An element is a pure chemical substance that contains
one kind of atom only. For example, pure carbon contains
only carbon atoms, normally joined together by covalent
bonds. Each atom of carbon shares electrons with other
atoms of carbon. Even tiny particles of soot will contain
millions of carbon atoms bonded together.
All elements are made of atoms, and an atom is made
up from 3 basic particles: Electrons
are particles with a negative charge. The electrons
are said to orbit the nucleus in energy levels (sometimes
called shells). This is the Bohr model - there are fixed
energy levels in an atom and each one has a specific
maximum number of electrons: the first is 2, the second
is 8, the third is 18, and the fourth is 36. The arrangement
of electons in an atom is called the electron
configuration, or electronic structure.
In a stable atom there are the same numbers of electrons
as protons - together they make atoms electrically neutral.
Electrons were the first sub-atomic particle to be identified.
The mass of an electron is thought to be 2,000 times
lighter than a hydrogen atom!
Protons have a positive charge. Every
atom contains one or more proton and the number of protons
- called the atomic number - is different for each element.
Protons were the second sub-atomic particle to be discovered.
Neutrons and protons form the tightly
packed nucleus of an atom. Neutrons are electrically
neutral; in other words they have no charge. The mass
of a neutron is equal to the mass of the proton in the
Scientists thought there was nothing smaller than the
proton in the nucleus of the atom. Then, in 1968, more
particles were discovered. Particles, called quarks,
were found inside the proton.
An ion is an atom that has either
gained or lost one or more of its electrons. Metal elements
tend to lose electrons forming positively charged ions.
Non-metal elements usually gain electrons to form negatively
charged ions. A crystal of ordinary table salt consists
of positively charged sodium ions attracted to negatively
charged chloride ions. Dropping a small piece of pure
sodium into a jar of pure chlorine gas will produce
a spectacular explosion as the two elements react to
form a compound. In this reaction each atom of chlorine
loses one electron and each atom of sodium gains an
500BC, Greek philosophers Democritus and Leucippus proposed
that everything was made up of tiny, indivisible particles
in constant motion. However, their theory was not accepted,
and it was ignored for centuries.
Modern discovery of atomic structure began in 1808
when an English school teacher and scientist called
John Dalton published his experimental findings: that
all the atoms of an element are exactly the same size
and weight, and are unlike the atoms of any other element.
In 1897 J.J. Thompson discovered the first component
part of the atom: the electron. Seven years later he
proposed a model - nicknamed Thompson's pudding - where
he imagined the atom to be a sphere of positive substance
mixed with negative powered electrons 'like raisins
in a cake'.
In 1911 Ernest Rutherford explained an atoms'
structure in terms of a positively charged nucleus with
negatively charged electrons orbiting around it. But
it wasn't until 1932 that the final particle of the
atomic structure was discovered, when British physicist
James Chadwick detected the neutron.
Isotopes are atoms of the same element
containing the same number of protons and electrons
but different numbers of neutrons. There are 300 naturally
Sodium, for example, has 2 isotopes.
The atoms of both isotopes contain 11 protons, but
sodium-23 has 12 neutrons whereas sodium-24 has 13.
Sodium-23 is quite stable (for example, it is one of
the elements in common salt - NaCl, sodium chloride).
Sodium-24, however, is not stable. When an isotope is
unstable, it loses particles in order to become more
stable. This is called radiation. Unstable
isotopes emit different types of radiation. The 3 main
types are alpha, beta and gamma radiation.
Radiation can be dangerous to living things because when radioactive
particles come into contact with other molecules, they
can become charged (ionised). If molecules in your body
become ionised, they can cause the cell that they are
in to mutate, which can lead to cancer. Our bodies are
pretty good at dealing with radiation though. They have
to be really, as we are constantly in contact with low
levels of radiation from space, from rocks, from x-rays
and even from each other! We are also learning to use
radiation for our benefit. Because radioactive particles
can damage cells, we can use them in a closely controlled
way to kill cancer cells. Sodium-24 emits gamma radiation
and is used in medicine as a radioactive tracer to examine
organs and the blood system.