Atoms have a small central nucleus, which is made up of protons and neutrons, around which there are electrons
Before this was established, the accepted model was the “plum pudding model” wherein the atom is composed of electrons surrounded by a soup of positive charge; the Geiger-Marsden experiment in 1909 directed a beam of alpha particles normally onto a sheet of very thin gold foil: under the plum pudding model, the particles should all have been scattered only slightly, but while this was mostly the case, a small percentage of particles were deflected back to the source, at angles much greater than 90°; This proves that the atoms has a small concentrated centre of positive charge and of relatively large mass, with the electrons situated in the mostly open space around it
The nucleus
The nucleus of an atom consists of protons and neutrons
The proton number Z is the number of protons in the nucleus
The nucleon number A is the number of nucleons, protons and neutrons, in the nucleus
The nuclide is the atomic species characterised by the specific constitution of its nucleus
It is represented by the nuclide notation AZX
Isotopes
Isotopes of an element are the set of nuclides with the same proton number but different neutron numbers
Practical applications
Americium-241 is used in smoke detectors by releasing alpha particles that ionise air particles in an ionisation chamber that is connected to an electric circuit, allowing a current to flow: when smoke enters the chamber, the ions attach to the smoke particles, causing the current to drop so an alarm sounds
Uranium-lead dating determines the age of an object by measuring the percentage of 2 Uranium isotopes that have decayed into 2 respective Lead isotopes
Radiocarbon dating determines the age of an object by measuring the percentage of the radioactive Carbon-14 isotope that has decayed