Isotopes

•Are All Atoms of an Element the Same?• Not necessarily. Some atoms have an unequal number of protons and neutrons. We call these atoms isotopes.

• In an isotope, the number of protons in an element always remains the same. The neutrons change.

• Let’s look at an example…

How are they different?

Proton

Electron

Proton

Electron

This is a Hydrogen Atom This is a Hydrogen Atom Isotope

Neutron

The isotope has 2 particles which means it has greater mass.

Properties of Isotopes

•Each element has a limited number of isotopes that occur naturally

•Some isotopes have unique properties because they are unstable

Properties of Isotopes

•An atom that is unstable means that the nucleus has the ability to change the numbers of neutrons that are in the nuclear space.

• Isotopes that are unstable are also called radioactive.

Properties of Isotopes•Simply stated…Isotopes share almost all of the same physical and chemical characteristics.

• For example, Oxygen isotopes are colorless, odorless gases at room temperature (physical).

How Can We Tell One Isotope from

another?•Each element has a mass number which identifies the isotope of the element.

•Mass number = protons + neutrons in an atom

•Electrons have such small mass they are not included in this calculation

Naming Isotopes• Let’s go back to our earlier example of the hydrogen atom and the hydrogen isotope.

Name That Isotope!

Proton

Electron

Proton

Electron

There is 1 proton and 0 neutrons so the name of this

atom is Hydrogen-1

There is now 1 proton AND 1 neutron so the name changes

to Hydrogen-2

Neutron

So How Do I Figure Out The Number of Protons And Neutrons?•As an example, we’ll look at the carbon isotope, with a mass number of 12 (found on the Periodic Table) It’s called Carbon-12.

Mass Number 12

- # Protons (atomic number) -6 Number of neutrons 6

Try One• Determine the number of protons, neutrons, and electrons in an aluminum-27 (Al).

• 13 protons, 14 neutrons, and 13 electrons

• How did we get the number of electrons?

• It’s the same as the number of protons.

Let’s Try One!•Use what you know about Carbon-12 to create Carbon-13.

•Using your Periodic Table • create Hydrogen-2 (H), Helium-3 (He), and Carbon-14 (C).

Where Have I Heard Carbon-14 Before?

•The isotope carbon-14 is used in radio-carbon-dating of animal and plant fossils.

•Uranium-238, uranium-235, and thorium-232 are isotopes that scientists use to tell the age of rocks and meteorites.

BrainPOPIsotopes

Calculating the Mass of an Element•Most elements found in nature contain a mixture of two or more stable (nonradioactive) isotopes

•The Atomic Mass is the average of the masses of all the naturally occurring isotopes of an element.

Let’s try it!•The metal thallium occurs naturally as 30 percent thallium-203 and 70 percent thallium-205. Calculate the atomic mass.

204.4 amu