Transcript Chapter 19 Molecules and Compounds

Chapter 19
Molecules and
Compounds
Section 19.1
Most matter is in the
form of compounds.
Most atoms are
unstable unless
combined.
Atoms combine in
molecules.
Chemical bonds are
formed when atoms
combine.
Valence shell
electrons (outermost)
are involved in
bonding.
What are valence
shell e ?
e
are found in
energy levels (E.L.)
within electron cloud
surrounding nucleus.
Highest E.L., highest
energy.
Energy Levels
First E.L.
can hold
up to 2
e.
What is this element?
Second E.L. can hold
up to 8 e .
What is
this
element?
Third E.L. can hold up
to 8 valence shell e .
How many
more e can
this atom
hold in its
valence
shell?
Octet Rule
Chemically stable atoms
have 8 valence electrons
st
(2 for 1 E.L. only).
Atoms form bonds to
complete their octet and
become stable.
Known as octet rule.
Use the PT to find # of
valence e .
Column (group / family)
tells how many valence e .
All atoms in a group have
same # of valence e-, with
exception of transition
metals.
Dot Diagrams
Dot diagrams are used
to represent valence e-.
Element symbol
represents nucleus &
all e- except for
valence e-.
Dot Diagrams, cont.
– Dots
around symbol =
valence e .
– Place one dot for each
valence e on each side
of the symbol before
pairing up the e .
Examples:
H
C
Mg
N
Al
O
Cl
Ar
Types of Chemical
Bonds
Recall that atoms
gain, lose, or share e
to gain 8 valence e .
so that they become
stable.
Ionic Bonds - formed
when atoms gain or
lose electrons.
Ex: Na has 1 valence
=
e
When Na loses that
electron, its 2nd E.L.
becomes full and stable
with 8 e .
sodium atom
sodium ion
Atoms with an electrical
charge are called ions.
Chlorine has 7 valence
electrons.
If Cl gains 1 electron,
its valence shell will be
full and stable.
chlorine atom
chlorine ion
Opposites attract, so…
Are these two ions
likely to bond?
Covalent Bonds
Most atoms share
electrons to gain an
octet.
Covalent bonds are
formed when e- are
shared.
Covalent Bonds, cont.
Covalent bonds can
form between 2
different types of
atoms or between 2
or more atoms of the
same type.
Diatomic molecules:
– covalently
bonded
atoms
of the same type.
Exs: H2 Cl2 F2 I2
O2 Br2 N2
How can you tell
whether a bond is
ionic or covalent?
In GENERAL:
Ionic – metal / nonmetal
Covalent – 2 nonmetals
Electronegativity
e
Sometimes are
shared unequally in
covalent bonds.
Electronegativity –
attraction atom has for
shared pair of e .
Chapter 19: Molecules
and Compounds
Section 19.2
Chemical Formulas
Chemical Formula:
Ratio of atoms bonded
together in a compound,
i.e. X:Y
General Form:
AxBy
where x and y are called
subscripts.
Recall NaCl (sodium
chloride)…
Formula shows that
atoms combine in a 1:1
ratio.
Na1Cl1 = 1:1
Why in that ratio?
To be stable…
the net
electrical
charge of
compounds
must be
zero.
Oxidation Number:
Indicates how many
valence e are lost, gained,
or shared when bonding.
(+) or (–) symbol is
written after the number,
i.e. 1+ or 2-
Writing Chemical Formulas –
monatomic ions
1.Symbol of (+) ion always
written 1st.
2.Symbol of (–) ion always
written 2nd.
3.Add subscripts so sum of
oxidation #’s is zero.
Example:
Write formula for
binary (2 element)
compound made of
iron(III) and oxygen.
1. Find oxidation #’s of
elements:
iron(III) =
oxygen =
3+
Fe
2O
How do you make a
cmpd electrically neutral?
3+
Fe ions
Calculate
needed
to combine with O2- ions
to make electrical charges
equal zero.
2
3+
(Fe )
added to 3
2(O )
=0
2 (3+) added to 3 (2-) = 0
2. To determine ratios
write chemical
formulas…Use the
Criss-cross Method
3+
Fe
O
2
3
2-
to
Naming binary
ionic compounds
Write name of 1st element
or polyatomic ion.
Write root name of 2nd
element and add -ide.
Exs: chlor-ine = chlor-ide
phosph-orus = phosph-ide
Naming ionic cmpds with
polyatomic ions
Write name of (+) ion
Use PT or Table 19.2
Write name of (-) ion
Use PT or Table 19.2
st
1 .
nd
2 .
Naming binary
covalent compounds
Specify number of each
element by using prefixes
(Figure 19.25, pg 332).
If only one atom of 1st
element, don’t use mono-
Examples:
CO carbon monoxide
CO2 carbon dioxide
PCl5 phosphorus
pentachloride
N2S6 dinitrogen hexasulfide
Chapter 19:
Molecules and
Compounds
Section 19.3
Comparing
Molecules
COMPARING DIFFERENT
MOLECULES
How does the
mass of different
molecules
compare?
Do you think that a
molecule of water has the
same mass as a molecule
of calcium carbonate?
Recall atomic mass
units (amu)…
Atoms are assigned a
relative mass based on
carbon as the standard.
Known as
atomic
mass unit
2
4
6
C
Carbon
12.0107
Chemical formula
gives 3 pieces of info:
types / numbers of atoms.
–if polyatomic ions are
present.
– allows calculation of mass
of 1 molecule of a
compound relative to mass
of other compounds.
Formula Mass
Way to compare masses
of molecules of different
compounds.
Calculate by adding up
atomic masses of all
atoms in a compound.
Example: Figuring
Formula Mass
H2O means 2H and 1O
2 (1.01 amu) =
2.02
+1 (16.00
amu) = 16.00
•Formula mass = 18.02
amu of H2O
An amu is very small,
so to be usable in
measurements, we
equate the number
value of the formula
mass in amu to an
equal amount in
grams.
Avogadro’s Number
The formula mass in grams
of any element or
compound contains 6.02 x
1023 atoms or molecules.
Known as Avogadro’s # or
a “mole” of the substance.
Calculate the formula mass
of calcium carbonate.
1. Write formula
2. List number of
atoms and atomic
mass of each: CaCO3
1 Ca = 1(40.08) = 40.08
1 C = 1(12.01) = 12.01
3 O = 3(16.00) = 48.00
3. Add up values to
calculate formula
mass
40.08
12.01
+ 48.00
100.09 amu
for CaCO3
So, how do we
use this value?
If you measure out
100.09 grams of CaCO3,
you have 6.02 x 1023
molecules of CaCO3.
Likewise, 18.02 g of H2O
23
contains 6.02 x 10
molecules of water.
Hydrates (BaCl2
2H2O)
Some molecules contain
precise numbers of H2O
molecules chemically
bonded to their ions.
Called hydrates.
Can remove H2O by
heating.
When H2O is gone,
the compound is
known as anhydrous
(BaCl2).
To calculate formula
mass, simply add the
mass of the attached H2O
molecules to that of the
anhydrous mass.
Example: BaCl2 2H2O
1 Ba = 1(137.30) = 137.30
2 Cl = 2(35.45) = 70.90
4 H = 4(1.01) = 4.04
2 O = 2(16.00) = 32.00
Formula mass = 244.24
amu