Compounds & Molecules
Molecule: The smallest identifiable
unit that retains the physical and
chemical properties of the pure
substances.
NaCl, salt
Ethanol, C2H6O
Buckyball, C60
Compounds & Molecules
• COMPOUND is a combination of 2 or
more elements in definite ratios by mass.
• The character of each element is lost when
forming a compound (e.g., think of NaCl).
• MOLECULES are the smallest units of a
compound that retains the characteristics of
the compound.
MOLECULAR FORMULAS
• Formula for glycine is C2H5NO2
(description of the composition)
• In one molecule there are
– 2 C atoms
– 5 H atoms
– 1 N atom
– 2 O atoms
CONDENSED FORMULAS
• Formula for glycine is NH2CH2CO2H
(composition and functional groups)
• In one molecule there are
– 1 NH2 (amine group)
– 1 CH2 group
– 1 CO2H group
STRUCTURAL FORMULAS
• Show how the atoms are attached within a
molecule
• The lines between atoms represent chemical
bonds that hold the atoms together.
WRITING FORMULAS
• Can also write glycine as the condensed
formula
H2NCH2COOH
showing functional groups (atom ordering and
connectivity)
• or in the form of a structural
formula
H H O
H N C C O H
H
showing how atoms are attached to each other
(bond orders)
MOLECULAR MODELING
An even higher level of structural detail
H H O
H N C C O H
H
Ball & stick
Drawing of glycine
Space-filling
Resources for
Molecular Modeling
• Modeling software
–CAChe (General Chemistry Interactive
CD-ROM)
–Rasmol
–Molden
–Gaussview
–Maestro
MOLECULAR WEIGHT
AND MOLAR MASS
Molecular weight = sum of the
atomic weights of all atoms in the
molecule.
Molar mass = molecular weight in
grams per mol.
What is the molar
mass of ethanol,
C2H6O?
1 mol contains
2 moles of C (12.01 g C/1 mol) = 24.02 g C
6 moles of H (1.01 g H/1 mol) = 6.06 g H
1 mol of O (16.00 g O/1 mol) = 16.00 g O
TOTAL = molar mass = 46.08 g/mol
Tylenol
• Formula = C8H9NO2
• Molar mass = 151.2 g/mol
Molar Mass
How many moles of alcohol (C2H6O)
are there in a “standard” can of beer
if there are 21.3 g of C2H6O?
(a) Molar mass of C2H6O = 46.08 g/mol
(b) Calc. moles of alcohol
1 mol
21.3 g •
= 0.462 mol
46.08 g
How many molecules of alcohol are
there in a “standard” can of beer if
there are 21.3 g of C2H6O?
We know there are 0.462 mol of C2H6O.
6.022 x 1023 molecules
0.462 mol •
1 mol
= 2.78 x 1023 molecules
How many atoms of C are there in
a “standard” can of beer if there are
21.3 g of C2H6O?
There are 2.78 x 1023 molecules.
Each molecule contains 2 C atoms.
Therefore, the number of C atoms is
2 C atoms
23
2.78 x 10 molecules •
1 molecule
= 5.57 x 1023 C atoms
Molecular & Ionic Compounds
Heme
NaCl
Molecular compounds consist of discrete molecules
Ionic compounds consist of discrete ions
IONS AND IONIC COMPOUNDS
• IONS are atoms or groups of atoms with a positive
or negative charge.
• Taking away an electron from an atom gives a
CATION with a positive charge
• Adding an electron to an atom gives an ANION
with a negative charge.
Forming Cations & Anions
A CATION forms
when an atom
loses one or
more electrons.
Mg -->
Mg2+
oxidation
+ 2 e-
An ANION forms
when an atom
gains one or
more electrons
F + e- --> Freduction
oxidation
reduction
PREDICTING ION CHARGES
In general
• metals (Mg) lose electrons ---> cations
• nonmetals (F) gain electrons ---> anions
Charges on Common Ions
Cation charge=group #
+1
Anion charge=group #-8
-4 -3 -2 -1
+2
+3
By losing or gaining e-, atom has same number of
electrons as nearest Group 8A atom.
Predicting Charges on Monatomic Ions
METALS
M ---> n e- + Mn+
where n = periodic group
Na+
sodium ion
Mg2+
magnesium ion
Al3+
aluminum ion
Transition metals --> M2+ or M3+
are common
Fe2+
iron(II) ion
Fe3+
iron(III) ion
NONMETALS
NONMETAL + n e- ------> Xnwhere n = 8 - Group no.
Group 4A
Group 5A
Group 6A
Group 7A
C4-,carbide N3-, nitride O2-, oxide F-, fluoride
S2-, sulfide Cl-, chloride
Br-, bromide
I-, iodide
Ion Formation
Reaction of
aluminum
and
bromine
POLYATOMIC IONS
CD Screen 3.6
Groups of atoms with a charge.
MEMORIZE the names and formulas of common
polyatomic ions listed in Table 3.1, page 107 (next slide)
Polyatomic Ions
NH4+
ammonium ion
One of the few common
polyatomic cations
Polyatomic Ions
(oxoanions)
HNO3
nitric acid
NO3nitrate ion
Prefix per- and suffix –ate: largest #
Suffix -ate : greater # of oxygen atoms
Suffix -ite : smaller # of oxygen atoms
Prefix hypo- and suffix –ite: smallest #
Polyatomic Ions
SO42sulfate ion
SO32sulfite ion
Polyatomic Ions
NO3nitrate ion
NO2nitrite ion
Polyatomic Ions
CO32carbonate ion
HCO3bicarbonate ion
hydrogen carbonate
Polyatomic Ions
PO43phosphate ion
CH3CO2acetate ion
COMPOUNDS
FORMED
FROM IONS
CATION +
ANION --->
COMPOUND
Na+ + Cl- --> NaCl
A neutral compd.
requires
equal number of +
and - charges.
IONIC COMPOUNDS
NH4
+
Cl
ammonium chloride, NH4Cl
Some Ionic Compounds
Ca2+ + 2 F- ---> CaF2
Mg2+ + NO3- ----> Mg(NO3)2
magnesium nitrate
calcium fluoride
Fe2+ + PO43- ----> Fe3(PO4)2
iron(II) phosphate
Properties of Ionic Compounds
Forming NaCl from Na and Cl2
• A metal atom can
transfer an
electron to a
nonmetal.
• The resulting
cation and anion
are attracted to
each other by
electrostatic
forces.
Electrostatic Forces
The oppositely charged ions in ionic compounds are
attracted to one another by ELECTROSTATIC
FORCES.
These forces are governed by COULOMB’S LAW.
Electrostatic Forces
COULOMB’S LAW
(charge on +)(charge on -)
Force of attraction =
2
(distance between ions)
As ion charge increases, the attractive force
_______________.
As the distance between ions increases, the
attractive force ________________.
This idea is important and will come up
many times in future discussions!
Importance of Coulomb’s Law
NaCl, Na+ and Cl-,
m.p. 804 oC
MgO, Mg2+ and O2m.p. 2800 oC
ELEMENTS THAT EXIST AS
MOLECULES
Allotropes of C
See SCREEN 3.2
on the CD-ROM
Screen 3.2
ELEMENTS THAT EXIST AS
DIATOMIC MOLECULES (gases)
ELEMENTS THAT EXIST AS
POLYATOMIC MOLECULES
S8 sulfur
molecules
White P4 and polymeric
red phosphorus
Molecular Compounds
Compounds without Ions
CO2 Carbon dioxide
CH4 methane
BCl3 boron trichloride
Naming Molecular
Compounds
CO2 Carbon dioxide
CH4 methane
BCl3
boron trichloride
All are
formed from
two or more
nonmetals.
Ionic
compounds
generally
involve a metal
and nonmetal
(NaCl)
Empirical & Molecular
Formulas
A pure compound always consists of the
same elements combined in the same
proportions by weight.
Therefore, we can express molecular
composition as PERCENT BY
WEIGHT
Ethanol, C2H6O
52.13% C
13.15% H
34.72% O
Percent Composition
Consider some of the family of nitrogenoxygen compounds:
NO2, nitrogen dioxide and closely
related, NO, nitrogen monoxide (or
nitric oxide)
Chemistry of NO,
nitrogen monoxide
Structure of NO2
Percent Composition
Consider NO2, Molar mass = ?
What is the weight percent of N and of O?
Wt. % N =
14.0 g N
• 100% = 30.4 %
46.0 g NO2
Wt. % O  2 (16 .0 g O per mole ) x 100 %  69 .6%
46 .0 g
What are the weight percentages of N and O in NO?
Determining Formulas
In
chemical analysis we determine
the % by weight of each element in a given
amount of pure compound and derive the
EMPIRICAL or SIMPLEST formula.
PROBLEM:
A compound of B and H
is 81.10% B. What is its empirical
formula?
A compound of B and H is 81.10% B. What is
its empirical formula?
• Because it contains only B and H, it
must contain 18.90% H.
• In 100.0 g of the compound there are
81.10 g of B and 18.90 g of H.
• Calculate the number of moles of each
constitutent.
A compound of B and H is 81.10% B. What is
its empirical formula?
Calculate the number of moles of each
element in 100.0 g of sample.
1 mol
81.10 g B •
= 7.502 mol B
10.81 g
1 mol
18.90 g H •
= 18.75 mol H
1.008 g
A compound of B and H is 81.10% B. What is
its empirical formula?
Now, recognize that atoms combine in the
ratio of small whole numbers
Find the ratio of moles of elements in
the compound.
A compound of B and H is 81.10% B. What is
its empirical formula?
Take the ratio of moles of B and H. Always
divide by the smaller number.
18.75 mol H
2.499 mol H
2.5 mol H
=
=
7.502 mol B
1.000 mol B
1.0 mol B
But we need a whole number ratio.
2.5 mol H/1.0 mol B = 5 mol H to 2 mol B
EMPIRICAL FORMULA = B2H5
A compound of B and H is 81.10% B.
Its empirical formula is B2H5. What is
its molecular formula ?
Is the molecular formula B2H5, B4H10,
B6H15, B8H20, etc.?
B 2H 6
B2H6 is one example of this class of compounds.
A compound of B and H is 81.10% B. Its empirical
formula is B2H5. What is its molecular formula?
We need to do an EXPERIMENT to find
the MOLAR MASS.
Here experiment gives 53.3 g/mol
Compare with the mass of B2H5
= 26.66
g/unit
Find the ratio of these masses.
53.3 g/mol
2 units of B2H5
=
26.66 g/unit of B2H5
1 mol
Molecular formula = B4H10
How to Determine the molar mass?
Mass spectrometer
Mass Spectrum of Ethanol
Mass Spectrum of Ethanol
(from the NIST site)
CH2O+
31
CH3CH2O+
45
CH3CH2OH+
46
Determine the formula of a
compound of Sn and I using the
following data.
•
•
•
•
Reaction of Sn and I2 is done using excess Sn.
Mass of Sn in the beginning = 1.056 g
Mass of iodine (I2) used = 1.947 g
Mass of Sn remaining = 0.601 g
Tin and Iodine Compound
Find the mass of Sn that combined with
1.947 g I2.
Mass of Sn initially =
1.056 g
Mass of Sn recovered = 0.601 g
Mass of Sn used =
0.455 g
Find moles of Sn used:
1 mol
0.455 g Sn •
= 3.83 x 10-3 mol Sn
118.7 g
Tin and Iodine Compound
Now find the number of moles of I2 that
combined with 3.83 x 10-3 mol Sn. Mass
of I2 used was 1.947 g.
1 mol
1.947 g I2 •
= 7.671 x 10-3 mol I2
253.81 g
How many moles of iodine atoms?
-3
7.671 x 10
2 mol I atoms 
mol I2 

 1 mol I2

= 1.534 x 10-2 mol I atoms
Tin and Iodine Compound
Now find the ratio of number of moles of moles
of I and Sn that combined.
1.534 x 10-2 mol I
4.01 mol I
=
1.00 mol Sn
3.83 x 10-3 mol Sn
Empirical formula is
SnI4