An atom is the smallest part of an element that can exist on its own. Copper, for example, is made from copper atoms, which are different to the oxygen atoms that make up oxygen. Atoms are so tiny that even the full stop at the end of this sentence has a width of around 20 million atoms. Inside each atom are even smaller particles, called subatomic particles. These include a nucleus, which contains protons and neutrons, and electrons that whizz around the nucleus.
Imagine an atom magnified to the size of a football stadium. The nucleus of the atom would be the size of a pea in the centre of the stadium, and the electrons would be whizzing around the outer stands. Everything in between would be empty space.
The nucleus is a tightly bound cluster of protons and neutrons. This carbon atom nucleus has 6 protons and 6 neutrons. Protons have a positive electric charge and neutrons nave no charge. Positively charged protons would normally repel each other, but the nucleus is held together by a powerful force called the strong nuclear force.
Every element has a different atomic number, depending on the number of protons its atoms have in their nuclei. A carbon atom, for example, has 6 protons in its nucleus and so carbon has an atomic number of 6. If the number of protons in the nucleus changes, the atom becomes a completely different element with different properties (characteristics).
Atoms of different elements vary in mass. Their mass depends on the number of protons and neutrons in their nucleus. A hydrogen atom has one proton and no neutrons, so it has an atomic mass of one. The greater the atomic mass of an atom, the smaller the atom is.
An atom is usually electrically neutral, which means that it has exactly the same number of positively charged protons as it does negatively charged electrons. In this way, the charges cancel one another out. A carbon atom, for example, always has 6 protons and 6 electrons, and usually has 6 neutrons (although different carbon atoms may contain slightly different numbers of neutrons).
In 1913, Bohr published his model of atomic structure in which electrons travelled in orbits around the central nucleus. He also introduced the idea of electron shells, saying that the properties of an atom depended on how its electrons were arranged in the shells. In 1922, Bohr was awarded the Nobel Prize for Physics.