The ratio and mass of elements refer to how elements combine in fixed proportions by number of atoms (ratio) and by mass, based on their atomic masses.

1. Ratio of Elements:

This is the mole ratio of atoms in a compound.

Example: In water (H₂O), the ratio of hydrogen to oxygen atoms is 2:1.

2. Mass of Elements:

Based on the atomic mass of each element.

Example: In water (H₂O):

Hydrogen has an atomic mass ≈ 1 u.

Oxygen has an atomic mass ≈ 16 u.

So, the mass ratio of hydrogen to oxygen is 2:16 = 1:8.

Key Point:

Even if the atomic ratio is 2:1, the mass ratio can be different due to different atomic masses.

This is governed by the Law of Definite Proportions — elements in a compound always combine in fixed mass ratios.