Isotopes & Atomic Mass

Isotopes are the same element by proton number but differ in neutron count, so their mass numbers differ. Nature mixes them in fixed abundances, and average atomic mass is the abundance-weighted average of those mass numbers. The average leans toward the more abundant isotope, not the simple midpoint. Here you move the abundance slider and watch the average atomic mass change.

Same Element, Different Mass

Abundance of the lighter isotope (%)75 %

👀 See It

①Isotopes have the same proton count (same element) but different neutron counts, so different mass numbers

②In nature, isotopes occur in fixed proportions

③The average atomic mass is the weighted mean of mass numbers by abundance

What Isotopes Are

Isotopes

same protons, different neutrons ⇒ different mass number

Chemical properties are identical (same electron configuration); only the mass differs

Average atomic mass

average = Σ (mass number × abundance)

Use abundance as a decimal (sum 1) or percent (sum 100%) for the weighted mean

Intuition for the Weighted Mean

⚖️ It Leans Toward the Majority

①The average is not the simple midpoint of the two mass numbers

②It leans toward the more abundant isotope

③So chlorine’s average (35.5) is closer to 35

Compute It Directly

Example 1

If ³⁵Cl is 75% and ³⁷Cl is 25%, find the average atomic mass of chlorine.

Multiply each mass number by its abundance (decimal) and add.

35 × 0.75 + 37 × 0.25

Compute.

= 26.25 + 9.25 = 35.5

▸ 35.5

The average leans toward the more abundant 35, giving 35.5.

Example 2

An element has isotopes of mass 10 (20%) and mass 11 (80%). Find the average atomic mass.

Set up the weighted mean.

10 × 0.2 + 11 × 0.8

Compute.

= 2 + 8.8 = 10.8

▸ 10.8

It leans toward the more abundant 11, giving 10.8 — above the midpoint 10.5.

Wrap-up

Key result

average atomic mass = Σ (mass number × abundance)

Isotopes differ only in mass number; the average is the abundance-weighted mean

2020 CSAT Science (Chemistry Ⅰ) type, adapted

If copper has ⁶³Cu at 70% and ⁶⁵Cu at 30%, what is the average atomic mass?

①63

②63.6

③64

④64.4

⑤65

▸ ② 63.6

Set up the weighted mean.

63 × 0.7 + 65 × 0.3

Compute.

= 44.1 + 19.5 = 63.6

🎯 Exam Points

①isotopes: same protons, different neutrons (mass number)

②chemical properties identical

③average = Σ(mass × abundance)

④leans toward the more abundant

⑤convert percent to decimal (÷100) to compute

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Periodic Table

Ionic Bond

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