Answer : The atomic mass of the silicon is, 28.09 amu
Explanation :
Average atomic mass of an element is defined as the sum of masses of each isotope each multiplied by their natural fractional abundance.
Formula used to calculate average atomic mass follows:

As we are given that,
Mass of isotope, Silicon-28 = 27.98 amu
Percentage abundance of isotope, Silicon-28 = 92.23 %
Fractional abundance of isotope, Silicon-28 = 0.9223
Mass of isotope, Silicon-29 = 28.98 amu
Percentage abundance of isotope, Silicon-29 = 4.68 %
Fractional abundance of isotope, Silicon-29 = 0.0468
Mass of isotope, Silicon-30 = 29.97 amu
Percentage abundance of isotope, Silicon-30 = 3.09 %
Fractional abundance of isotope, Silicon-30 = 0.0309
Now put all the given values in above formula, we get:
![\text{Average atomic mass of element}=\sum[(27.98\times 0.9223)+(28.98\times 0.0468)+(29.97\times 0.0309)]](https://tex.z-dn.net/?f=%5Ctext%7BAverage%20atomic%20mass%20of%20element%7D%3D%5Csum%5B%2827.98%5Ctimes%200.9223%29%2B%2828.98%5Ctimes%200.0468%29%2B%2829.97%5Ctimes%200.0309%29%5D)

Therefore, the atomic mass of the silicon is, 28.09 amu