How fractional abundance?

atomic weight are average atomic masses, the sum of the mass of each naturally occurring isotope of an element times its fractional abundance. OR All elements exist as isotopes in nature. This means...

Multiply number of x adn ave mass per x

It is the ratio of the total mass of a specified element in the earth's crust of ten expressed as a percentage. For example, the abundance of aluminum in the earth's crust is 8 percent

1:100

That means that the exponent is a fraction, such as 21/2 = square root of 2, 101/3 = cubic root of 10, etc.