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What's inside an element?

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.

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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 same atom.

Scientists thought there was nothing smaller than the proton in the nucleus of the atom. Then, in 1968, more sub-atomic particles were discovered. Particles, called quarks, were found inside the proton.

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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 electron.

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What's inside an atom?
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The history of the atom

DemocritusIn 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.

What is an isotope?

Isotopes are atoms of the same element containing the same number of protons and electrons but different numbers of neutrons. There are 300 naturally occurring isotopes.

Sodium, for example, has 2 isotopes.

sodium 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.

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