Very good! Because Oxygen is heavier than Hydrogen, 1000 atoms of Oxygen is heavier than 1000 atoms of Hydrogen. So, Object B is heavier than Object A.
Let us look at the mass of an actual atom. The mass of a Carbon atom is 0.00000000000000000000000001994 kg. As you can see, it is very difficult to write such small numbers. So, atomic masses are measured in amu (atomic mass units).

1 amu is \(\frac{1}{12}\) th the mass of a Carbon atom. So, the number you see above is \(12 \, \mathrm{amu}\). The table alongside gives the atomic weights of different elements.
Which is heavier, two atoms of C, or one atom of Mg?
  • Two atoms of C
  • One atom of Mg
  • They are both equally heavy
  • Can't say