Chemistry
10/21/11
Brainteaser
• Name these ionic compounds:
– Fe2S3
– Ca(OH)2
– MnCrO4
• Write the correct formula of these
compounds
– Zinc sulfate
– Magnesium nitrate
– Potassium permanganate
Gaining an Octet
• There are two ways that elements want to be
like the nearest noble gas:
1) Gain or lose electrons to form an ionic
compound.
2) Share electrons with other elements to
form covalent compounds.
How does this happen?
• Whenever two nonmetals bond to each other,
electrons don’t get transferred because both
elements have similar electronegativities (i.e.
they both want to gain electrons to be like the
nearest noble gas).
– In ionic bonding (as with NaCl, Cl is
electronegative and Na isn’t, so Na doesn’t mind
giving electrons to Cl). This way, both fill their
octets.
– In covalent bonding (as with F2), both elements
have similar electronegativities so neither will give
electrons to the other. As a result, they’re forced to
share electrons.
Definitions
• Covalent compound: A compound formed
when nonmetals bond by sharing two or more
valence electrons.
–Share valence electrons  co”valent”
compound
• Covalent bond: A chemical bond formed
when nonmetal atoms share two valence
electrons.
The Octet Rule and
Covalent Compounds
 Covalent compounds tend to form so that each
atom, by sharing electrons, has an octet of
electrons in its highest occupied energy level.
 Covalent compounds involve atoms of nonmetals
only.
 The term “molecule” is used exclusively for
covalent bonding
The Octet Rule:
The Diatomic Fluorine Molecule
F
F
1s
2s
2p
1s
2s
2p
F F
Each has seven
valence
electrons
The Octet Rule:
The Diatomic Oxygen Molecule
O
O
1s
2s
2p
1s
2s
2p
O O
Each has six
valence
electrons
The Octet Rule:
The Diatomic Nitrogen Molecule
N
N
1s
2s
2p
1s
2s
2p
N N
Each has five
valence
electrons
Lewis Structures
Lewis structures show how valence electrons
are arranged among atoms in a molecule.
Lewis structures Reflect the central idea that
stability of a compound relates to noble gas
electron configuration.
Shared electrons pairs are covalent bonds and
can be represented by two dots (:) or by a
single line ( - )
Properties of Compounds
• All properties of covalent compounds are
determined by the fact that covalent
compounds form molecules, while ionic
compounds form crystals.
• A good model for thinking of this:
–Ionic compounds are like stacks of Legos all
locked together into a big block.
–Covalent compounds are like rubber balls
thrown together into the same bucket.
Properties contd.
• Low Melting and Boiling Points
–In covalent compounds, the molecules only
have very weak forces (called Van der Waals
forces) holding them to each other. As a
result, covalent compounds can be found as
solids, liquids, or gases at standard room
conditions.
• BONDS ARE NOT BROKEN WHEN THERE IS A
STATE CHANGE!!!
• How is this different than ionic compounds?
–Magnetically stuck together in big crystals
Naming Binary Covalent
Compounds
• Write the name of the first nonmetal
• Write the name of the second nonmetal with the
ending changed to the suffix “–ide.”
• Insert prefixes into the name to reflect subscripts
in the formula.
mono =1
di = 2
tri = 3
tetra = 4
penta = 5
hexa= 6
hepta= 7
octa = 8
nona= 9
deca = 10
• Never start a name with the prefix “mono-”
Practice
• Name the following binary molecular
compounds
– CCl4
– CO
– SO2
– NF3
Practice
• Write the formula of these binary
molecular compounds
– Dihydrogen monoxide
– Tricarbon tetrachloride
– Diphosphorus pentoxide
– Sulfur dioxide
Review
• Rules for naming ionic compounds
– Metal (+) and Non-metal (-)
– Monatomic (only one atom present)
• O2-
– Polyatomic (more than one atom present)
• Example: NO3-
• Rules for naming covalent compounds
– Non-metal(-) and Non-metal(-)
• Hydrogen is a non-metal (if it comes after a non-metal)
– CH4
– Use Prefixes (Never start with Mono in the first non-metal)