Relative masses of atoms and molecules

Relative masses

• The relative atomic mass ($A_r$) of an element is the average mass of its atoms, compared to $\tfrac{1}{12}$ of a $^{12}\text{C}$ atom.
• The relative molecular mass ($M_r$) is the sum of the $A_r$ of every atom in the molecule. (For ionic compounds we call it the relative formula mass.)

Reacting masses by proportion

• You can sometimes find a mass by simple proportion.
• If 24 g of magnesium makes 40 g of magnesium oxide, then 12 g (half) makes 20 g.

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