+92 Well, I haven't seen this sort of thing before...Ah well. 
First, let's convert it into moles. We can do this by dividing the number of atoms by Avogadro's Number, which is 6.022*10^23. This number is basically how many atoms are in a mole. I'd like to note that I'm using a different calculator, so it might be different from web2.0calc.
1 atom Si / 6.022*10^23 mol^-1 = 1.66057788*10^-24 mol Si
Next, we can convert this straight into grams using silicon's molar mass, which is 28.09 (There might be more accurate masses on other sites, but this is the molar mass I have on my table). We multiply our previous answer by 28.09. Also, for maximum accuracy it's preferred that you don't round the answer until the end.
1.66057788*10^-24 mol Si * 28.09 g/mol = 4.66456327*10^-27 g Si
Now we have to round for significant figures. If we assume that there's only one figure in the information given, then our answer can only have one figure.
5*10^-27 g Si
However, if the problem used an exact number, then the significant figures don't matter.
+3454 I don't really know much on this topic, but this might help you out: https://answers.yahoo.com/question/index?qid=20100919160021AAmtrS8
+92 Well, I haven't seen this sort of thing before...Ah well.
First, let's convert it into moles. We can do this by dividing the number of atoms by Avogadro's Number, which is 6.022*10^23. This number is basically how many atoms are in a mole. I'd like to note that I'm using a different calculator, so it might be different from web2.0calc.
1 atom Si / 6.022*10^23 mol^-1 = 1.66057788*10^-24 mol Si
Next, we can convert this straight into grams using silicon's molar mass, which is 28.09 (There might be more accurate masses on other sites, but this is the molar mass I have on my table). We multiply our previous answer by 28.09. Also, for maximum accuracy it's preferred that you don't round the answer until the end.
1.66057788*10^-24 mol Si * 28.09 g/mol = 4.66456327*10^-27 g Si
Now we have to round for significant figures. If we assume that there's only one figure in the information given, then our answer can only have one figure.
5*10^-27 g Si
However, if the problem used an exact number, then the significant figures don't matter.
