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Physics.

the smallest component of an element having the chemical properties of the element, consisting of a nucleus containing combinations of neutrons and protons and one or more electrons bound to the nucleus by electrical attraction; the number of protons determines the identity of the element.
an atom with one of the electrons replaced by some other particle:
muonic atom; kaonic atom.

Energy. this component as the source of nuclear energy.
a hypothetical particle of matter so minute as to admit of no division.
anything extremely small; a minute quantity.
Contemporary Examples

Check: “This atom smashing business is going to herald the final victory of the machine.”
Mailer’s Letters Pack a Punch and a Surprising Degree of Sweetness Ronald K. Fried December 13, 2014

Of course, an atom bomb is about to drop on American Idol: Simon Cowell will leave at the end of this season.
The Next Simon Cowell Is… Richard Rushfield May 11, 2010

If we could split the atom and build the bomb, we could certainly mitigate the consequences at home.
At the Hanford Nuclear Reservation, a Steady Drip of Toxic Trouble Eric Nusbaum February 23, 2013

Each type of atom and molecule has its own unique spectrum, according to the rules of quantum mechanics.
SAMI Is Like Google Earth for the Universe Matthew R. Francis July 26, 2014

Uranium is enriched to make fuel for power reactors, but the same process also is used to make the explosive core of atom bombs.
Behind the Smiles in Geneva, No Concrete Progress in Iran Negotiations Michael Adler October 16, 2013

Historical Examples

The atom when it is acting under the direction of a fixed law is a giant in strength.
Nature’s Miracles, Volume 1 Elisha Gray

Fictions or realities, could they survive the touchstone of this atom of common sense?
The Uncommercial Traveller Charles Dickens

No writer concerning Hook can claim for him an atom of respect.
The Atlantic Monthly, Volume 15, No. 90, April, 1865 Various

Even the good things that the atom had brought were destroyed in the panic that ensued.
Now We Are Three Joe L. Hensley

It is these jets which occasionally sweep away an atom from the surface of the sphere.
Occult Chemistry Annie Besant and Charles W. Leadbeater

noun

the smallest quantity of an element that can take part in a chemical reaction
this entity as a source of nuclear energy: the power of the atom See also atomic structure

any entity regarded as the indivisible building block of a theory
the hypothetical indivisible particle of matter postulated by certain ancient philosophers as the fundamental constituent of matter See also atomism
a very small amount or quantity; minute fragment: to smash something to atoms, there is not an atom of truth in his allegations
n.

late 15c., as a hypothetical indivisible body, the building block of the universe, from Latin atomus (especially in Lucretius) “indivisible particle,” from Greek atomos “uncut, unhewn; indivisible,” from a- “not” + tomos “a cutting,” from temnein “to cut” (see tome). An ancient term of philosophical speculation (in Leucippus, Democritus), revived 1805 by British chemist John Dalton. In late classical and medieval use also a unit of time, 22,560 to the hour. Atom bomb is from 1945 as both a noun and a verb; cf. atomic.

atom at·om (āt’əm)
n.

A unit of matter, the smallest unit of an element, having all the characteristics of that element and consisting of a dense, central, positively charged nucleus surrounded by a system of electrons. The entire structure has an approximate diameter of 10-8 centimeter and characteristically remains undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons.

This unit regarded as a source of nuclear energy.

A part or particle considered to be an irreducible constituent of a specified system.

The irreducible, indestructible material unit postulated by ancient atomism.

An extremely small part, quantity, or amount.

a·tom’ic (ə-tŏm’ĭk) adj.
atom
(āt’əm)
The smallest unit of an element, consisting of at least one proton and (for all elements except hydrogen) one or more neutrons in a dense central nucleus, surrounded by one or more shells of electrons. In electrically neutral atoms, the number of protons equals the number of electrons. Atoms remain intact in chemical reactions except for the removal, transfer, or exchange of certain electrons. Compare compound. See also ion, isotope, orbital.

A unit of matter; the smallest unit of a chemical element. Each atom consists of a nucleus, which has a positive charge, and a set of electrons that move around the nucleus. (See Bohr atom.)

Note: Atoms link together to form molecules.