IsotopesMs FedorArts and College Preparatory Academy

ChemCatalyst

An atom has a mass of 22 and 11 protons. 1. What element is this atom?2. How many neutrons and

electrons does it have?

ChemCatalystDraw a Bohr Diagram for magnesium

A chemist investigating a sample of lithium found that some lithium atoms have a lower mass than other lithium atoms. The chemist drew models of the three different types of lithium atoms.

1. What is different about the two atoms?2. What is the atomic number of each

atom?3. What is the atomic mass of each atom?

What is an isotope?ISOTOPE Atoms of the same element that have different numbers of neutrons.

Atomic mass

Protons and neutrons each have a mass of 1 amu (atomic mass unit)

ATOMIC MASS is equal to the mass of the nucleus (since electrons have very little mass)

Nucleus = protons + neutrons so…. Atomic mass = # protons + #

neutrons

ChemCatalyst

Why did Ernest Rutherford conclude that the nucleus was so small?

Atoms with the same number of protons, but a different number of neutrons are called Isotopes

The nuclei of these 3 hydrogen atoms are different, because they have different numbers of neutrons

Most hydrogen atoms are this

Isotopes of the same element still have the same properties and behave the same way in a chemical reaction.

=

This carbon is actually a mixture of C-12, C-13, and C-14

Symbols associated with isotopes

Isotopes can be represented using their atomic symbol

What is the atomic mass and atomic number of chlorine?

Mass = 35

Atomic number = 17

We also write the atomic mass after the name to distinguish them

Carbon-12 6 P+, 6 N0

Carbon-13 6 P+, 7 N0

Carbon-14 6 P+, 8 N0

Percent Abundance

NATURAL PERCENT ABUNDANCE: The percentage that each isotope of an element occurs in nature

Atomic mass is an average value

It is calculated based on how much of that element is made up of the isotope.

The average mass is closest to the mass of the most common isotope

For example: Most Carbon is Carbon-12, so the mass will be closer to 12 than 13 or 14.

Calculating Average Atomic Mass

1. Multiply the atomic mass of each isotope by its percent abundance.

2. Add them all up.

For example: If the abundance of O-16 is 75%, and the abundance of O-17 is 25% than the atomic mass would be:

(16 x 0.75) + (17 x 0.25) = 12 + 4.25 = 16.25

Practice: Calculating Atomic Mass

What is the atomic mass for neon (Ne) if 20Ne has a relative abundance of 19.91%, and 21Ne has a relative abundance of 80.09%?

(20 x 0.1991) + (21 x 0.8009) = 1.991 + 8.8099 = 10.8009

Known 1: 10Ne mass = 2010Ne abundance = 19.91%

Known 2: 11Ne mass = 2111Ne abundance = 80.09%

unknown:

Atomic mass of Ne

Q: Which copper isotope is more abundant: Cu-63 or Cu-65 (atomic mass of copper is 63.546?

A: The atomic mass of 63.546 is closer to 63 than to 65.

Because the atomic mass is a weighted average of these two isotopes, Cu-63 must be more abundant than Cu-65

ChemCatalyst

Draw Bohr diagrams for the three isotopes of hydrogen: hydrogen-1 (atomic mass of 1), hydrogen-2 (atomic mass of 2), and hydrogen-3 (atomic mass of 3)

ChemCatalyst

Magnesium has three isotopes, Magnesium-24, Magnesium-25, and Magnesium 26.

Which do you think is the most common isotope of magnesium and why?

ChemCatalyst

How many protons, neutrons, and electrons are in phosphorus-32?

ChemCatalystWhich of the following are isotopes of copper, Cu? Explain your reasoning.

63 197 63 87

Cu Cu Cu Cu29 79 28 29

34 65

Cu Cu29 29

Quiz test corrections

1. For each multiple choice question missed:

◦ Explain which answer is correct and why

2. For each short answer missed: ◦ retry the question, and explain why

your answer was incorrect/insufficient

ChemCatalyst

What would be the average atomic mass of chlorine given the following information:

Chlorine with 18 neutrons: 75.77%Chlorine with 20 neutrons: 24.23%

How are “atoms”, “isotopes”, and “elements” related?

Remember: All matter is made of atomsAtoms with the same number

of protons belong to the same element. ◦ This means that all atoms with

the same number of protons, have the same properties.

Different forms of an element are called isotopes. ◦ This means that atoms of the

same element can differ slightly in the number of neutrons, and consequently, the mass.

Naturally Occurring Isotopes

The neutrons stabilize the nucleus.

Lower numbered elements (40 and below), have an even number of P+ and N0

Higher numbered elements have more neutrons than protons.

Radioactive IsotopesAtoms that are unstable are called “radioactive”Unstable nuclei

will emit particles and break down over time

Those atoms are lost over time because they decay into smaller atoms.

Some elements have naturally occurring radioactive isotopes

Elements above bismuth (84 and up) are all radioactive

Elements above uranium (93 and up) are all man-made elements

ChemCatalyst

Write the isotope for neon with 11 neutrons in two ways.