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P6 Radioactivity (Atoms in General (Why don’t atoms have an overall…
P6 Radioactivity
Atoms in General
Protons, neutrons and electrons. At the centre of every atom is a nucleus containing protons and neutrons. All atoms of the same element have the same number of protons. This number is used to arrange the elements in the periodic table, beginning with hydrogen, which has just one proton.
Protons and neutrons have the same mass, which is about 2,000 times larger than the mass of an electron.
Protons and electrons have an electrical charge. This electrical charge is the same size for both, but protons are positive and electrons are negative.
Neutrons have no electrical charge; they are neutral.
These properties are summarised in the table:
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The "Plum Pudding" Atom
1897 J.J. Thompson discovered that electrons could be removed from atoms, so atoms must be made up of smaller bits.
He suggested the "plum pudding" model- that atoms were spheres of positive charge with tiny negative electrons stuck in them.
In 1909 , Rutherford and Marsden tried firing a beam of alpha particles at a thin gold foil. They expected it to go straight through however some went straight through and some reflected of the sheet. Rutherford realised most of the matter was concentrated at the centre of the nucleus.
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Niels Bohr tweaked Rutherford's idea a few years later by proposing a model where the electrons were in fixed orbits and in set distances from the nucleus
He suggested that electrons can only exist in these fixed orbits(shells), and not anywhere in between.
This model is also know as the Bohr model and is pretty close to our currently accepted model of the atom.
Isotopes
Each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element.
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