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Chemical Bonding

1. Lewis Dot Structures

2. Electronegativity

3. VSEPR

4. Polarity

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Chemical Bonds

• Chemical bond– A force that holds

atoms together

– Ionic Bonds

– Covalent Bonds

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Figure 8.2

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Table 8.02

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Ionic Bond

• electrostatic attraction • oppositely charged ions• metal and a nonmetal• Electrons transferred • Extremely strong bonds

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Structures of Ionic Crystals• Ionic crystal

– regular geometric pattern that maximizes the attractive forces and minimizes the repulsive forces.

– Hard and brittle– Can shatter if struck forcefully – Charge and size of ions determines structure

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Ionic Bonding

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Structures of Ionic Crystals• Crystal lattice

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Practice – Lewis SymbolsDetermine the valence electrons

and place themLi

Ca

Fe

Cl

P

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Covalent Bonds• sharing of electrons • nonmetals • a neutral overall charge• Electrons not

transferred• Electrons shared • Weaker bonds than

ionic bonds

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Figure 8.2 or another

molecule picture

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Diatomic molecules

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Covalent Bonding•

– shared by two atoms

– Each atom contributes one electron to the bond

• The orbitals overlap • electron pair is between

two atoms

– Lewis formula• atoms are shown

separately • valence electrons are

represented by dots

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Figure 8.14

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Covalent Bonding• Multiple covalent bonds

– more than one pair of e- shared

– Double bond• two pairs of electrons (4 e-

total)• a double bond is represented

by 4 dots or 2 parallel lines

– Triple bond• three pairs of electrons (6

electrons total)• a triple bond is represented by

6 dots or 3 parallel lines.8 -

Figure 8.17

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Practice – Identifying Types of Bonding

Between Calcium and Fluorine

Ca F

Between carbon and hydrogen

C H

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Polar vs. Nonpolar Covalent Bonds

– Polar • Unequal sharing of

electrons• different elements

bonded – different

electronegativities

– Nonpolar • Equal sharing of

electrons • same elements form

a bond– Same

electronegativities

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Type of bond predictions

• Electronegativity difference

• 0-0.4 nonpolar covalent equal sharing

• 0.4-2.0 polar covalent unequal sharing

• > 2.0 ionic bond transfer of electrons

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Polar vs. Nonpolar

• Nonpolar covalent bonds are:

– Typically longer bonds

– Weaker bonds

• Polar covalent bonds are: – Typically shorter

bonds – Stronger bonds

(more ionic )

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Practice – Polar Bonds

Water

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• Polarity– polar covalent molecules

– degree of transfer of electrons in a covalent bond

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Sigma bond

• When two atomic orbitals combine to form a molecular orbital that is symmmetrical along the axis connecting two atomic nuclei, a sigma bond is formed.

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Pi bond

• The side by side overlap of atomic p orbitals produces what are called pi molecular orbitals. When a pi molecular orbital is filled with two electrons, a pi bond results.

• pi bonds tend to be weaker than sigma bonds.

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Greater difference in E-negativity between atoms

– A polar covalent bond• water

– Increased ionic character• NaCl

– Decreased bond length and increased bond strength

• nonpolar covalent bond– e.g. H2 (g)

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Electronegativity

Atoms attract bonding electrons

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Electronegativity

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Figure 8.5

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Trends in Electronegativity

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Figure 8.6

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Polar bonds in Non-polar molecules

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Lewis Dot Symbols & Octet Rule– Dots represent valence electrons

– Pair electrons as needed

– Octet rule• 8 valence electrons

– Same e configuration as noble gas

– 8 e- exist in 4 pairs

– Ions - 8 e- by losing or gaining electrons– Ex. H reacts to obtain a total of 2 electrons like He.

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Steps for Writing Lewis Dot Structures

1. Write an atomic skeleton.• atoms usually symmetrical.

• The central atom, the one surrounded by the other atoms, tends to be the one that is less electronegative and is present in the least quantity.

• Hydrogen atoms are generally on the outside of the

molecule.

• The chemical formula may give clues

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Writing Lewis Dot Structures2. Add up all the atom’s electrons to get the total

3. Place two electrons between each pair of bonded atoms.

4. Add any remaining electrons as unshared • electron pairs, consistent with the octet rule.• first to complete the octet of atoms the central atom.

5. Make double or triple bonds to satisfy octet rule

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Shapes of Molecules• electron determine shape.• electrons repel one another• orbitals stay as far apart as

possible.

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• Valence shell electron pair repulsion (VSEPR) theory– electron pairs adjust their orbitals – maximize the distance– bonded atoms and unshared pairs stay

as far apart as possible• Bond angle

– bond angle between the central atom and the atoms bonded to it

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VSEPR Parent Structures

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Table 8.5

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VSEPR Derivative Structures

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Steps for VSEPR Structures1. Draw a Lewis formula.

2. # atoms bonded to the central atom

3. count # unshared pairs on central atom

4. # of atoms bonded + # of unshared e-

5. The total indicates the parent structure.

Lone pairs + bonding pairs = shape

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Natural Applications of VSEPR Theory• Heme molecule

– Oxygen is carried throughout the body via red blood cells containing heme molecules.

– Histidine, an amino acid in the heme molecule, just fits into the space next to the oxygen molecule.

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Polarity of Molecules

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Figure 8.32

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Polarity of Molecules• Diatomic molecules

– Polarity lies along the plane of the bond

• Polyatomic molecules– A nonpolar molecule = polar

bonds that cancel out

– A polar molecule is one that has polar bonds that DO NOT cancel out

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