The protons and neutrons in each atom are tightly packed in a positively charged nucleus. Negatively charged electrons move around the nucleus. The number of protons in a nucleus defines the type of atom and is the same asthe atomic number. The number of neutrons is found by subtracting the atomic number from the mass number. In an atom because there is no overall charge the number of electrons equals the number of protons.
In chemical reactions the nucleus remains unchanged.
Geiger and Marsden bombarded a thin gold foil with a beam of alpha particles.
Most of the particles passed through the foil without deflection and were detected by a flash of light when the alpha particle struck a zinc sulphide screen, surrounding the gold foil.
A few were deflected and some of these were deflected at angles greater than 900, suggesting they had been repelled by large positive charges within the foil - nuclei of atoms of gold.
Arrangement of electrons around the nucleus
From GCSE you should be familiar with the Bohr model of electrons arranged around a nucleus. The electrons are in certain energy levels and each energy level can hold only up to a maximum number of electrons.
This is summarised in the table below:
Energy level or 'shell'Max no of electrons 1st 2 2nd 8 3rd 18 4th 32 5th 50
A sodium atom containing 11 electrons has an electron arrangement of 2,8,1. Two electrons filling the first shell, eight electrons filling the second shell and one electron in the outer third shell.