mass of an element we need to know the masses of the different isotopes and their relative abundancies. This is done using a Mass Spectrometer. It works in 4 stages. 1) The element sample is introduced as a vapour to the ionisation chamber. Here it is subjected to a beam of fast moving electrons that are created by a heated filament.