Changes through time

 We cannot see atoms, but we know they exist because of other things we observe  We use atomic models to explain what the structure must be, to explain how atoms behave  Making models is common in Science; these change as we get new ideas

 Models change because science ideas change We get new evidence that the old model doesn’t fit Social ideas change so new evidence is accepted We get new technology so we can test ideas more carefully  This means old models are incomplete (not wrong)

 Democritus was a Greek philosopher from about 400BC  Atoms can not be divided and are separated from each other by empty space  They are indestructable and always in motion  They come in an infinite variety of types

 His ideas were revolutionary  Sadly, he had no evidence to back up his ideas and did no experiments to prove them  He did back up his model with philosophical ideas about how matter works  Other philosophers made fun of him and put down his ideas  His model was mostly forgotten after his death

 He believed that everything was made of round, hard particles called atoms  Atoms of one element have different properties to those of other elements  Atoms cannot be subdivided, created, or destroyed.  Atoms of different elements combine in simple whole-number ratios to form chemical compounds  In chemical reactions, atoms are combined, separated, or rearranged

 Dalton based his ideas on experiments he did with chemical reactions  His model worked well for most simple chemical reations  Some of his ideas were based on faith that nature was as simple as possible  We still use Dalton’s model when we talk about collision theory and rates of reactions

 Dalton’s model couldn’t explain the behaviour of cathode rays  These occur when you put a really high voltage across a piece of metal, and tiny negative charges jump off the metal

 Negative charges jump off the piece of metal and complete the circuit  They must have come from inside the atom  An atom must be made up of two parts: these very small, very light negative parts, and a larger positive part

 Thomson’s model – the plum pudding model – incorporated these tiny negatively charged particles  The model has spherical, positively charged atoms with tiny negative charges (electrons) stuck in the positive charges  Imagine them being a little bit like sultana scones

 Thomson did not clearly define his model of what the positively charged part of the atom was like  It was sometimes likened to a soup or cloud, not solid and elastic like Dalton’s model  Thomson based his model on cathode ray experiments from the 1890s

 Scientists wanted to know more about the positively charged part of the atom  Rutherford supervised an experiment conducted by two others: the Geiger-Marsden experiment

 Alpha particles were fired at a thin sheet of gold, all in a vaccuum

 Some alpha particles were deflected almost straight back – these particles must have been repelled from a big, positively charged mass  Most alpha particles went straight through or were deflected only slightly – most of the atom must be empty space

 Rutherford took the results of the Geiger- Marsden experiment and came up with a model  The centre of the atom must be positively charged, relatively massive and very dense  The rest of the atom must be mostly empty space  The tiny electrons surround the nucleus

 Rutherford’s model was unclear about how the electrons were arranged  Some believed they were in a cloud, others believed they were in a ring structure  Over time, the rings became more popular, based on a theory by Nagaoka

 The Rutherford model was not stable – the electrons would all be attracted to the positively charged nucleus  Experiments were done with light, where electrons absorbed and gave out light in fixed frequencies  This meant that the electrons could not be in a cloud or in simple, unfixed rings

 This model was based on ideas about how electrons absorb and lose energy  An electron that has absorbed energy can be excited – if it absorbs too much, it will escape from the atom  When the excited electrons relax back, they release light with certain energies  Other energies don’t get released at all, meaning that the electron can’t have them

 Electrons can only be stable in some orbitals, or shells  Each shell is a certain, fixed distance from the nucleus  Each shell has a particular energy that depends on how far it is from the nucleus  Electrons gain and lose energy by jumping between shells

 The Bohr model only really worked for Hydrogen  The model was extended to give an approximation for other elements  We use these extensively in Chemistry  The Bohr model did not help understand how the nucleus works  The Bohr model was one of the first quantum models

 Which model you use depends on what purpose you have  Dalton’s model is still useful for rates of reaction and simple states of matter  Thomson’s model is still useful for electricity  Rutherford’s model is still useful for nuclear reactions where we aren’t interested in electrons  Bohr’s model is still useful for chemical reactions

 Ivanenko realised that the nucleus was made of protons and neutrons  Soon after, scientists realised they were held together by the Strong Force  Current models of the nucleus are far more complicated than this model

 Current models in use in Physics and Chemistry are very complicated  For this level, we use the Bohr model, with a nucleus made up of protons and neutrons held together by the strong force