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An atom is the smallest component of an element, characterized by a sharing of the chemical properties of the element and a nucleus with neutrons, protons and electrons. (noun)

Hydrogen and helium are examples of elements that have an atom.

See atom in Webster's New World College Dictionary

noun

Obsolete any of the indivisible particles postulated by philosophers as the basic component of all matter
a tiny particle of anything; jot
Chem., Physics any of the smallest particles of an element that combine with similar particles of other elements to produce compounds: atoms combine to form molecules, and consist of a complex arrangement of electrons revolving about a positively charged nucleus containing (except for hydrogen) protons and neutrons and other particles

See atom in American Heritage Dictionary 4

(ătˈəm)

noun

a. A part or particle considered to be an irreducible constituent of a specified system.
b. The irreducible, indestructible material unit postulated by ancient atomism.
An extremely small part, quantity, or amount.
Physics & Chemistry
a. 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.
b. This unit regarded as a source of nuclear energy. See Table at subatomic particle.

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