ch 9-section-1
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Chapter 9
COVALENT BONDING
Review….What is a chemical bond?
Force that holds two atoms togetherWhat is an ionic bond?
An electrostatic force that holds oppositely charged particles together in an ionic compound
Compounds formed from metal & nonmetal
Forms when….?What are atoms always trying to
achieve?StabilityComplete set of valence electrons…
OCTECT
What is a covalent bond?Chemical bond that results from sharing
of valence electronsOccurs b/w nonmetal & a nonmetalBalance b/w attractive and repulsive forces
2 Hydrogen Atoms
Sharing their 1 Ve-
MoleculesCompound made when 2 or more atoms are
bonded covalentlyDiatomic molecules
In nature, sometimes two atoms of the same element are more stable when they are covalently bonded than the individual atom alone…
BrINClHOF (pronounced “Brinkle Hoff”)Br2 I2 N2 Cl2 H2 O2 F2
Single Covalent BondsA single covalent bond –Atom shares 1 pair (2)
electrons.Shared pairs – both elements count the electron
pair to achieve octetLone pairs – pair of electrons that are not shared
b/w the atomsLewis structures- Use electron dot diagram to
show how atoms are arranged in a molecule.
H Cl
shared or Bond pair
Unshared orLone pair (LP)
. .
. .. .
In the Fluorine Molecule…..How many bonding pairs are there in
each?1
How many lone pairs are there each?3
Multiple Covalent Bonds
Double covalent bonds share two pairs of electrons. CO2 O=C=O
Triple covalent bonds share three pairs of electrons.N2 :N=N:
Covalent Bond Formation in Hydrogen
Increased overlap brings the electrons and nuclei closer together while simultaneously decreasing electron-electron repulsion.
However, if atoms get too close, the internuclear repulsion greatly raises the energy.
The attractive and repulsive forces in covalent bonding must be balanced.
– Bond Length - In general, the closer the electrons are held by the atoms, the shorter the bond length and the
higher the bond energy.
– Multiple bonds result in stronger, shorter bonds.
– Bond Energy - The amount of energy required to break a bond. The greater the energy, the stronger the bond.
– Bond breaking is an endothermic process, so bond breaking enthalpies are positive.
Comparing Bond Length and Bond Strength
Using the periodic table, but not Tables 9.2 and 9.3, rank the bonds in each set in order of decreasing bond length and bond strength:
(a) S F, S Br, S Cl
(b) C = O, C O, C O
Sigma () Bonds
Sigma bonds are characterized byHead-to-head overlap.Cylindrical symmetry of electron density
about the internuclear axis.
Pi () BondsPi bonds are
characterized bySide-to-side
overlap.Electron density
above and below the internuclear axis.
Single Bonds vs. Multiple bonds
Single bonds are always bonds, because overlap is greater, resulting in a stronger bond and more energy lowering.
In a multiple bond one of the bonds is a bond and the rest are bonds.
Orbital overlap
Lewis Dot Structures1. Determine the number of Valence e- for all
atoms in the moleculea. Divide the Ve- by 2 to get pairs (2 dots or 1 line)
2. Decide on central atom (least electronegative or furthest to the left).
a. Hydrogen & halogens are terminal atoms
3. Connect all atoms to the central atom by a bonding pair (single line)
4. Place remaining pairs around all atoms before moving on to central atom.
5. Check for octet (not H)a. If atom does not have an octet, move lone pairs
from a terminal atom to create a double or a triple bond (except grp 7).