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Sub atomic heavyweights. Isotopes. ChemCatalyst. A chemist investigating a sample of lithium found that some atoms have a lower mass than other atoms. The chemist determined that the structures of the two types of atoms would be similar to the two drawings below. (cont.).

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chemcatalyst
ChemCatalyst

A chemist investigating a sample of lithium found that some atoms have a lower mass than other atoms. The chemist determined that the structures of the two types of atoms would be similar to the two drawings below.

(cont.)

Unit 1 • Investigation III

slide3
What is different about the two atoms?

What is the atomic number of each atom?

What is the mass number of each atom?

Do you think they are both lithium atoms? Why or why not?

(cont.)

Unit 1 • Investigation III

the big question
The Big Question

How do isotopes of an atom account for the atomic weight of an element?

Unit 1 • Investigation III

isotopes
Isotopes

Atoms of the same element that have different numbers of neutrons

Ex: Most hydrogen atoms have no neutrons. But a rare form of hydrogen atoms will have a neutron

differences in isotopes
Differences in Isotopes
  • Different number of neutrons
  • Different mass values
  • Some isotopes are more stable (not radioactive)
similarities in isotopes
Similarities in isotopes
  • All isotopes of an element have the basically the same characteristics:

Color

Reactivity

What elements they react with

Conductivity

How shiny or dull

Etc…

isotope names
Isotope names

Two ways

1st: list the name or symbol of the element and the mass number of the isotope:

Ex Uranium-237, U-238 (can use symbol)

2nd: Write symbol of element and place the mass number on top and the atomic number below

Ex 12

C

6

write both versions of the isotope names for each of the following
Write both versions of the isotope names for each of the following:
  • Molybdenum (Mo) atom with 55 neutrons
  • Tungsten atom with a mass number of 183
  • An atom of the most common isotope of Xenon
subatomic heavyweights
Subatomic Heavyweights

Work with a partner to complete the chart and compare answers to the questions in the handout

activity
Activity

Goal: Complete the chart on back. Determine the average atomic weight for the 10 atoms shown

(cont.)

Unit 1 • Investigation III

slide15
(cont.)

(cont.)

Unit 1 • Investigation III

atomic weight
Atomic weight

The weighted average of the atomic masses of different isotopes taking into account their abundance

determining atomic weight from
Determining Atomic weight from %
  • Need to know the percentage of the element’s atoms that conform to each isotope

Ex: 68% of Cu is Cu-63, 32% is Cu-65

To determine atomic weight:

1) convert % into decimal equivalent

2) multiply decimal to mass #

3) add resulting values

(0.68)(63) + (0.32)(65) = 63.7 = atomic weight

slide19
Try
  • Magnesium has 3 isotopes
  • 70% of atoms are Mg-24
  • 29% of atoms are Mg-25
  • 1.0% of atoms are Mg-26
  • What is the atomic weight of Mg?
check in
Check in

Predict the isotopes of Carbon, C.

Which isotope is more abundant?

How do you know?

most common isotopes
Most common isotopes
  • Look at atomic weight on PT
  • Most common isotopes are generally found by rounding the weight up OR down

Ex: Os (Osmium) = 190.2 so

The 2 most common isotopes are Os-190 and Os-191

Which is there more of?

slide24
Complete chart

Unit 1 • Investigation III

questions for section
Questions for section

Label sketches in question 1

How do you average values?

answers to lab questions
Answers to lab questions

Fill in chart

2 different isotopes: Boron-10, Boron-11

2 atoms of Boron-10, 8 atoms of Boron-11

10, 11 amu, add protons and neutrons

(10+11+11+11+11+10+11+11+11+11)/10 = 10.80 amu

10.80 compared to 10.81

more answers to lab questions
More answers to lab questions

6a) 2 different isotopes

6b) 20 Boron-10 and 80 Boron-11

6c) 20%, 80%

even more answers to lab questions
Even more answers to lab questions

8) 3 isotopes

9) Argon-40 is the most common

10) 4 neutrons, because Lithium-7 is most common isotope

making sense
Making Sense

Explain why the atomic weights listed in the periodic table are not usually whole numbers.

Unit 1 • Investigation III

slide31
While the element iron is defined as being made up of neutral atoms with 26 protons and 26 electrons, not every iron atom has the same number of neutrons.

Atoms that have the same number of protons but different numbers of neutrons are called isotopes.

Notes

(cont.)

Unit 1 • Investigation III

slide32
What we call the atomic weight on the periodic table is actually the average atomic mass of that element’s naturally occurring isotopes.

Isotopes have similar chemical properties in that they combine with other elements to form similar compounds.

Notes(cont.)

(cont.)

Unit 1 • Investigation III

slide33
Atomic Weight is the weighted average of the atomic masses of different isotopes taking into account their abundance.

(cont.)

(cont.)

Unit 1 • Investigation III

slide34
(cont.)

Unit 1 • Investigation III

if you finish early
If you finish early

Cu

Has 2 naturally occurring isotopes

69% with 34 neutrons,

31% with 36 neutrons

Calculate average atomic mass of Cu

slide36
Isotopes of a single element exhibit similar properties in that they form similar compounds.

Isotopes are referred to by their mass numbers.

(cont.)

Unit 1 • Investigation III

wrap up
Wrap-Up

Elements may have anywhere from 2 to 10 naturally occurring isotopes.

The atomic weight of an element listed on the periodic table is actually the average mass of the naturally occurring isotopes of that element.

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

(cont.)

Unit 1 • Investigation III

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