An atom (Greek ?t?µ?? from ?: non and t?µ??: divisible) is a submicroscopic structure found in all ordinary matter. It is the smallest unit of an element to retain all the chemical properties of that element. The word atom originally meant a smallest possible particle of matter, not further divisible. Later, the objects that had been called atoms were found to be further divisible into smaller subatomic particles, but the word atom nonetheless continues to refer to them.
Most atoms are composed of three types of massive subatomic particles which govern their external properties:
electrons, which have a negative charge and are the least massive of the three;
protons, which have a positive charge and are about 1836 times more massive than electrons; and
neutrons, which have no charge and are about 1838 times more massive than electrons.
Together, protons and neutrons form the nucleus of an atom, which is surrounded by the electrons.
Atoms can differ in the number of each of the subatomic particles they contain. Atoms of the same element have the same number of protons, but can differ in the number of neutrons, in which case they are called isotopes of that element. Atoms are electrostatically neutral if they have an equal number of protons and electrons. Electrons that are furthest from the nucleus are less tightly bound to the atom and are relatively easily transferred to other nearby atoms or even shared between atoms. Atoms which have either a deficit or a surplus of electrons are called ions. Particles in the nucleus also sometimes escape atoms, in a process known as radioactive decay.
Atoms are the fundamental building blocks of chemistry, and are conserved in chemical reactions. Atoms are able to bond into molecules and other types of chemical compounds. Molecules are made up of multiple atoms; for example, a molecule of water is a combination of two hydrogen and one oxygen atom.
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