Molecules and Ions
AP Chemistry, Unit 2, Presentation 1
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Molecules
Two or more atoms of the same or
different elements, covalently bonded
together.
Molecules are discrete structures, and
their formulas represent each atom
present in the molecule.
Pentane, C5H12
Covalent Network Substances
Covalent network substances have covalently
bonded atoms, but do not have discrete
formulas.
Why Not??
Graphene – carbon allotrope
Ions
Cation: A positive ion
Mg2+, NH4+
Anion: A negative ion
Cl-, SO42Ionic Bonding: Force of attraction
between oppositely charged ions.
Ionic compounds form crystals, so their
formulas are written empirically (lowest
whole number ratio of ions).
Predicting Ionic Charges
Group 1: Lose 1 electron to form 1+ ions
H+
Li+ Na+
K+
Rb+
Cs+
Predicting Ionic Charges
Group 2: Loses 2 electrons to form 2+ ions
Be2+
Mg2+
Ca2+
Sr2+
Ba2+
Predicting Ionic Charges
B3+ Al3+
Ga3+
Group 13: Loses 3
electrons to form
3+ ions
Predicting Ionic Charges
C22-
C4-
Caution!
and
are both called carbide
Group 14: Loses 4
electrons or gains
4 electrons
Predicting Ionic Charges
N3- Nitride
P3- Phosphide
As3- Arsenide
Group 15: Gains 3
electrons to form
3- ions
Predicting Ionic Charges
O2- Oxide
S2- Sulfide
Se2- Selenide
Group 16: Gains 2
electrons to form
2- ions
Predicting Ionic Charges
F1-
Fluoride
Cl1-
Chloride
Br1-
Bromide
I1-
Iodide
Group 17: Gains 1
electron to form
1- ions
Predicting Ionic Charges
Group 18: Stable
Noble gases do
not form ions!
Predicting Ionic Charges
Groups 3 - 12: Many transition metals have
more than one possible oxidation state.
Iron(II) = Fe2+
Iron(III) = Fe3+
Predicting Ionic Charges
Groups 3 - 12: Some transition metals
have only one possible oxidation state.
Zinc = Zn2+
Silver = Ag+
Writing Ionic Compound Formulas
Example: Barium nitrate
1. Write the formulas for the cation and anion,
including CHARGES!
2. Check to see if charges are balanced.
3. Balance charges , if necessary, using subscripts.
Use parentheses if you need more than one of a
polyatomic ion.
Ba2+ ( NO3- ) 2
Not
balanced
Writing Ionic Compound Formulas
Example: Ammonium sulfate
1. Write the formulas for the cation and anion,
including CHARGES!
2. Check to see if charges are balanced.
3. Balance charges , if necessary, using subscripts.
Use parentheses if you need more than one of a
polyatomic ion.
( NH4
+)
2
SO42-
Not
balanced
Writing Ionic Compound Formulas
Example: Iron(III) chloride
1. Write the formulas for the cation and anion,
including CHARGES!
2. Check to see if charges are balanced.
3. Balance charges , if necessary, using subscripts.
Use parentheses if you need more than one of a
polyatomic ion.
Fe3+ Cl-
3
Not
balanced
Writing Ionic Compound Formulas
Example: Aluminum sulfide
1. Write the formulas for the cation and anion,
including CHARGES!
2. Check to see if charges are balanced.
3. Balance charges , if necessary, using subscripts.
Use parentheses if you need more than one of a
polyatomic ion.
3+
Al
2
2S
Not
balanced
3
Writing Ionic Compound Formulas
Example: Magnesium carbonate
1. Write the formulas for the cation and anion,
including CHARGES!
2. Check to see if charges are balanced.
2+
Mg
CO32-
They are
balanced
Writing Ionic Compound Formulas
Example: Zinc hydroxide
1. Write the formulas for the cation and anion,
including CHARGES!
2. Check to see if charges are balanced.
3. Balance charges , if necessary, using subscripts.
Use parentheses if you need more than one of a
polyatomic ion.
Zn2+ ( OH- ) 2
Not
balanced
Writing Ionic Compound Formulas
Example: Aluminum phosphate
1. Write the formulas for the cation and anion,
including CHARGES!
2. Check to see if charges are balanced.
Al3+ PO43They ARE balanced
Naming Ionic Compounds
Cation first, then anion
Monatomic cation = name of the
element
Ca2+ = calcium ion
Monatomic anion = root + -ide
Cl- = chloride
CaCl2 = calcium chloride
Naming Ionic Compounds
Metals with multiple oxidation states
 some metal forms more than one cation
 use Roman numeral in name
 PbCl2
 Pb2+ is cation
 PbCl2 = lead(II) chloride
Binary Molecular Compounds
 Compounds between two nonmetals
 First element in the formula is named first.
 Keeps its element name
 Gets a prefix if there is a subscript on it
 Second element is named second
 Use the root of the element name plus the
-ide suffix
 Always use a prefix on the second element
 1 = mon(o)
 2 = di
 3 = tri
 4 = tetra
 5 = penta
 6 = hexa
 7 = hepta
 8 = octa
 9 = nona
 10 = deka
List of
Prefixes
Naming Binary Compounds
P2O5 = diphosphorus pentoxide
CO2 = carbon dioxide
CO = carbon monoxide
N2O = dinitrogen monoxide
Practice – Write the Formula
Compound Name
Compound Formula
Carbon dioxide
Carbon monoxide
Diphosphorus pentoxide
Dinitrogen monoxide
Silicon dioxide
Carbon tetrabromide
Sulfur dioxide
Phosphorus pentabromide
Iodine trichloride
Nitrogen triiodide
Dinitrogen trioxide
Check next slide for answers
Answers – Write the Formula
Compound Name
Compound Formula
Carbon dioxide
CO2
Carbon monoxide
CO
Diphosphorus pentoxide
P2O5
Dinitrogen monoxide
N2O
Silicon dioxide
SiO2
Carbon tetrabromide
CBr4
Sulfur dioxide
SO2
Phosphorus pentabromide
PBr5
Iodine trichloride
ICl3
Nitrogen triiodide
NI3
Dinitrogen trioxide
N2O3
Practice – Name the Compounds
Compound Formula
Compound Name
N2O4
SO3
NO
NO2
As2O5
PCl3
CCl4
H2O
SeF6
Check next slide for answers
Answers – Name the Compounds
Compound Formula
Compound Name
N2O4
dinitrogen tetroxide
SO3
sulfur trioxide
NO
nitrogen monoxide
NO2
nitrogen dioxide
As2O5
diarsenic pentoxide
PCl3
phosphorus trichloride
CCl4
carbon tetrachloride
H2O
dinitrogen monoxide
SeF6
selenium hexafluoride