Average Atomic Mass & Percent Abundance

A set of vocabulary flashcards covering key concepts related to average atomic mass and percent abundance in chemistry.

Atomic mass unit (amu)

Defined as 1/12 the mass of a carbon-12 atom.

Average atomic mass

The weighted average of all naturally occurring isotopes of an element.

Why is average atomic mass rarely a whole number?

Because it’s an average of isotope masses weighted by abundance.

Percent abundance

The relative percentage of each isotope in a natural sample.

How is average atomic mass calculated?

Multiply each isotope’s mass by its fractional abundance and add the results.

Chlorine isotopes

Cl-35 (34.969 amu, 75.77%) and Cl-37 (36.966 amu, 24.23%). Average atomic mass is 35.45 amu.

Boron isotopes

B-10 (10.013 amu, 19.91%) and B-11 (11.009 amu, 80.09%). Average atomic mass is 10.81 amu.

X isotopes

X-20 (19.992 amu, 90.48%) and X-22 (21.991 amu, 9.52%). Average atomic mass is 20.18 amu.

Element with isotopes 62.93 amu and 64.93 amu

Identified as Copper (Cu) with an average atomic mass of 63.55 amu.

Isotopic abundance effect

The more abundant isotope has a greater effect on the average atomic mass.

Abundant isotope of Neon

Ne-20, since the average is closest to 20.18 amu.

Isotope definition

Isotopes have the same number of protons but different numbers of neutrons.

Percentage of isotopes A and B

80% isotope B (11.0 amu) and 20% isotope A (10.0 amu) for an average atomic mass of 10.8 amu.

Isotope contribution to average atomic mass

The isotope with higher abundance contributes more to the average.

Carbon isotopes

C-12 (98.89%) and C-13 (1.11%). Average atomic mass is 12.01 amu.

Magnesium isotopes

Mg-24 (78.70%), Mg-25 (10.13%), Mg-26 (11.17%). Average atomic mass is 24.31 amu.

Importance of isotopic composition

Important in fields like medicine or archaeology for radiometric dating and tracing pathways.

Atomic mass vs. mass number

Atomic mass is the weighted average of isotopes; mass number is total protons + neutrons of one isotope.

Weighted average in chemistry

Similar to calculating a grade-point average (GPA) based on contribution.

Standard atomic weight

The average atomic mass of an element’s isotopes as found naturally on Earth.