Particles Constituents of the atom Rutherford’s experimentThursday, 10 November 2011
Thompson’s modelThompson believed that the atom was made by:• A distribution of positive charge spread over a relatively large area• Small negatively charged particles (Electrons) immersed in the positive distribution• These electrons are like raisin in a Christmas cake (plum pudding) and are distributed in a way that the overall charge is neutral
Rutherford’s ExperimentRutherford shot a beam of -particles to a thin target of gold. He was expecting the -particles to be slightly deflected by the positive charge of the atoms of gold. Can you explain why he expected such an outcome?The problem was that the positive charge of the atoms of gold was too spread out (in Thompson’s model) to produce a strong deflection, or even the scattering backwards of -particles that was observed. Possible trajectory Impossible trajectory
Rutherford’s Model of the AtomWhat did Rutherford observed?1) Most α-particles passed straight through the foil undeflected2) Some α-particles were slightly deflected (small angles)3) Few α-particles were deflected at very large angles or even scattered backwards
Rutherford’s Model of the AtomWhat were the implications of Rutherford’s observations?1) The atom is mainly made of empty space2) The positive charge of an atom is concentrated inside a very small nucleus in the centre of the atom.3) The atom has a nucleus in its centre in which the positive charge and most of the mass is concentrated
The Structure of the AtomRutherford’s experiment revealed that the atom was made of a positive massive nucleus surrounded by electrons orbiting around it. However, they didn’t know what was in the nucleus. More experiments showed the whole structure of the atom. Neutron: neutral particle inside the nucleus of mass slightly bigger than a proton. Different no of There is empty space between neutrons for the same element make different nucleus and electrons. In scale the ISOTOPES of that element nucleus is a pea at the centre of a football field. Nucleon no: given by the sum of no of + protons and no of neutrons. Using this no + + you can identify the different isotopes of the same element. Electron: atomic particle orbiting around the nucleus with same and opposite charge as the proton and mass 1/1800 of a proton Proton: a +ve particle in 7 Li the nucleus with mass similar to neutron Proton no: it tells the no of protons (and 3 also electrons) in the atom. This no is a property of the element, so the atomic no identifies what element the atom is.
Sub-atomic particles properties Mass Relative Charge RelativeParticle (kg) Mass (C) ChargeProton 1.67x10-27 1 +1.6x10-19 +1Neutron 1.68x10-27 About 1 0 0Electron 9.11x10-31 1/2000 -1.6x10-19 -1
IsotopesThanks to the mass spectrometer Physicists were able to measure the mass of the atoms of elements. In this way they made the remarkable discovery that the atoms of a particular element do not always have the same mass. What they all have in common is their positive charge!
IsotopesIn other words, isotopes are atoms with the same atomic number, but different mass numbers. This lead to the search for an additional subatomic particle that would be responsible for the extra mass, but have no charge. This additional particle is the NEUTRON.Match the terms with the correct definitions. The number of protons in the nucleus. Nuclide Tells what element the nuclide is. Mass Number An atom of a particular nuclear structure.Atomic Number A nuclear particle Nucleon Total number of nucleons in the nucleus
Isotopes Circle the isotopes of the same element with the same colour. That’s right Mass number A =N+ZAtomic numberZ = no of protons Chemical symbol for the element
IsotopesSome elements have a large number of isotopes. Some isotopes are unstable, which means that they will decay spontaneously into more stable nuclei by the emission of particles and energy.However, the percentage of isotopes of a particular element mined on the Earth is remarkably constant no matter what part of the World it has been extracted from.