Isotopes & Atomic Mass

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

Was this helpful? Support seegongsik