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 nuclide 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, 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.
