Which of the following elements has the highest atomic mass?

1. Helium: 2 protons and 2 neutrons

2. Nitrogen: 7 protons and 7 neutrons

3. Oxygen: 8 protons and 8 neutrons

4. hydrogen: 1 proton and no neutrons

Ununoctium is the heaviest element, but it is man-made. The heaviest naturally-occurring element is Uranium (atomic number 92, atomic weight 238.0289). Another way to look at heaviness is in terms of density, which is mass per unit volume

Oxygen has the highest atomic mass out of these four elements.