ATOMIC STRUCTURE
STRUCTURE OF THE ATOM
Typical radius of an atom: 1x10^-10m
o The radius of the nucleus is 10,000 times smaller
o Most of the mass of the atom is concentrated at the nucleus
Isotopes: atoms of the same element, with different masses, that have the same number of
protons but a different number of neutrons
ATOMIC MODEL
1800 – Dalton said that everything was made of tiny spheres that could not be divided
1897 – JJ Thompson discovered the electron
o Plum Pudding Model: overall charge of an atom is neutral, so the negative electrons
are dispersed through the positive ‘pudding’ to cancel out the charges
1911 – Rutherford realised most of the atom was empty space
o Gold Foil Experiment:
most alpha particles went straight through the foil, meaning most of the
atom is empty space
Some alpha particles were slightly deflected, meaning the nucleus must be
charged
Few alpha particles deflected significantly, meaning the nucleus contained
most of the mass
1913 – Rutherford Model: a positive nucleus at the centre and negative electrons existing in
a cloud around the nucleus
1913 – Bohr produced the final model of the atom
o The electrons existed in fixed ‘orbitals’ to not cause the atom to collapse
RADIATION
Activity is the rate at which a source of unstable nuclei decays, measured in Becquerels (Bq)
Radioactive decay is a random process
Countr-rate: the number of decays recorded by a detector per second (e.g. a Geiger-Muller
Tube)
ALHPA
o Charge of +2
o Range of a few centimetres
o Heavy
o Stopped by paper/skin
o Highly ionising
STRUCTURE OF THE ATOM
Typical radius of an atom: 1x10^-10m
o The radius of the nucleus is 10,000 times smaller
o Most of the mass of the atom is concentrated at the nucleus
Isotopes: atoms of the same element, with different masses, that have the same number of
protons but a different number of neutrons
ATOMIC MODEL
1800 – Dalton said that everything was made of tiny spheres that could not be divided
1897 – JJ Thompson discovered the electron
o Plum Pudding Model: overall charge of an atom is neutral, so the negative electrons
are dispersed through the positive ‘pudding’ to cancel out the charges
1911 – Rutherford realised most of the atom was empty space
o Gold Foil Experiment:
most alpha particles went straight through the foil, meaning most of the
atom is empty space
Some alpha particles were slightly deflected, meaning the nucleus must be
charged
Few alpha particles deflected significantly, meaning the nucleus contained
most of the mass
1913 – Rutherford Model: a positive nucleus at the centre and negative electrons existing in
a cloud around the nucleus
1913 – Bohr produced the final model of the atom
o The electrons existed in fixed ‘orbitals’ to not cause the atom to collapse
RADIATION
Activity is the rate at which a source of unstable nuclei decays, measured in Becquerels (Bq)
Radioactive decay is a random process
Countr-rate: the number of decays recorded by a detector per second (e.g. a Geiger-Muller
Tube)
ALHPA
o Charge of +2
o Range of a few centimetres
o Heavy
o Stopped by paper/skin
o Highly ionising
