An atom has a nucleus, made of protons and neutrons, which is surrounded by electrons.
The modern model of the atom developed over time as scientists rejected earlier models and proposed new ones to fit the currently available evidence.
Each stage relied on scientists using reasoning to propose models which fitted the evidence available at the time.
Models were rejected, modified and extended as new evidence became available.
After the discovery of the electron in the 19th century by Thomson scientists imagined that atoms were small particles of positive matter with the negative electrons spread through, like currants in a cake.
This was the model used until 1910 when the results of the Rutherford-Geiger-Marsden alpha particle scattering experiment provided evidence that a gold atom contains a small, massive, positive region (the nucleus).
Atoms are small – about 10–10 m across, and the nucleus is at the centre, about a hundred-thousandth of the diameter of the atom.
Each atom has a nucleus at its centre and that nucleus is made of protons and neutrons.
For an element, the number of the protons is always the same but the number of neutrons may differ. Forms of the same element with different numbers of neutrons are called the isotopes of the element.
Interpreting the unexpected results of the Rutherford-Geiger-Marsden experiment required imagination to consider a new model of the atom.
Some substances emit ionising radiation all the time and are called radioactive.
The ionising radiation (alpha, beta, gamma, and neutron) is emitted from the unstable nucleus of the radioactive atoms, which as a result become more stable.
Alpha particles consist of two protons and two neutrons, and beta particles are identical to electrons.
Gamma radiation is very high frequency electromagnetic radiation.
Radioactive decay is a random process.
For each radioactive isotope there is a different constant chance that any nucleus will decay.
Over time the activity of radioactive sources decreases, as the number of undecayed nuclei decreases.
The time taken for the activity to fall to half is called the half-life of the isotope and can be used to calculate the time it takes for a radioactive material to become relatively safe. |
1. Describe the atom as a positively charged nucleus surrounded by negatively charged electrons, with the nuclear radius much smaller than that of the atom and with almost all of the mass in the nucleus
You should know
 How has our understanding of the structure of atoms developed over time.
How the development of the nuclear model of the atom is an example of how scientific explanations become accepted
In your practical work you will have:
Collected data to calculate the half-life of a radioactive isotope.
Used a random event such as dice-throwing to model radioactive decay. |
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