Isotopes and Atomic Mass (4.5 Timberlake)

Isotopes and Atomic Mass (4.5 Timberlake)

What are isotopes?

• Atoms of the same element that have the same atomic number but different numbers of neutrons.

• Because they have different numbers of neutrons, they have different mass numbers.

• To distinguish between different isotopes, we write the mass number after the element symbol or name. For example, Lithium - 6, Lithium - 7, or Lithium - 8.

Multiple Choice

How many neutrons does Lithium - 6 have?

• 3

• 4

• 6

Multiple Choice

What is the mass number of Lithium - 7?

• 3

• 4

• 7

Multiple Choice

How many protons does Lithium - 8 have?

• 3

• 5

• 8

Multiple Choice

All isotopes of lithium have the same

• mass number

• atomic number

• number of neutrons

Isotope Abundance

Not all isotopes are "created equal"

Take lithium for example

• If you had a sample of lithium, over 90 % would be lithium - 7. So lithium-7 is far more abundant.

Isotope Stability

Not all isotopes are "stable".

Going back to our lithium example...

• Lithium - 8 is not stable. The ratio of neutrons to protons is too high. When this happens the element is radio active and "breaks apart".

• This is the topic of Chapter 16 which I hope that we'll get to in the 2nd semester.

Atomic Mass

• Chemists generally use samples that contain all the different isotopes of an element.

• Because each isotope has a different mass, chemists have calculated an atomic mass for an "average atom".

• This is just a weighted average ... much like your grade is in this class.

Consider the following example:

Chlorine-35 has a mass of 34.97 amu and chlorine-37 has a mass of 36.97 amu.

Chlorine-35 is 75.76 % abundant, whereas chlorine-37 is 24.24% abundant.

Can you figure out the atomic mass?

Multiple Choice

chlorine-35: 34.97 amu

chlorine-37: 36.97 amu

chlorine-35: 75.76 % abundant

chlorine-37: 24.24% abundant

Atomic mass?

• 34.97 amu

• 35.45 amu

• 35.97 amu

• 36.97 amu