Which of the following has the greatest mass, in grams? 1 atom of gold (au) 0.5 mol lead (pb) atoms 2.0 mole of chlorine (cl) atoms 25 atoms of mercury (hg)

The atomic mass of each element from the periodic table can help you find those masses. The atomic mass is the molar mass of 6.02 x 10^23 atoms of that element, so... 

A. Atomic mass of Au is 196.97 g/mol. Dividing by the Avogadro constant (6.02 x 10^23), or 1 mol of atoms, would give you the weight in grams for one atom of gold: 3.27 x 10^-22 

B. The atomic mass of mercury is 200.59 g/mol, so one mol of mercury would weigh 200.59 g. 

C. Chlorine weighs 35.45 g/mol, so two mols would be twice that, or 70.90 g. 

D. We know that 6.02 x 10^23 atoms are more than 25 atoms of mercury, so D would already be far less than B, but let's do the calculations just in case. 1 mol of Hg weighs 200.59 g/mol, divide by Avogadro's number to get the weight per atom (3.33 x 10^-22 g), and multiply by 25 - 8.33 x 10^-21 g. 

By the results, we can see that 1 mol of Hg has the greatest mass of all the possible choices. Generally, the weight of one or a few single atoms is minuscule, so we could have eliminated A and D immediately, but it's good to understand and do the math,

They are all decomposition reactions.

Yes it would be a physical change.