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Atoms are the fundamental unit of chemistry, and were for nearly one hundred years between 1803 and 1897 considered the smallest indivisible particle of matter in the universe. In modern chemistry, atoms are important as they are the smallest unit that can be said to have chemical properties anything smaller is the realm of modern quantum physics. As such, atoms are the building blocks of chemistry, and these blocks come in different varieties called elements: including hydrogen, helium, oxygen, carbon, iron, uranium and 111 others discovered.

While the word atom means 'indivisible', atoms are comprised of a combination of smaller particles: neutrons, protons and electrons. There are many possible equilibria (balances of force) of these particles, and so two atoms could have different numbers of these subatomic particles. Protons and electrons are only found in equal numbers within atoms, such that their opposing charges cancel. A different number of protons in an atom constitutes a different element, whereas the same number of protons and a different number of neutrons in an atom constitutes a different isotope (isotopes are unimportant to most of chemistry as chemical behaviour does not depend on neutrons).

The question of how atoms "work" must start at the question of how atoms are composed of these three subatomic particles, and what forces exist to allow them to be stable.

The majority of what we think of as an atom is actually empty space, as discovered by Ernest Rutherford in 1909. In the centre of this empty space is a small, concentrated mass of matter called the nucleus (the "inner part") which is a ball of protons and neutrons. Protons are positively charged, and two positive charges repel each other it seems natural, then, that protons wouldn't want to be close to each other. However, while the electromagnetic force in the nucleus pushes the protons away from each other, the nuclear force pulls them back together.

The nuclear force is an unusual force, as it has a very limited range which explains why we can live perfectly ordinary lives without ever stumbling across it. It arises as a consequence of an understanding of the constituent parts of protons and neutrons and the realms of modern quantum physics. Its key natures are that it is insignificant at a range greater than the size of an atom, and attracts protons and neutrons (together called nucleons) together. This attractive force balances the electromagnetic repulsion, allowing the nucleus to

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