Download Ppt05(PS2-3)_PeriodicLaw_CountingByWeighing_AtomicWeight

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]
**See “Mystery of Matter” Part 2 Unruly Elements
https://www.youtube.com/watch?v=yG0G_NkwOMc
• Counting by Weighing Idea
– And relation to (Average) Atomic Mass (Tro, 2.8)
[Beginning of PS3 material (#1-3 on PS3)]
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!!
3
– Used pattern to predict undiscovered elements (& properties)
Tro: Chemistry: A Molecular Approach, 2/e
Periodic Pattern
4
Tro: Chemistry: A Molecular Approach, 2/e
A fewof
About
Most
elements
¾the
of the
remaining
elements
are classified
elements
are classified
asare
metalloids.
classified
as metals.
Their
as nonmetals.
They
solidshave
havea
reflective
Their
some
solids
characteristics
surface,
have aconduct
non-reflective
of metals
heatand
and
surface,
some
electricity
ofdononmetals.
not
better
conduct
than other
heat
elements,
and
electricity
and well,
are malleable
and are brittle.
and ductile
5
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?
85200 lb
 4260 BB
20.0 lb/BB
or
1 BB
 4260 BB
85200 lb x
20.0 lb
Counting Objects by Weighing—Jelly
Beans example
•
•
•
•
Customer wants 1000 jelly beans
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:
**Does each bean have
5.03 g
x 1000 beans  5030 g
bean
a mass of 5.03 g?
Unlikely. 5.03 g is the
AVERAGE value. OK?
• 785 g of beans contains how many beans?
785 g
 156 j.b. (156.06...)
5.03 g/j.b.
Mass (of objects) ≠ # of objects!
• 785 g of beans contains how many beans?
156 j.b.
• 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)?
• For every 100 BB, 10 are 22.0 lb, 90 are 20.0 lb
• Average mass =
total mass 90(20.0 lb)  10(22.0 lb)

100 BB
# BB
2020 lb 20.2 lb


100 BB
BB
Does any bowling ball
have a mass of 20.2 lb?

90
100
90%
NO! But for counting, it’s as if
each one has that mass. “abundance”
of 20 lb
(20.0 lb) 
10
100
10%
“abundance”
of 22 lb
(22.0 lb)
Isotopic masses
(applied to each
isotope of an
element)
This is just like Ne!*
Atomic mass
(applies to each
element)
20.2 amu
(on average )
(20.0 amu) 
(22.0 amu) 
Ne atom
100
100
90.5
9.5
90.5%
“abundance”
of 20Ne
9.5%
“abundance”
of 22Ne
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.
*Values used here are only approximate to help you see the concept.
EXAMPLE 2.5 Atomic Mass
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.
SOLUTION
Convert the percent natural abundances
into decimal form by dividing
by 100.
Compute the atomic mass using the equation given in
the text (UNDERSTAND IT—don’t “need” it!!).
FOR PRACTICE 2.5
Magnesium 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.
© 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)
Atomic mass is
35.45 amu because a
greater % of atoms
have a mass close to
35 (vs. 37) amu!
(average)
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”