Molecules, Ions, and
Their Compounds
Goals:
1. Know formulas for ionic and molecular
compounds.
2. Name compounds.
3. Understand some properties of ionic compounds.
4. Calculate and use molar mass.
5. Calculate % composition for a compound and
derive formulas from experimental data.
What are compounds?
What are molecules?
• COMPOUNDS
are a combination of 2 or more elements in
definite _____________. The character of
each element is _________when forming a
compound.
• MOLECULES
are the smallest unit of a compound that
retains the characteristics of the compound.
Compounds: Ionic and Molecular
NaCl, salt
CH3CH2OH, ethanol
What is a Molecular Formula?
• Molecular Formulas describe the composition
of molecules:
• Formula for glycine is C2H5NO2
• In one molecule there are
– 2 C atoms
– 5 H atoms
– 1 N atom
– 2 O atoms
Writing Molecular Formulas
• Can also write glycine formula as
– H2NCH2COOH
(condensed formula) to show atom ordering.
• or in the form of a structural
formula:
H H O
H N C C O H
H
Molecular Modeling
• Representations of
glycine:
Ball & stick
Write structural formula of glycine:
Space-filling
What is Molecular Weight?
What is Molar Mass?
Molecular weight: ______________
weights of all ________ in the
molecule.
Molar mass: molecular weight in
________.
What is the molar mass of ethanol,
C2H6O?
Molar Mass
Students should become familiar with calculations
and use of molar mass.
How many moles of alcohol are there in a “standard”
can of beer if there are 21.3 g of C2H6O? How many
molecules? How many C atoms?
1. Calculate the number of moles: This is a grams to moles
conversion. Need to know how many grams are there per mol of
ethanol.
2. Calculate the number of molecules:
3. Calculate the number of C atoms: There are 2 C per
ethanol molecule.
Compounds: Ionic and Molecular
NaCl
Water
Caffeine
Citric acid
How to know?
MgBr2
CCl4
K2S
H2O
CuSO4
C6H12O6
• 1. Most metal-containing compounds are ionic.
• 2. If there is no metal in the formula, it is
likely that the compound is not ionic (except
compounds from polyatomic ions based on
nonmetals, ex: NH4NO3).
• Learn the formulas of polyatomic ions.
What are Ions?
How are Ions formed?
• 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 Ions
Predicting Ions Charges
Look at Fig. 3.7 (page 105).
In general,
Metals (Li) loose electrons to become
CATIONS.
Nonmetals (F) gain electrons to
become ANIONS.
Charges on Common Ions
-4 -3 -2
+1
-1
+2
+3
By losing or gaining e-, the atom has the same
number of e-’s as nearest Group 8A atom.
Charges on Monoatomic Ions
Students should become familiar with the
charges of common ions.
Metals and their Ions
M ---> n e- + Mn+
where n = periodic group
Na – Group 1
Na+ sodium ion
Mg – Group 2
Mg2+ magnesium ion
Al – Group 3
Al3+ aluminum ion
Transition metals --> M2+ or M3+
are common
Fe2+
iron(II) ion
Fe3+
iron(III) ion
Nonmetals and their Ions
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
Name of monoatomic
anion is derived by
adding -ide to stem
Br-, bromide
I-, iodide
Polyatomic Ions
• Groups of atoms with a charge.
• MEMORIZE the names and formulas in Table
3.1, page 107.
Students should become familiar with formulas,
names, and charges of polyatomic ions.
Polyatomic Ions
Note: many O
containing anions have
names ending in –ate
(or -ite).
Naming Polyatomic Ions
NH4+
ammonium ion
One of the few common
polyatomic cations
Compounds formed from Ions
CATION + ANION --->
COMPOUND
Na+ + Cl- --> NaCl
+1 + (-1) = 0
A neutral compound
requires
equal number of
+ and - charges.
What are Ionic Compounds?
• Ionic compounds are composed of ions.
NH4+
ClStudents should become
familiar with formulas and
names ionic compounds.
ammonium chloride, NH4Cl
Formation of Ionic Compounds
Ca2+ + 2 F- ---> CaF2
Mg2+ + NO3- ---->
Mg(NO3)2
magnesium nitrate
Fe2+ + PO43- ---->
Fe3(PO4)2
iron(II) phosphate
calcium fluoride
Formation of Ionic Compounds
• A metal atom
transfers an
electron to a
nonmetal.
• The resulting cation
and anion are
attracted to each
other by
electrostatic
forces.
Formation of Ionic Compounds
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 and Ionic
Compounds
• What happens to the attractive force when
the charge on the ion increases?
• What happens to the attractive force when
the distance between ions increases?
Electrostatic Forces
Coulomb’s Law
Importance of Coulomb’s Law.
Why is the m.p. of MgO higher than that
of NaCl?
NaCl, Na+ and Cl-,
m.p. 804 oC
MgO, Mg2+ and O2m.p. 2800 oC
Melting Point
• Melting point is the temperature at which a
substance goes from solid to liquid.
• In a solid: particles are fixed, in ionic solid
they are fixed by very strong electrostatic
attractions.
• In a liquid: particles move.
• In order to change the substance from solid
to liquid (get the particles moving) the
particles need some energy: just enough to
break the electrostatic attractions.
• The stronger the attraction, the more energy
is needed to overcome it: a higher melting
point results.
Practice
Select the compound with the higher melting point:
NaCl, AlN
KCl, CsI
Some general properties
• The compound is ionic:
– The compound would conduct electricity if molten.
– The compound dissolved in water it would be a
strong electrolyte (produce plenty of ions).
– The compound would be expected to be a solid at
room temperature and pressure.
– The compound would be expected to have a
relatively high melting point.
• The compound is molecular:
– If the compound dissolved in water it would be a
non-electrolyte.
– The compound would be expected to have a
relatively low melting point.
Elements that exist as Molecules
Allotropes of Carbon
C
C
C60
Elements that exist as
Diatomic Molecules
Elements that exist as
Polyatomic Molecules
S8 sulfur
molecules
White P4 and polymeric
red phosphorus
What are Molecular Compounds?
• Compounds without ions.
Carbon dioxide, CO2
Methane, CH4
Boron trichloride, BCl3
Molecular compounds
are formed from two
or more nonmetals.
Students should become familiar with formulas
and names of molecular compounds.
Practice
• Which of the following are ionic, and which
are molecular? Give their name.
NaI
CCl4
Ca(NO3)2
FeCl3
NF3
NH4CH3COO
NaBr3
Empirical and 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
Mass percent
• Expression of molecular composition in
terms of the mass of each element in
the compound relative to the total mass
of the compound.
NO2
Mass % N in NO2 =
Mass of N in 1 mol NO2
Mass of 1 mol NO2
Percent Composition
Consider some of the family of
nitrogen-oxygen compounds:
NO2, nitrogen dioxide and closely related,
NO, nitrogen monoxide (or nitric oxide).
What is the weight percent of N and of
O in NO2? In NO?
Students should become familiar with
calculations of % weight, empirical
and molecular formulas.
What is the weight percent of N and of O
in NO2? In NO?
How can Molecular Formulas be
determined?
• Molecular formulas can be determined
from chemical analysis or instrumental
methods, such as mass spectrometry.
CH2O+
31
CH3CH2O+
45
CH3CH2OH+
46
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.
Benzene
-Aromatic compound
H
C
HC
CH
HC
CH
C
H
C6H6
CH
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.
1. Calculate the number of moles of each
constituent in 100 g of sample.
A compound of B and H is 81.10% B.
What is its empirical formula?
2. Find the ratio of moles of elements in the
compound. Take the ratio of moles of B and H. Always
divide by the smallest number. Find a whole number ratio.
Deriving Formulas
• Steps from % composition to formula:
Convert weight
percent to mass
Convert mass
to moles
%A
gA
X mol A
%B
gB
X mol B
Ratio gives formula
AxBy
Find mole ratio
x mol A
y mol B
Always divide the
larger number by
the smaller one.
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
1. Find the ratio of these masses.
Determine the MOLECULAR Formula from
Combining Masses (see page 125).
Sn(s) + some I2(s) ---> SnIx
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
1. Find the mass of Sn (used in excess) that
combined with 1.947 g I2 (limiting reactant).
2. Find moles of Sn used:
3. Now find the number of moles of I2 that
combined with the mol Sn. Mass of I2 used
was 1.947 g.
4. Calculate the mol of Iodine atoms:
5. Find the ratio of the number of moles of I
and S that combined.
What are Hydrated Compounds?
• Hydrated compounds are those in which
molecules of water are associated with the
ions of the compound.
Anhydride
CoCl2. 6H2O
CoCl2
Hydrated Compounds
CoCl2.6H2O
Atoms in formula of compound:
Practice
• Ten grams of each substance contains the
largest number of moles?
H2O
Kr
BaCl2 . H2O
Br2
Using molar masses, calculate number of moles.
Know that the larger the molar mass, the
fewer number of moles.
Practice
• How many nitrate ions are found in 1.50 moles
of calcium nitrate?
Particles in a mole?
Avogadro’s number
Remember
• Go over all the contents of your
textbook.
• Practice with examples and with
problems at the end of the chapter.
• Practice with OWL tutor.
• Practice with the quiz on CD of
Chemistry Now.
• Work on your assignment for Chapter 3.