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Atom Facts

• Atoms are tiny particles which build together to make every substance. An atom is the tiniest bit of any pure substance or chemical element.

• You could fit two billion atoms on the full stop after this sentence.

• The number of atoms in the Universe is about 10 followed by 80 zeros.

• Atoms are mostly empty space dotted with a few even tinier particles called subatomic particles.

• In the centre of each atom is a dense core, or nucleus, made from two kinds of particle: protons and neutrons. Protons have a positive electrical charge, and neutrons none. Both protons and neutrons are made from different combinations of quarks (see quarks).

• If an atom were the size of a sports arena, its nucleus would be just the size of a pea.

• Around the nucleus whizz even tinier, negatively-charged particles called electrons (see electrons).

• Atoms can be split but they are usually held together by three forces: the electrical attraction between positive protons and negative electrons, and the strong and weak 'nuclear' forces that hold the nucleus together.

• Every element is made from atoms with a certain number of protons in the nucleus. An iron atom has 26 protons, gold has 79. The number of protons is the atomic number.

• Atoms with the same number of protons but a different number of neutrons are called isotopes.

• The nucleus of an atom is made up of two kinds of particle: protons (red) and neutrons (green). Protons have a positive electric charge while neutrons have none. Tiny electrons (blue) whizz around the nucleus.

• In the 1890s scientists thought that atoms were solid like billiard balls and completely unbreakable.

• In 1897 J. J. Thomson discovered that atoms contained even smaller particles, which he called electrons (see electrons).

• In 1900 scientists thought atoms were like plum puddings with electrons like currants on the outside.

• In 1909 Ernest Rutherford was firing alpha particles (see radioactivity) at a sheet of gold foil. Most went straight through, but 1 in 8000 particles bounced back!

• Rutherford concluded that the atom was mostly empty space (which the alpha particles passed straight through) but had a tiny, dense nucleus at its centre.

• In 1919 Rutherford managed to split the nucleus of a nitrogen atom with alpha particles. Small atoms could be split.

• In 1932 James Chadwick found the nucleus contained two kinds of particle: protons and neutrons.

• In 1933 Italian Enrico Fermi bombarded the big atoms of uranium with neutrons. Fermi thought the new atoms that then formed had simply gained the neutrons.

• In 1939 German scientists Hahn and Strassman repeated Fermi's experiment and found smaller atoms of barium.

• Austrian Lise Meitner realized that Hahn and Strassman had split the uranium atoms. This discovery opened the way to releasing nuclear energy by fission (see nuclear energy).

• Atoms are the building blocks of the Universe, the invisibly small particles from which matter is made.

• Atoms are so small that you could fit a billion on the full stop at the end of this sentence.

• Atoms are the very smallest identifiable piece of a chemical element (see elements).

• There are as many different atoms as elements.

• Atoms are mostly empty space dotted with tiny sub-atomic particles (subatomic is 'smaller than an atom').

• The core of an atom is a nucleus made of a cluster of two kinds of subatomic particle – protons and neutrons.

• Whizzing around the nucleus are even tinier particles called electrons.

• Electrons have a negative electrical charge, and protons have a positive charge, so electrons are held to the nucleus by electrical attraction.

• Under certain conditions atoms can be split into over 200 kinds of short-lived subatomic particle. The particles of the nucleus are made from various even tinier particles called quarks.

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