(a) Atomic mass unit (amu) is 1/12 of the mass of one atom of C-12. The relative atomic mass of the atom of an element is the average mass of the atom as compared to 1/12th of the mass of the C-12 atom.

(b) Atoms of most of the elements do not exist independently. However, they combine in specific numbers to form molecules or ions which we can feel and know about their presence.

For example,

A molecule of H₂SO₄ consists of 2 atoms of H + 1 atom of S + 4 atoms of O

A molecule of C₁₂H₂₂O₁₁ consists of 12 atoms of C + 22 atoms of H + 11 atoms of O