history of the atom

• radius of an atom


  around 0.1 nanometres or 1x10^-10
• radius of the nucleus


  approxiamtely 1x10^-14 metres
• radius of the nucleus is less than 1/10,000 the radius of the atom
• relative mass of a proton and a neutron is 1
• relative mass of an electron is very small
• 1800's John Dalton described atoms as solid spheres and different types of spheres made up different elements
• 1897 JJ thomson measurements of charge and mass showed atoms contained negatively charged particles - electron.
• plum pudding model showed the atom as a ball of positive charge with discreet electrons in it
• In 1909 Rutherford and his student fired positively charged alpha particles at a thin sheet of gold
• from the plum pudding model they were expecting the particles to pass straight through the sheet or be slightly deflected at most
• the positive charge of each atom was though to be spread out through the 'pudding' of the atom
• most of the particles did go straight through the gold sheet, some were deflected more than expected and a small number were deflected backwards proving the plum pudding model wrong
• the nuclear model of the atom describes the atom as a tiny positively charged nucleus at the centre where most of the mass is concentrated. A 'cloud' of negative electrons surrounds this nucleus - so most of the atom is empty space
• when alpha particles came near the concentrated, positive charge of the nucleus they were deflected, if directly at the nucleus they were deflected backwards
• scientists realised that electrons couldnt be in a 'cloud' around the nucleus of an atom because nothing would stop the electrons from rushing in towards positive nucleus as they would be attracted to it and cause the atom to collapse.
• 1931 Niels Bohr suggested electrons orbited the nucleus in fixed shells and each shell is a fixed distance from the nucleus.
• about 20 years after scientists had accepted that atoms have nuclei, James Chadwick carried out an experiment which provided evidence for neutral particles in the nucleus, now called neutrons.
• further experiments by rutherford and others showed that the nucleus can be divided into smaller particles which each have the same charge as a hydrogen nucleus. These particles were named protons.