Exp 6 Prelab “Warning”!

• See D2L Announcement

Plan

• The Periodic Law and the Periodic Table – (Tro, 2.7) [End of PS2 material; problems 17-18]

• Counting by Weighing Idea – And relation to (Average) Atomic Mass (Tro, 2.8)

[Beginning of PS3 material (#1-3 on PS3)]

3

Mendeleev (~1869)

• Ordered elements by atomic mass• Saw a repeating pattern of properties • Periodic Law – when the elements are arranged in

order of increasing atomic mass, certain sets of properties recur periodically

• Ordered them in a table (Periodic Table!)• New row = new “Period”

– Start a new row when properties start to “recur”)

• Elements with similar properties ended up in the same column (column = “Group” or “Family”)

• Very Powerful!!– Used pattern to predict undiscovered elements (& properties)

Tro: Chemistry: A Molecular Approach, 2/e

4

Periodic Pattern

Tro: Chemistry: A Molecular Approach, 2/e

5

About ¾ of the elements are classified as metals. They have a reflective surface, conduct heat and electricity better than other elements, and are malleable and ductile

Most of the remaining elements are classified as nonmetals. Their solids have a non-reflective surface, do not conduct heat and electricity well, and are brittle.

A few elements are classified as metalloids. Their solids have some characteristics of metals and some of nonmetals.

Tro: Chemistry: A Molecular Approach, 2/e

Periodic Table Trends--Upshot

• metals—left of stepladder (except for H)

• nonmetals—right of stepladder (except for H)

• metalloids—touching stepladder (except Al, Po)

Other Periodic Table Ideas

• Families or Groups are COLUMNS– These elements have similar properties

• “observation” at this point—explanation will come later with quantum mechanical model of atoms!

• Rows are called PERIODS– These generally do not have similar

properties• But there are trends in properties that we’ll learn

later (and these, too, will be explained by the QM model of atoms!)

• [End of PS2 material]

• Next Slide Begins PS3 material

Counting Objects by Weighing

• If a freight train car carries a load of bowling balls (BBs) weighing 85200 lb, and each BB weighs 20.0 lb, how many BBs are in the car?

BB 4260lb/BB 20.0

lb 85200

BB 42601 BB

85200 lb x 20.0 lb

or

Counting Objects by Weighing—Jelly Beans example

• Customer wants 1000 jelly beans

g 5030beans 1000 x bean

g 5.03

• Much easier to weigh out than to count!• What do you need to do this?• Grams per jelly bean! Then multiply by 1000

(beans) to get grams:

• 785 g of beans contains how many beans?

.06...)6(15 j.b. 156 g/j.b. 5.03

g 785

**Does each bean have a mass of 5.03 g? Unlikely. 5.03 g is the AVERAGE value. OK?

Mass (of objects) ≠ # of objects!

• 785 g of beans contains how many beans?

j.b. 156

• Be careful not to confuse grams with “how many (units)” of something!!

• 785 g j.b. ≠ 785 j.b. (look closely at the units!)

Average Mass—Back to Bowling Balls

• What if it turned out that 1 out of every 10 BB had a mass of 22.0 lb (the rest are 20.0 lb)?

BB #

mass total

BB 100

lb) 10(22.0 lb) 90(20.0

BB

lb 20.2

BB 100

lb 2020

lb) (22.0lb) (20.0100

10

100

90

90% “abundance”

of 20 lb

10% “abundance”

of 22 lb

• For every 100 BB, 10 are 22.0 lb, 90 are 20.0 lb

• Average mass =

Does any bowling ball have a mass of 20.2 lb?

NO! But for counting, it’s as if each one has that mass.

This is just like Ne!*

amu) (22.0amu) (20.0100

9.5

100

90.5

90.5% “abundance”

of 20Ne

9.5% “abundance”

of 22Ne

) (on atom Ne

amu 20.2average

*Values used here are only approximate to help you see the concept.

Quiz: Does any atom of Ne have a mass of 20.2 amu?

Ans: NO! But a sample with lots of Ne atoms in it (from Earth!) would act for counting purposes as if each atom had this mass.

Isotopic masses (applied to each isotope of an element)

Atomic mass (applies to each element)

FOR PRACTICE 2.5Magnesium has three naturally occurring isotopes with masses of 23.99 amu, 24.99 amu, and 25.98 amu and natural abundances of 78.99%, 10.00%, and 11.01%, respectively. Calculate the atomic mass of magnesium.

EXAMPLE 2.5 Atomic Mass

Compute the atomic mass using the equation given in the text (UNDERSTAND IT—don’t “need” it!!).

Convert the percent natural abundances into decimal form by dividing by 100.

SOLUTION

Copper has two naturally occurring isotopes: Cu-63 with mass 62.9396 amu and a natural abundance of 69.17%, and Cu-65 with mass 64.9278 amu and a natural abundance of 30.83%. Calculate the atomic mass of copper.

© 2011 Pearson Education, Inc.

Atomic mass of

“Cl”, in amu

Quiz: Does any atom of Cl have a mass of 35.45 amu?

(34.9689 amu)

(36.9659 amu)

(average) Atomic mass is 35.45 amu because a greater % of atoms have a mass close to 35 (vs. 37) amu!

Miscellaneous

• Although # of protons determines the type of atom, the electrons are what determine the kinds of chemistry that an atom tends to undergo.– Electrons are either transferred or shared with

other atoms when atoms of different elements “bond” to one another.

– “Repartnering” involves changing of “bonding”