C1 - Relative Atomic Mass

What are isotopes?

Atoms of the same element with different numbers of neutrons.

Why do isotopes of an element have different mass numbers?

Because they have different numbers of neutrons.

What is the relative atomic mass of an element?

The weighted average mass of the isotopes of the element based on their abundance.

Why is the relative atomic mass of chlorine 35.5 instead of a whole number?

Because it is a weighted average of its isotopes, mainly chlorine-35 and chlorine-37.

What does the term "abundance" mean in the context of isotopes?

How common each isotope is in a natural sample.

How do scientists calculate the relative atomic mass of an element?

By taking the sum of each isotope's mass number multiplied by its percentage abundance, divided by 100.

Write the formula to calculate relative atomic mass using isotopes.

Relative atomic mass = (mass of isotope 1 × % abundance of isotope 1 + mass of isotope 2 × % abundance of isotope 2 + ...) ÷ 100

If chlorine-35 has an abundance of 75% and chlorine-37 has an abundance of 25%, what is the relative atomic mass of chlorine?

((35 × 75) + (37 × 25)) ÷ 100 = 35.5

Boron has two isotopes: mass 10 (20% abundance) and mass 11 (80% abundance). What is the relative atomic mass?

((10 × 20) + (11 × 80)) ÷ 100 = 10.8

Magnesium has three isotopes: 24 (79%), 25 (10%), and 26 (11%). Calculate its relative atomic mass.

((24 × 79) + (25 × 10) + (26 × 11)) ÷ 100 = 24.3

Why might the relative atomic mass on the periodic table be a rounded number?

To simplify values for practical use in calculations.

If an element has isotopes with equal abundance and mass numbers 10 and 12, what is its relative atomic mass?

(10 + 12) ÷ 2 = 11

What does a relative atomic mass of 35.5 tell us about chlorine?

That it is mostly chlorine-35 with some chlorine-37.

How does the weighted average account for isotopes?

It gives more influence to isotopes that are more abundant in nature.

Why do we use relative atomic mass instead of just the mass number of the most common isotope?

Because natural samples contain a mix of isotopes, so relative atomic mass reflects the average.