Atomic Structure 2

Dalton used small wooden balls and sticks to explain his theory. It took many years for the theory to become accepted - 'Atoms are round bits of wood invented by Mr Dalton' wrote a critic of John Dalton on reading his theory of 'atomism'. Dalton's theory had four parts; first that chemical elements are made of atoms, secondly and thirdly, that atoms of an element are identical to each other, but different from those of different elements. The new science here was that Dalton was able to work out the weight or mass of these atoms. The final part of his theory was that atoms combine in whole number ratios. Although it had taken some time for Dalton's theory to be accepted, it is now the basis for all chemical calculations. The models of atoms that you have seen, and possibly used in lessons, are not much different to those of Dalton's models.

One of the things that Dalton believed was that the atoms of an element were identical to each other. Something that seems obvious to you, but was revolutionary at the time. We now understand why this is the case and can include the number of protons in the definition of an element as follows - atoms that have the same number of protons are the same element. So it doesn't matter about how many electrons or neutrons there are, atoms of the same element always contain the same number of protons. Where two atoms exist that have the same number of protons but different numbers of neutrons, we say that they are isotopes. Isotopes are forms of the same element with a different atomic mass, one example being a radioactive form of an element.

Now try these question about atomic structure - you will find it helpful to have your copy of the Periodic Table handy.