Define:

(a) Atom

(b) Orbit

(c) Atomic number

(d) Atomic Mass number.

(a) Atom: Atom is the smallest complete unit of matter. Atom has 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.

(b) Orbit: The path of an electron around the nucleus of an atom is termed as the orbit of an atom.

(c) Atomic number: Atomic number refers to the number of protons and electrons present in the atom.

For a neutral atom, the atomic number is same as the number of electrons present in an atom.

Hence for a neutral atom, Number of protons = Number of electrons

For example, atomic number of hydrogen atom is 1. So, in this atom the number of electrons is 1 and the number of protons is also 1.

(d) Atomic mass number: The atomic mass of an element is the mass of total number of protons and neutrons present in the nucleus of an atom. It is determined with reference to another element, which is carbon. It is usually expressed in atomic mass units (amu).