P2 - Physics Radiation

P2 – Physics - Aims (HIGHER TIER ONLY) – to explain how the Rutherford and Marsden scattering experiment led to the ‘plum pudding’ model of the atom being replaced by the nuclear model

P2 – Physics - Structure of the atom A hundred years ago people thought that the atom looked like a “plum pudding” – a sphere of positive charge with negatively charged electrons spread through it… Ernest Rutherford, British scientist: I did an experiment (with my colleagues Geiger and Marsden) that proved this idea was wrong. I called it the “Scattering Experiment”

P2 – Physics - The Rutherford Scattering Experiment Alpha particles (positive charge, part of helium atom) Thin gold foil Most particles passed through, 1/8000 were deflected by more than 900 Conclusion – the atom is made up of a small, positively charged nucleus surrounded by electrons orbiting in a “cloud”.

P2 – Physics - The structure of the atom ELECTRON – negative, mass nearly nothing PROTON – positive, same mass as neutron (“1”) NEUTRON – neutral, same mass as proton (“1”)

P2 – Physics - The structure of the atom Particle Relative Mass Relative Charge Proton 1 +1 Neutron Electron -1 MASS NUMBER = number of protons + number of neutrons He 2 4 SYMBOL PROTON NUMBER = number of protons

P2 – Physics - Radiation In an atom the number of electrons is equal to the number of protons in the nucleus. The atom has no net electrical charge All atoms of a particular element have the same number of protons Atoms of different elements have different numbers of protons Atoms of the same element that have different numbers of neutrons are called isotopes

O 8 16 O 8 17 O 8 18 P2 – Physics - Isotopes An isotope is an atom with a different number of neutrons: Notice that the mass number is different. How many neutrons does each isotope have? O 8 16 O 8 17 O 8 18 Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more. A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating.

P2 – Physics - Background Radiation 13% are man-made Radon gas Food Cosmic rays Gamma rays Medical Nuclear power

P2 – Physics - Types of radiation 1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of a ______ atom) Unstable nucleus New nucleus Alpha particle 2) Beta () – an atom decays into a new atom by changing a neutron into a _______ and electron. The fast moving, high energy electron is called a _____ particle. Beta particle Unstable nucleus New nucleus 3) Gamma – after  or  decay surplus ______ is sometimes emitted. This is called gamma radiation and has a very high ______ with short wavelength. The atom is not changed. Words – frequency, proton, energy, neutrons, helium, beta Unstable nucleus New nucleus Gamma radiation