What is Atom ?

An atom is the smallest unit of an element that retains the chemical properties of that element. Atoms are extremely small — one grain of sand contains about 43 million million atoms. Atoms combine to form molecules and compounds.

Structure of an Atom:

An atom consists of three subatomic particles:

Proton (+):
Found in the nucleus. Has a positive electric charge. The number of protons in an atom is called the atomic number and determines what element the atom is. Mass = 1 atomic mass unit (amu).

Neutron (neutral):
Also found in the nucleus alongside protons. Has no electric charge. Neutrons add mass to the nucleus and help hold it together. Mass = 1 amu.

Electron (-):
Found in the space around the nucleus, in regions called electron shells or orbitals. Has a negative electric charge and a very tiny mass (nearly negligible). The number of electrons equals the number of protons in a neutral atom.

Nucleus:
The dense central core of the atom that contains protons and neutrons. Almost all of an atom's mass is concentrated in the nucleus.

Key Terms:

Atomic Number: Number of protons in the nucleus (identifies the element)
Mass Number: Protons + Neutrons
Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons