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Question

Why do all elements have fractional atomic mass?


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Solution

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)
IsotopeCl1735Cl1737
Mass number35 u37 u
Number of neutrons35-17= 1837- 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 Cl1735× Atomic weight of Cl1735+ Fraction of Cl1737× Atomic weight of Cl1737
  • Fractional mass of chlorine is 35.5 u.

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