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Covalent BondsBoth atoms involved (typically
nonmetal) “want” to gain e- to become stable
Electrons are shared in order to allow this to happen
The number of e- shared depends on the element
When atoms share two, or more, electron(s), they form a molecule
Compounds formed with covalent bonds are neutral and, generally, follow the OCTET rule!
All atoms are most stable when they have the electron configuration of a noble gases (THIS MEANS HAVING EIGHT* VALENCE ELECTRONS)* For nonmetals the most common
exception to the octet rule (when forming bonds) is HYDROGEN.
Highly Stable = Noble gas configuration
Hydrogen - will share 1 e-
Oxygen - will share 2 e-
Nitrogen - will share 3 e-
Chlorine - will share 1 e-
Carbon - will share 4 e-
Covalent Bonding in Hydrogen
Electron sharing can occur only when electron orbitals from two different atoms overlap.
Formation of a Covalent Bond
FG08_006.JPG
The number of covalent bonds formed by a nonmetallic element is often directly related with the number of electrons it must share (commonly equivalent to the number of lone pairs of e- the atom has) in order to obtain an octet of electrons.
Types of Covalent BondsSingle - ONE pair of e- sharedDouble - TWO pair of e- sharedTriple - THREE pair of e- sharedCoordinate - Both electrons
being shared originate from a single atom
(a) A “regular” covalent single bond is the result of overlap of two half-filled orbitals. (b) A coordinate covalent single bond is the result of overlap of a filled and a vacant orbital.
- atoms participating in cc bonding generally do not form their normal # of covalent bonds
Ex.: HO2Cl, CO
Types of Covalent Bonds
Electron Dot Structuresof HCl
FG08_009.JPG
Lewis StructuresStructures which represent in a
drawing the arrangement of the atoms and the types of covalent bonds
There are FIVE basic steps to follow.
We’ll use water as an example:).
Step 1:Arrange the atoms!
Remember, Hydrogens always on the periphery
Use the expected bonding patterns to arrange the atoms
• the atom that forms the most bonds is typically in
the middle of the structure.
Step 2:
Count the electronsObtain a total to work from
Step 3:Add the bonds & lone
pairsRemember to give each atom (except H) an “octet” of e- (but, don’t exceed the number of e- available)
Step 4:
Use multiple bonds to fill octets when neededConvert one lone pair to a bonding pair for each pair of e- needed to complete an octet
Step 5:Exceptions to the octet
ruleLess than an octet
Hydrogen & BoronMore than an octet
Phosphorus & Sulfur, Noble gases
Lewis Structures - examples
CCl4PBr3
F2
H2S
NH4+
SO4-2
O3
Double BondsO2
C2H4
CO2
CH2CHCHCH2
NO3-
CO3-2
Triple bondsN2
C2H2
HCN
Exceptions (more than an octet) - Only elements in rows 3 and beyond Why?PCl3 and NCl3
PCl5 but not NCl5
XeF4
SF6
Resonance
Some molecules have measured values of bond lengths which do not support the Lewis structure drawn for the molecule
Example: Ozone, O3
To adequately represent such molecules with Lewis structures, you should draw all possible arrangements of ELECTRONS.
Naming Binary Molecular Compounds
Used only with the second element
Ex. Dihydrogen monoxide
Ex. Tetraphosphorus decoxide
Common vs. Chemical Names
Chemical Name
3-D arrangements of electron pairs
Arrangement of valence electron pairs about a central atom that minimize repulsions between the pairs.Since double & triple bonds are multiple electron pairs in the same location, they act like a single pair when determining the geometry of the molecule
3-D Models of Molecules
(a) Acetylene molecule. (b) Hydrogen peroxide molecule. (c) Hydrogen azide molecule.
Bonding
Molecular Geometry - VSEPR
Bond Polarity & Electronegativity
The difference in the electronegativities (ability of an atom to attract electrons) of the two bonded atoms can be used to define the “polarity” of the bond.
Abbreviated periodic table showing Pauling electronegativity values for selected representative elements.
Bond Polarity
(a) In the nonpolar covalent bond present, there is a symmetrical distribution of electron density. (b) In the polar covalent bond present, electron density is displaced because of its electronegativity.
•Bond LengthsBond lengths are measured
using nucleus-nucleus distances.
For bonds between the same two atoms:Single > Double > Triple
Example: C-O
Molecular Polarity
(a) Methane is a nonpolar tetrahedral molecule.
(b) Methyl chloride is a polar tetrahedral molecule.
Hint: Is the molecule SYMMETRICAL?
Determining Molecular Geometry & Polarity- a Shortcut!Geometry Polarity
A. Look @ center atom1. All bonds non-polar = NON-
POLAR molecule2. Bonds are Polar
a) Only VSEPR bonding groups - go to B.
b) Has VSEPR nonbonding groups = POLAR molecule
B. Look @ attached atoms1. All attached atoms the
same = NON-POLAR molecule
2. One or more different element’s atoms attached = POLAR molecule
Examples- use Lewis structures to guide you
Determine the geometry & polarity (look for symmetry) of these “molecules”:
1. HF2. H2O
3. SF2
4. NI35. SiBr4
6. SeO3
7. CO2
8. CO3-2
9. Na2SO4