Atomic Number and Atomic Mass

Atomic Number

Ø Atomic number of an element is the number of protons present inside the nucleus of that element.

Ø It is denoted by Z.

Ø Atomic number cannot have a fractional value.

 

Mass Number

Ø Sum of the number of protons and neutrons inside the nucleus of an atom is called its mass number.

Ø It is denoted by A.

 

Atomic Mass

Ø The average mass of an atom of an element in atomic mass units is called its atomic mass.

Ø Atomic mass is denoted by A.

Ø The atomic mass of hydrogen is 1.008 atomic mass unit (or 1.008 u).

Commonly, the term ‘relative atomic mass (Ar)’ is also used.

 

Relative Atomic Mass

Ø The relative atomic mass (A_r) of an element is defined as the average relative mass of its atom as compared with that of an atom of ¹²C (carbon-12) taken as 12 atomic mass units.

Ø Relative atomic mass has no units.

Ø It is a pure number.

Number of electrons, protons and neutrons in an atom of any element are related to its atomic number and mass number as:

Number of electrons (e) = Number of protons (P) = Atomic number (Z)

 

The atomic masses, number of electrons, protons and neutrons in the atoms of certain common elements is given below.

Mass number (A) = Number of protons (P) + Number of neutrons (N)

Number of Electrons, Protons and Neutrons in an Atom