LOJ Feb 2004 You need to know: In a neutral atom, the number of electrons is equal to the number of protons in the nucleus. The atom as a whole has no electrical charge. The number of protons determines which element you are dealing with The number of neutrons determines which isotope of the element you are dealing with.
LOJ Feb 2004 Plum Pudding Model JJ Thomson’s model was sensible! They knew that electrons were ‘tiny specks’ that could be removed from atoms – so they were seen as tiny ‘plums’ in a large plum pudding. The whole atom was thought of as a positive sphere embedded with negative electrons just as a plum pudding was embedded with plums. This was the pre - 1911 understanding of what the atom was like!
Rutherford’s Experiment The ‘plum pudding’ atom indicated that the atom was made of smeared out positive charge with concentrated specks of negative charge embedded in it So if you fired alpha particles at it you expected them to go straight through Any slight change in trajectory would indicate how the charge gradient altered within the sphere Was it getting gradually more dense towards the centre? Or was it denser around the edge – like a shell? How was the charge distributed? In 1911 Rutherford hoped to find out!
LOJ Feb 2004 Rutherford’s Experiment His findings astounded him! The fact that the vast majority of the alpha particles got straight through led Rutherford to propose that the atom was composed primarily of empty space.
LOJ Feb 2004 Rutherford’s Experiment The fact that backscattering occurred in 1 in 8000 alpha particles indicated that the nucleus in the centre was: small (that was why so few were affected) massive (meaning containing lots of mass - he knew the electrons had very little mass and the fact that all of the positive charges were concentrated into a small area meant that the mass was concentrated there too) positively charged (because it repelled the alpha particles)
LOJ Feb 2004 Why did his new model become so widely accepted? Because it was backed up by experimental evidence! It wasn’t common sense – but the evidence showed it was true! Others were able to repeat the experiment and find this out for themselves So the old model was replaced by the new one
LOJ Feb 2004 The proton number Z (or atomic number) indicates the element to which the atom belongs All atoms of a particular element X have the same number of protons. Atoms of different elements have different numbers of protons. The total number of protons and neutrons (nucleons) in an atom is called its nucleon number A.