GCSE physics

• Rutherford replaced the plum pudding model with the nuclear model
• JJ Thompson discovered particles called electrons
• Thompson also suggested atoms were spheres of positive charge with electrons embedded in them (plum pudding model)
• in 1909, Rutherford carried out the alpha scattering experiment where he fired beams of alpha particles at gold foil
• if the plum pudding model was correct, all the alpha particles would have passed straight through
• most alpha particles did go straight through, however a lot were deflected and some bounced back the way they came.
• because a few of the alpha particles were deflected back, scientists realised that most of the mass of the atom must be concentrated at the centre in a nucleus.
• since some alpha particles deflected and repelled, the nucleus must have been positively charged.
• since most alpha particles passed straight through, this meant that most of the atom was empty space.
• the alpha scattering experiment introduced the nuclear model of the atom
• nuclear model stated that the atom was a positively charged nucleus surrounded by negative electrons.
• Niels Bohr said that electrons orbiting the nucleus do so at certain distances called energy shells/levels.
• James Chadwick proved the existence of neutrons within the nucleus of an atom which explained the imbalance between atomic and mass numbers.
• protons have a +1 charge and mass of 1
• neutrons have a 0 charge but a mass of 1
• electrons have a -1 charge but a mass of 0 / (rlly small mass)
• nucleus is tiny and makes up the mass of an atom because it contains protons and neutrons so it has an overall positive charge.
• most of the atom is empty space

  A) electron

  B) proton

  C) neutron

  D) electron shells

  E) nucleus

  
• number of protons = number of electrons
• electrons can move within the shells , if they absorb electromagnetic radiation = they move to higher electron shell (away from nucleus) and if they release EM radiation = they move to lower shell (closer to nucleus)
• isotopes have the same proton number but different number of neutrons
• all elements have isotopes but there are only a few stable ones.
• Radioactive decay - when unstable isotopes tend to decay into other elements (to become stable) and therefore give out radiation.
• radioactive substances release ionising radiation from there nucleus (alpha, beta, gamma)

  They can also release neutrons when they decay to rebalance atomic and mass numbers
• ionising radiation is radiation that knocks electrons off atoms which creates positive ions
• the ionising power of a radiation source is how easily it can knock the electron off an atom.
• alpha radiation is when an alpha particle (α) is emitted from the nucleus.
• alpha particles dont penetrate far and are stopped quickly - can only travel a few cm in air and absorbed by a sheet of paper.
• alpha particles are strongly ionising (because of their size)
• beta particles ( β ) are high speed electrons released by the nucleus.
• beta particles are moderately ionising and penetrate moderately far into materials before colliding (with atoms)
• beta particles are absorbed by a sheet of aluminium
• for every beta particle emitted, a neutron has turned into a proton.
• Gamma rays are electromagnetic waves with a short wavelength.
• Gamma rays are waves of electromagnetic radiation released by the nucleus.
• Gamma rays are highly penetrating and travel a long distance in air
• Gamma rays are weakly ionising because they pass through atoms rather than colliding. (Eventually they do hit an atom and damage.)
• Gamma Rays can be absorbed by a thick sheet of lead or metres of concrete.
• nuclear equations show radioactive decay by using element symbols.

  written: atom before decay → atom after decay + radiation emitted
• the mass and atomic numbers on both sides of the nuclear equation must be the same.