Ever wondered why the atomic mass on the periodic table...
Chemistry816 views·Updated 29 Jun 2026·1 pageUnderstanding Isotopes and Calculating Relative Atomic Mass
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zuzanna antoniak@zuzannaantoniak_oasl
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Isotopes and Relative Atomic Mass
Isotopes are basically different versions of the same element that have identical numbers of protons but different numbers of neutrons. Think of them as siblings in the same family - they're related but not identical.
Since different isotopes have different masses, chemists calculate the relative atomic mass (Ar) to find the average mass of all isotopes that make up an element. This explains why atomic masses on the periodic table are decimal numbers rather than whole numbers.
The formula is straightforward: Sum of (isotope abundance × isotope mass) ÷ Sum of abundance of all isotopes. Abundance tells you how common each isotope is in nature.
Let's look at copper as an example. Copper-63 has an abundance of 69.2% and copper-65 has 30.8%. Using the formula: (69.2 × 63) + (30.8 × 65) ÷ (69.2 + 30.8) = 63.6. That's why copper's relative atomic mass is 63.6, not a whole number.
Quick Tip: Remember that mass number = total protons + neutrons, while atomic number = number of protons only.
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Chemistry816 views·Updated 29 Jun 2026·1 pageUnderstanding Isotopes and Calculating Relative Atomic Mass
Z
zuzanna antoniak@zuzannaantoniak_oasl
Ever wondered why the atomic mass on the periodic table isn't a whole number? It's because elements exist as different isotopes - variations of the same element with different masses - and we need to calculate their average mass.
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of 1
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Isotopes and Relative Atomic Mass
Isotopes are basically different versions of the same element that have identical numbers of protons but different numbers of neutrons. Think of them as siblings in the same family - they're related but not identical.
Since different isotopes have different masses, chemists calculate the relative atomic mass (Ar) to find the average mass of all isotopes that make up an element. This explains why atomic masses on the periodic table are decimal numbers rather than whole numbers.
The formula is straightforward: Sum of (isotope abundance × isotope mass) ÷ Sum of abundance of all isotopes. Abundance tells you how common each isotope is in nature.
Let's look at copper as an example. Copper-63 has an abundance of 69.2% and copper-65 has 30.8%. Using the formula: (69.2 × 63) + (30.8 × 65) ÷ (69.2 + 30.8) = 63.6. That's why copper's relative atomic mass is 63.6, not a whole number.
Quick Tip: Remember that mass number = total protons + neutrons, while atomic number = number of protons only.
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