Why do all elements have fractional atomic mass?

Fractional atomic mass:

• When the value of atomic mass exists as a fraction instead of a whole number, it is called as fractional atomic mass.

Reason:

• Majority of elements have fractional atomic mass as most of the elements exist as isotopes i.e.; atoms of the same element which have more than one value of atomic mass. Example: Chlorine (Cl). Some atoms of Chlorine have an atomic mass as 35 u and others have atomic mass of 37 u.
• This is due to the different number of neutrons present in these atoms as shown in the table below.

Mass number = Number of protons+ Number of neutrons

• (The mass number is indicated as superscript)

Number of neutrons= Mass number- Number of protons

• Number of protons= Atomic number (Indicated as subscript)

Isotope	${}_{17}{}^{35}\mathrm{Cl}$	${}_{17}{}^{37}\mathrm{Cl}$
Mass number	35 u	37 u
Number of neutrons	35-17= 18	37- 17= 20

Fractional mass of Chlorine (Cl):

• When the value of atomic mass exists as a fraction instead of a whole number, it is called as fractional atomic mass.
• Fractional atomic mass occurs for those elements where atoms of the same elements have more than one value of atomic weight. Example: Chlorine. Some atoms of Chlorine have atomic mass as 35 u and others have atomic mass as 37 u.
• Fractional atomic mass = Fraction of ${}_{17}{}^{35}\mathrm{Cl}$× Atomic weight of ${}_{17}{}^{35}\mathrm{Cl}$+ Fraction of ${}_{17}{}^{37}\mathrm{Cl}$× Atomic weight of ${}_{17}{}^{37}\mathrm{Cl}$
• Fractional mass of chlorine is 35.5 u.