Average Atomic Mass Calculations

​Average Atomic Mass

How are the atomic masses in the periodic table calculated?

Average Atomic

Mass

The average atomic mass of the elements as seen on the periodic
table is a weighted average of the masses of all naturally
occurring isotopes for the element.

• Average atomic masses that are on the periodic table are
calculated based on knowledge of the following information:

1. the mass of all the naturally occurring isotopes for the element

2. the natural abundance of each of these naturally occurring
isotopes (usually expressed as a percentage)

Formula

• 𝑨𝒗𝒆𝒓𝒂𝒈𝒆 𝑨𝒕𝒐𝒎𝒊𝒄 𝑴𝒂𝒔𝒔 =

% 𝒂𝒃𝒖𝒏𝒅𝒂𝒏𝒄𝒆 𝒐𝒇 𝒊𝒔𝒐𝒕𝒐𝒑𝒆 𝟏 𝑿 𝒎𝒂𝒔𝒔 𝒐𝒇 𝒊𝒔𝒐𝒕𝒐𝒑𝒆 𝟏 +( % 𝒂𝒃𝒖𝒏𝒅𝒂𝒏𝒄𝒆 𝒐𝒇 𝒊𝒔𝒐𝒕𝒐𝒑𝒆 𝟐 𝑿 𝒎𝒂𝒔𝒔 𝒐𝒇 𝒊𝒔𝒐𝒕𝒐𝒑𝒆 𝟐%

𝟏𝟎𝟎

Draw

If 75.0% of the isotopes of an element have a mass of 35.0 u and 25.0% of the isotopes have a mass of 37.0 u, what is the atomic mass of the element? Show your work.

Multiple Choice

Base your answer to this question on the information below.

The accepted values for the atomic mass and percent natural abundance of each naturally occurring isotope of silicon are given in the data table below.

• 1. (27.98)(92.22) + (28.98)(04.69) + (29.97)(3.09)

• 1. (27.98)(0.0469) + (28.98)(0.9222) + (29.97)(0.0309)

• 1. (27.98)( 04.69) + (28.98)(92.22) + (29.97)(0.0309)

• 1. (27.98)(0.9222) + (28.98)(0.0469) + (29.97)(0.0309)

Draw

Naturally occurring boron is composed of two isotopes. The percent abundance and the mass of each isotope are listed below.

• 19.9% of the boron atoms have a mass of 10.013 atomic mass units.

• 80.1% of the boron atoms have a mass of 11.009 atomic mass units. Show your work.

Draw

Two isotopes of Bromine naturally occur:

Isotopes of Bromine: Percent Abundance: Atomic Mass:

Bromine-79 50.69% 78.92 amu

Bromine-81 49.31% 80.92 amu

Calculate the average atomic mass for the two isotopes of Bromine.

Open Ended

Calculate the average atomic mass forthe two isotopes of copper.

Multiple Choice

24.1% of all the isotopes of a an element have a mass of 75.23 amu, 48.7% have a mass of 74.61 amu, and 27.2% have a mass of 75.20 amu.
What is the average mass of this element?

• 74.92 amu

• 24.97 amu

• 75.01 amu

• 74.51 amu

Multiple Choice

The atomic mass of an element is the ___.

• average of the mass number and the atomic number for the element

• weighted average of the masses of the isotopes of the element

• total mass of the isotopes of the element

• total number of subatomic particles in the nucleus

Multiple Choice

Nitrogen has three occurring isotopes: Nitrogen-13, Nitrogen-14, Nitrogen-15. Which isotope is the most abundant? Check the periodic table to find the weighted average atomic mass.

• Nitrogen-13

• Nitrogen-14

• Nitrogen-15

• Based on the information given, it cannot be determined