Atoms are incredibly tiny and have very little mass. Because of this, scientists use a special way to measure their mass called relative atomic mass (symbol: Ar).

The relative atomic mass of an element compares the mass of its atoms to the mass of a carbon-12 atom, which is used as a standard. The Ar values for different elements can be found on the periodic table. These values are just numbers and don't have any units.

When we talk about the mass of a compound (a substance made of more than one element), we use something called the relative formula mass, which has the symbol Mr.

To calculate the Mr of a compound, follow these steps:

1. Find the Formula: Look at the chemical formula of the compound to see how many atoms of each element are present.
2. Use Ar Values: Use the Ar values from the periodic table for each element in the compound.
3. Add Them Up: Multiply the Ar value by the number of atoms of that element in the formula, then add all these values together to get the Mr.

Relative Formula Mass of Ionic Compounds: Compounds like sodium chloride (NaCl) don't exist as individual molecules but as a lattice of ions. However, you can still calculate the Mr in the same way, using the numbers in the formula.

Key Points to Remember:

• Ar is the relative atomic mass of an element.
• Mr is the relative formula mass of a compound, and it's calculated by adding up the Ar values of all the atoms in the formula.
• Both Ar and Mr are just numbers with no units because they are relative.