The atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged
electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in
the case of Hydrogen-1, which is the only stable isotope with no neutron). The electrons of an atom are bound to the
nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other, forming a molecule.
An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive or
negative charge and is an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the
number of protons determines the chemical element, and the number of neutrons determine the isotope of the element.
The name atom comes from the Greek ἄτομος/átomos, α-τεμνω, which means uncuttable, something that cannot be
divided further. The concept of an atom as an indivisible component of matter was first proposed by early Indian and
Greek philosophers. In the 17th and 18th centuries, chemists provided a physical basis for this idea by showing that
certain substances could not be further broken down by chemical methods. During the late 19th and early 20th centuries,
physicists discovered subatomic components and structure inside the atom, thereby demonstrating that the 'atom' was
not indivisible. The principles of quantum mechanics were used to successfully model the atom.
Relative to everyday experience, atoms are minuscule objects with proportionately tiny masses. Atoms can only be
observed individually using special instruments such as the scanning tunneling microscope. Over 99.9% of an atom's
mass is concentrated in the nucleus,[note 1] with protons and neutrons having roughly equal mass. Each element has
at least one isotope with unstable nuclei that can undergo radioactive decay. This can result in a transmutation that
changes the number of protons or neutrons in a nucleus. Electrons that are bound to atoms possess a set of stable
energy levels, or orbitals, and can undergo transitions between them by absorbing or emitting photons that match the
energy differences between the levels. The electrons determine the chemical properties of an element, and strongly
influence an atom's magnetic properties.
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