silicon, which makes up about 25% of earths crust by mass, is used widely in the modern electronics…

Explanation:

Step1: Recall atomic - mass formula

The atomic mass ($A$) of an element with isotopes is calculated using the formula $A=\sum_{i}m_{i}p_{i}$, where $m_{i}$ is the isotopic mass of the $i$-th isotope and $p_{i}$ is the relative abundance of the $i$-th isotope (expressed as a decimal).

Step2: Convert percentages to decimals

The relative abundances are: $p_{1}=0.9222$ for $^{28}Si$, $p_{2}=0.0469$ for $^{29}Si$, and $p_{3}=0.0309$ for $^{30}Si$. The isotopic masses are $m_{1} = 27.976927$ amu, $m_{2}=28.976495$ amu, and $m_{3}=29.973770$ amu.

Step3: Calculate the atomic mass

$A=m_{1}p_{1}+m_{2}p_{2}+m_{3}p_{3}$ $A=(27.976927\times0.9222)+(28.976495\times0.0469)+(29.973770\times0.0309)$ $A = 27.976927\times0.9222+28.976495\times0.0469 + 29.973770\times0.0309$ $A=25.809323+1.359098 + 0.926189$ $A = 28.09461$ amu $\approx28.0855$ amu

Answer:

28.0855 amu