Molecular compounds are made of molecules.
each molecule contains anywhere from two atoms (diatomic molecules) to thousands (biological molecules).
each molecule has the same element composition and properties as the compound.
synonym: covalent compound
examples: H2O, CO2, C6H12O6, NH3, CH4
Ionic compounds are made of positive ions (cations) and negative ions (anions).
cations combine with anions in just the right numbers to give an electrically neutral compound.
metals form cations easily, and nonmetals form anions, so metal/nonmetal compounds are often ionic
cations and anions pack into orderly arrays in solids; they become mobile when the compound melts
individual molecules don't normally exist!
an ion pair
an ion cluster
an ion crystal
examples: NaCl, KBr, Na2S, MgBr2
synonym: salts
Molecular compounds
Ionic compounds
smallest particles
molecules
ions
elements present
close on the periodic table
widely separated on the periodic table
electrical conductivity
poor
good, when melted or dissolved
state at room temperature
solid, liquid, or gas
solid
other names
covalent compounds
salts
Formulas for molecular compounds
A molecular formula shows the type and number of atoms in a molecule
type of atom indicated by element symbol
number of atoms per molecule indicated by subscripts (if greater than one)
H2O contains 2 hydrogen atoms and one oxygen atom per molecule.
Carbon tetrafluoride, CF4, contains four fluorine atoms and one carbon atom per molecule.
atoms in formulas are sometimes grouped to show how they're connected in the molecule
Methanol is usually written as CH3OH to show that 3 hydrogens are bound to the carbon and another hydrogen is bound to the oxygen.
Acetic acid can be written as CH3COOH or as HC2H3O2 or C2H4O2. The first formula shows how the molecule is put together; the second formula emphasizes that one hydrogen is different from the others; the third formula is the least informative because it shows only the numbers and types of atoms in the molecule.
groups that appear more than once in the molecule are enclosed in parentheses
CH3(CH2)3CH3 could be written as C5H12, but all information about the structure of the molecule would be lost.
(CH3CH2)4P2O7 molecules contain 8 carbons, 20 hydrogens, two phosphoruses, and seven oxygens.
molecular weight = sum of weights of atoms in the molecule
The molecular weight of CH3OH is approximately 12 + 4*1 + 16 = 32 since the carbon, hydrogen, and oxygen have atomic weights of 12, 1, and 16 u, respectively.
Formulas for ionic compounds
empirical formula gives the elemental composition of a compound
formula lists elements present by element symbol
subscripts give ratios of ions or atoms in the compound
CuSO4 contains 4 atoms of O and 1 atom of S for every 1 atom of Cu.Na2CO3 contains 2 atoms of Na and 3 atoms of O for every 1 atom of C.
writing ionic empirical formulas
write the cation formulas, including charge
write the anion formulas, including charge
combine enough cations with enough anions to give a total charge of zero
trick: swap charges as subscripts
don't write charges when the ions are combined
use the simplest (lowest) cation-to-anion ratio possible
list cations first, anions last
Potassium ions (K+) and chloride ions (Cl-) combine to give potassium chloride, KClCalcium ions (Ca2+) and bromide ions (Br-) combine to give calcium bromide, CaBr2Aluminum ions (Al3+) and sulfide ions (S2-) combine to give aluminum sulfide, Al2S3
naming ionic compounds from formulas
name the anions
name the cations
recall that the names of transition metal and main group cations must include their charge as a Roman numeral.
the name of compound is cation name followed by anion name
Na2S contains sodium ion and sulfide ion. The compound is sodium sulfide. SnCl4 contains a tin cation and four chloride ions. Each chloride carries a -1 charge, so the tin must have a +4 charge. The compound is tin(IV) chloride.
the formula weight is the sum of atomic weights for atoms in the formula
NaOH
NaOH:23.0 u+ 16.0 u+ 1.0 u40.0 u
NaCO
Na2CO32×23.0 u+ 12.0 u+ 3×16.0 u106.0 u
NHSO
(NH4)2SO42× 14.0 u+ 2×4×1.0 u+ 32.0 u+ 4×16.0 u132.0 u
Polyatomic ions
definition: ions formed from more than one atom
examples: ammonium (NH4+), hydroxide (OH-), nitrate (NO3-), sulfate (SO4-)
polyatomic ions retain their identity within ionic compounds, and in many reactions
names, formulas and charges of common polyatomic ions should be memorized!
formulas for ionic compounds containing polyatomic ions are written as usual, except:
put parentheses around polyatomic ions whenever there are more than one
don't break up polyatomic ions (write Ca3(PO4)2, not Ca3P2O8)
Structural formulas
definition: a map that shows how atoms are bonded within a molecule
molecular formula
structural formula
molecular model
H2O
structural formulas of polyatomic ions are bracketed, with the charge indicated by a superscript
NH4+
Sticks indicate shared electron pairs. There can be more than one pair shared, as in the C=O group in this acetic acid molecule.
HC2H3O2
Finding chemical formulas experimentally
to obtain a molecular formula from an empirical formula:
you must know the molecular weight
compute the ratio of molecular weight to formula weight
multiply all subscripts in the empirical formula by this ratio
examples
Give the molecular formula for a compound with empirical formula CH and molecular weight 78.11 daltons.
Give the molecular formula for a compound with empirical formula CH2O and molecular weight 180 daltons.
Acids
release hydrogen ions when dissolved
leading H's in formula are acidic hydrogens
guidelines for naming acids
name the anion within the acid
change the anion ending to one of the following:
anion ending
in acid, replace with:
examples
*ide
hydro*ic acid
hydrochloric acid, HCl
-ite
-ous acid
nitrous acid, HNO2
-ate
-ic acid
sulfuric acid, H2SO4
Binary Covalent Compounds
naming binary covalent compounds
write the name of the first nonmetal
write the name of the second nonmetal with the ending changed to -ide
insert prefixes into the name to reflect subscripts in the formula:
1
hexa-
6
di-
2
hepta-
7
tri-
3
octa-
8
tetra-
4
nona-
9
penta-
5
deca-
10
never start a name with mono-
examples
N2S5
dinitrogen pentasulfide
NO2
nitrogen dioxide
S2Cl2
disulfur dichloride
N2O4
dinitrogen tetroxide
SF6
sulfur hexafluoride
N2O5
dinitrogen pentoxides
common names should be used for the following compounds:
formula
common name
H2O
water
H2O2
hydrogen peroxide
H2S
hydrogen sulfide
N2O
nitrous oxide
NO
nitric oxide
NH3
ammonia
N2H4
hydrazine
Addition Compounds
contain several compounds packed in regular way into a crystal
formulas of addition compounds
added compounds are separated by a dot
prefix compound with the number of times it occurs per formula unit
naming addition compounds
translate numbers after a dot into Greek prefixes
name each compound in order
cross out first redundant ion names
water in addition compounds is called hydrate
hydration waters can be driven off by strong heating
compound with all hydration waters driven off is called anhydrous
anhydrous salts usually absorb water from air to become hydrates again
examples
Formula
Name
Na2SO4 · 10H2O
sodium sulfate decahydrate
CuSO4 · 5H2O
copper(II) sulfate pentahydrate
CaCO3 · MgCO3
calcium magnesium carbonate
carbonate is named only once
CaHPO4 · 2H2O
calcium hydrogen phosphate dihydrate
Ga2(SO4)3 · 18H2O
gallium(III) sulfate 18-hydrate
Use numbers instead of prefixes when the number is larger than 12
ZnSO4 · (NH4)2SO4 ·6H2O
zinc ammonium sulfate hexahydrate
zinc is always +2, so zinc(II) is not necessary
