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).
BIOGRAPHY: NIELS BOHR Danish, 1885-1962
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.
