To find average atomic mass you multiply the mass of each isotope by its percentage, and then add the values up.
35 * 0.90 + 37 * 0.08 + 38 * 0.02 = 35.22
Average atomic mass closest to 35.22 amu.
I'm not sure what it asks, but it answers
- how many protons are in the nucleus
- how many electrons are in the atom
- where it sits in the periodic table
- If the periodic table is divided in the modern way, it gives a good indicator of whether you are speaking about a metal, a non metal or neither.
