Why are atomic mass of some elements are in fractions?

Why do atomic masses of most of the elements in atomic mass units involve fractions?

If the average atomic mass of an element having two isotopes is 71, the ratio of the natural abundance of the two isotopes of mass 70 and 74 will be

In each of the following question, an Assertion (A) is following by a corresponding Reason (R ). Use the following keys to choose the appropriate answer. Assertion : Average atomic mass of an element depends upon the atomic masses of different isotopes and their fractional abundances. Reason : Average atomic mass is obtained by multiplying the atomic mass of the heavier isotope with its fractional abundances.

Define atomic number of an element?

Calculate the atomic mass of an element which crystallises into a cubic structure with one atom on each corner of the cube. The volume of the unit cell is 24xx10^-24 cm^3 and density is 7.2 gcm^-3 .

Calculate the atomic mass of an element having cubic structure with one atom at each corner. The volume of the unit cell is 2.4xx10^(-23) cm^3 and density is 4.4 g cm^(-3 .

Emission of a beta -particle by an atom of an element results in the formation of

The atomic number of an element is derived from.