P7 Radioactivity


Alpha is stopped by paper, beta is stopped by thin aluminium, and gamma is stopped by concrete.

Alpha radiation is positive. Geiger and Marsden carried out investigations wherein narrow alpha particle beams were directed at a thin metal foil: some rebounded from it due to the positive nucleus containing most of the atom's mass. The idea of neutrons and protons being in the nucleus was formed.

Atoms of a radioactive substance have an unstable nucleus becoming stable by emitting alpha, beta, or gamma radiation, which causes it to decay.


Geiger and Marsden investigated how a thin metal foil scatters an alpha particle beam: the apparatus was in a vacuum chamber and the dectector had a microscope focused on a small glass plate and was moved to different positions counting the numbers of spots of light seen in a certain time; most alpha particles went through the metal foil and the number of alpha particles deflected each minute decreased as the angle of deflection increased. Rutherford spoke of a positive nucleus smaller than the atom containing most of the atom's mass, a theory quickly accepted since it agreed with measurements, it explained decay, and it predicted the neutron.

The plum pudding model had electrons embedded in a cloud of positive charge.

Niels Bohr spoke of energy levels in which electrons orbited. Electrons could move to another shell by absorbing EM radiation to get further away from the nucleus or emitting EM radiation to get closer to the nucleus.

The hydrogen nucleus has the least amount of charge and a nucleus' charge is shared equally between a whole number of particles with the same amount of positive charge.

James Chadwick discovered the neutron.


In alpha or beta, proton number changes. 

In alpha, neutron number changes. An alpha particle is a helium nucleus and when emitted, the atomic number decreases by 2 and the mass number goes down by 4. 

Beta is created when there are too many neutrons compared to protons, so one neutron changes into a proton and a beta particle (electron). Atomic number rises by 1 and the mass is unchanged. The nucleus' charge rises.

In gamma, EM radiation is emitted from an atom's nucleus. Mass and charge of the nucleus are unchanged.

Neutrons can be emitted by radioactive substances as a result of alpha particles colliding with unstable nuclei, causing the nucleus to become unstabler.


Alpha is stopped by skin & paper, beta is stopped by thin aluminium & penetrates the body deeply, and gamma passes through the body easily and is stopped by thick lead.

Radioactive substances contaminate other materials and radiation can ionise molecules and DNA causing unusual chemical reactions, mutations, and cancer.

While handling radiation, keep as far away as possible from it, spend as short a time as possible handling it, and shield yourself from the radiation.

Alpha particles are used in smoke alarms. Air is ionised, conducting electricity. If there's smoke, it interacts with alpha particle ions and


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