Chlorine has two isotopes, one with atomic mass 35 and one with atomic mass 37. Pick the correct combination of subatomic particles for the two isotopes.

The study of atomic structure includes many aspects such as the sub-atomic particles (protons, neutrons and electrons) and their arrangement within the atoms of different elements; atomic mass; atomic number; positive and negative charge; the nucleus; energy levels or electron shells - etc. The word atom comes from the ancient Greek word atomos meaning unsplittable or uncuttable. A philospher called Democrites carried out a thought experiment. He imagined taking a piece of rock and hitting it with a hammer. He knew the rock would break so he wondered what might happen if you took one of the... Show more

The study of atomic structure includes many aspects such as the sub-atomic particles (protons, neutrons and electrons) and their arrangement within the atoms of different elements; atomic mass; atomic number; positive and negative charge; the nucleus; energy levels or electron shells - etc.

The word atom comes from the ancient Greek word atomos meaning unsplittable or uncuttable. A philospher called Democrites carried out a thought experiment. He imagined taking a piece of rock and hitting it with a hammer. He knew the rock would break so he wondered what might happen if you took one of the broken pieces and hit that and so on. He arrived at the conclusion that you would eventually be left with a piece that was so small it could not be broken any more.

But that was as far as it went. The Greek philosophers did no experiments and believed everything to be made of earth, fire, air and water. These ideas were hard to get rid of and there are even people today who believe that the Greek ideas are correct! Gradually, science progressed and by the end of the 18th Century, scientists were coming up with ideas about the atom.

Early atomic models explained how many chemicals behaved, but there were times when it didn't work. It was these imperfections of the theory that lead to new work and new ideas. Eventually the realization came late in the 19th century - an atom could in fact be divided up into smaller particles - protons (with a positive charge), neutrons (with a neutral charge) and electrons (with a negative charge). We now see the structure of atoms as being a very complex arrangement of protons, neutrons and electrons - but mainly empty space.

One of the biggest hurdles to understanding chemistry is the idea of scale. With atomic structure, we are dealing with processes and objects that we cannot see directly which is why it was not until well into the 20th century that protons and neutrons were identified. Our knowledge of what lies inside an atom has been discovered by using indirect observations, for example, the scattering of alpha particles as they passed through a thin piece of gold foil demonstrated that the mass of an atom was concentrated into a very small area at the center - the nucleus.

The first of the sub-atomic particles that were identified was the electron. Experiments using 'cathode rays' in 1897 by J.J. Thompson showed that they were the same no matter what element they came from. He guessed that they were some kind of fundamental particle that was a structural part of all atoms. They had a negative charge and in 1904 he suggested that an atom consisted of these 'corpuscles' whizzing round in a 'sea' of positive charge. We now know them as electrons which do move round in atoms but they do so in well defined areas known as 'shells' or 'energy levels'. They remain part of the atom because of the attraction of the nucleus. We also now appreciate that each of the 'shells' contains a specific maximum number of electrons. You only need to know how to work out the arrangement of electrons in each of the first 20 elements and their ions for high school.

Towards the end of the 18th Century, the scientist John Dalton introduced his theory of atoms to the scientific community. He was trying to help science make sense of the world by bringing his ideas of the atom to the center of scientific theory. His ideas were not new but science was becoming more popular than before and so ideas like this were more likely to be noticed, discussed, modified and accepted more widely than in the past.

Dalton used small wooden balls and sticks to explain his theory. 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.

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