1) Silicon dioxide formula: SiO2 ....... 2 is a subscript for the O atom
2) From the formula you have 1 molecula of SiO2 contains 1 atom of SiO2
3) Then, 0.100 mol of SiO2 contains 0.1 mol of Si
4) Multiply by Avogadro's number: 0.100mol * 6.022*10^23 atoms/mol= 6.02*10^22 atoms
Answer: 6.02*10^22 atoms