Silicon has three naturally occurring isotopes. Based on the AVERAGE atomic mass, which is 28.085, what is the likely mass of the most common isotope 26
27
28
29
30

The most common isotope : Si-28, ²⁸Si

The elements in nature have several types of isotopes  

Isotopes are elements that have the same Atomic Number (Proton)  

Atomic mass is the average atomic mass of all its isotopes  

In determining the mass of an atom, as a standard is the mass of 1 carbon-12 atom whose mass is 12 amu  

An atomic mass unit = amu is a relative atomic mass of 1/12 the mass of an atom of carbon-12.  

The 'amu' unit has now been replaced with a unit of 'u' only  

Mass atom X = mass isotope 1 . % + mass isotope 2.% ..

The most common isotope is an isotope with a very large percentage which is indicated by the mass number closest to the average atomic mass

So from the above isotopes that are close to their average mass number is the Si-28

idk i’m struggling too