Early ideas

The Greeks philosopher Democritus suggested over 2500 years ago that matter was made up of solid tiny balls called atoms. Democritus thought of atoms as unbreakable spheres, in fact the word atom comes from the Greek word atomos which translates as - "can't be split". However we know today that it is possible to split the atom into smaller pieces and that atoms are not solid object but are mostly made up of empty space. It was only in the 20th century following the work of scientists such as J.J.Thomson, Ernest Rutherford and Neils Bohr, that scientists finally realised that atoms were made up of smaller particles and Democritus's idea of atoms as solid spheres was finally rejected.
J.J Thomson was responsible for discovered the electron in 1897. This discovery led the way for new ideas on the internal structure of atoms and allowed the brilliant scientist Ernest Rutherford to discover both the proton and the nucleus. Further work by Neils Bohr suggested that the electrons orbit the nucleus in shells or rings. The work of these scientists and others led to a new model of the atom, the nuclear atom.

The nuclear model of the atom

Atoms are very small! We might describe something small as microscopic, but microscopic objects have sizes of about 1mm (0.001m), atoms are much much smaller than this. Atoms may have a radius of around 1 x 10^-10m (0.0000000001m) or 0.1nm ( 1 nanometre= 1 x 10^-9m), about 1 million atoms stacked end to end would be about as thick as a human hair! The modern model we use to describe the internal structure of an atom is often called the Bohr atom, after the Danish physicist Neils Bohr. Bohr suggested that the electrons in an atom orbit the nucleus in a series of shells or levels. The nucleus, which was discovered by Ernest Rutherford, contains the protons and neutrons.

Charges and masses of protons, neutrons and electrons

	particle	relative mass	actual mass in Kg	charge	Where in the atom it is found
	proton	1	1.67 x 10-27	+1	nucleus
	neutron	1	1.67 x 10-27	0	nucleus
	electron	0	9.11 x 10-31	-1	electron shells or rings

Key Points

• Protons and electrons have equal but opposite charges.
• The mass of the atom is due to the protons and neutrons only. The mass of an electrons is very small compared to that of a proton or neutron.
• All the mass of an atom is concentrated in the nucleus. The nucleus is tiny when compared to the size of the atom. The nucleus is about 1_{/10 0000} th the size of the atom.
• Atoms are mostly empty space and not solid balls as imagined by Democritus.

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