Calculating Relative Atomic Mass Flashcards

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What is the relative atomic mass of an element?

The mean mass of its atoms, taking account of the masses and abundances of all its isotopes.

Why are the relative atomic masses of some elements not whole numbers?

Because the element exists as isotopes of different masses, and the relative atomic mass is the weighted mean of them.

What are the steps to calculate relative atomic mass from isotopic masses and percentage abundances?
e.g. 20% of mass 10 and 80% of mass 11

1. Multiply each isotope mass by its % abundance: $10 \times 20 = 200$, $11 \times 80 = 880$
2. Add the results: $200 + 880 = 1080$
3. Divide by 100: relative atomic mass $= 10.8$

What are the steps to find an unknown isotope abundance from the relative atomic mass?
e.g. isotopes of mass 63 and 65, relative atomic mass 63.5

1. Let $x$ = % of the lighter isotope, so $(100 - x)$ = % of the heavier one
2. Write the equation: $\frac{63x + 65(100 - x)}{100} = 63.5$
3. Solve for $x$: 75% of mass 63 and 25% of mass 65

True or false? Chlorine's relative atomic mass is 35.5 because each chlorine atom contains half a neutron.

False. It is the weighted mean of chlorine-35 and chlorine-37 atoms; individual atoms have whole-number mass numbers.

True or false? The mass number of a single atom can be a decimal number.

False. Mass number is a count of protons plus neutrons, so it is always a whole number; only relative atomic mass can be a decimal.