Relative atomic mass
The average mass of an atom of an element compared with carbon-12 (which is 12). It is the larger number next to each symbol on the periodic table. It has no units.
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Analogy
It is a score showing how heavy an atom is compared with a standard atom.
Example
H = 1C = 12O = 16Cl = 35.5
In practice
Oxygen has a relative atomic mass of 16. Magnesium has 24, so a magnesium atom is one and a half times as heavy.
Practice
Flashcards
- What is the Mr of water, H2O?18 (2 x 1 + 16).
Questions people ask
Why does chlorine have a relative atomic mass of 35.5?
Chlorine is a mixture of two isotopes, chlorine-35 and chlorine-37, in a ratio of about 3 to 1. The relative atomic mass is the average, weighted by how common each is. The result lies between 35 and 37, nearer 35, so 35.5.
Which number on the periodic table is the relative atomic mass?
It is the larger of the two numbers next to the symbol, and it is often a decimal. The smaller whole number is the atomic number. For sodium, 23 is the relative atomic mass and 11 is the atomic number. Check the labels on your own table.