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.