Covalent/Molecular Bonding

Ch. 16

The Nature of CovalentBonding

16-1Skip pgs 444 - 451

Covalent Bonds• Covalent (molecular) bond = the attraction

of two atoms for a shared pair of electrons– Neither atom will have an ionic charge– Usually between 2 nonmetals (some involve

metalloids)!• Covalent compound = a compound whose

atoms are held together by covalent bonds• Molecule = an uncharged group of two or

more atoms held together by covalent bonds

Single Covalent Bonds

• Single Covalent Bond = 2 atoms share one pair of electrons.– H2, F2, H2O

• Structural Formula = chemical formula that shows the arrangement of atoms.– H + H H H H – H Element Compound Structural dots dots formula

– F + F F F F - F

– H + O O H O - H H H H

F2:

H2O:

HCH4 C + 4H H C H

HH * H C HH

NH3 N + 3H H N HHH N HH

Double and TripleCovalent Bonds

• Double Covalent bonds = bonds that involve 2 shared pairs of electrons.– O2 O O

• Triple Covalent bonds = bonds that involve 3 shared pairs of electrons.– N2 N N

O2 O + O O O O O

N2 N + N N N N N

CO2 O + C + O O C O

O = C = OAdd to notes,above N2

Bonding Theories

16-2Skip pgs. 452–454 + 457-459

VSEPR Theory

• VSEPR Theory states that because electron pairs repel, molecular shape adjusts so that valence-electron pairs are as far apart as possible.– Ex: H2O bond is NOT linear!

2H + O O H H

VSEPR GeometricsA = Central Atom, X = Attached Species,

E = Extra Pair of e-’s on A

Total # of Attached Species

Species Type

Molecular Geometry

Example

2 AX2 Linear CO2

4 AX4

AX3E

AX2E2

TetrahedralPyramidal

Bent

CH4

NH3

H2O

A

AA

A A A

107° 105°

Triatomic120°

Pyramidal Bent

Linear Example

Tetrahedral Example

Pyramidal Example

Bent Example

Polar Bonds +Molecules

16-3 Part I

Bond Polarity• Bonding pairs of electrons are pulled, as in a

tug-of-war, between nuclei of atoms sharing electrons.

• If bonding pairs are shared equally it is a nonpolar covalent bond.– Atoms will have equal electronegativities (pg. 405)– Ex: N2, O2, H2, Cl2, CO2

• If bonding pairs are shared unequally it is a polar covalent bond.– Atoms have unequal electronegativity.– H2O, HCl, CO

Polar Molecules• Polar molecule = one end

of molecule is slightly negative and other end is slightly positive.Ex: HClElectronegativity:

H = 2.1, Cl = 3.0Difference = 0.9

Ex: H2O

H = 2.1, O = 3.5Difference = 1.4

Electronegativity Differences + Bond Types

Electronegativity Difference

Type of Bond Example

0.0 – 0.3 Nonpolar Covalent H – H (0.0)

0.4 – 1.0 Moderate PolarCovalent

∂+ ∂-H – Cl (0.9)

1.1 – 2.0 Very Polar Covalent

∂+ ∂- H – F (1.9)

> 2.0 Ionic Na+Cl- (2.1)

• The polarity of a molecule depends on the shape + orientation of the bonds.– Ex: CO2 polarity cancels

out since it is linear = nonpolar molecule

– Ex: H2O poles add up due to its bent shape = polar molecule

• Polar: F

H C H H

H N H H

OH H

• Nonpolar:H

H C HH

O C O

H HC CH H

Comparing Ionic + Molecular Properties

Characteristic Ionic Cmpd Covalent/Molecular Cmpd

Representative Unit Formula Unit Molecule

Bond Formation Transfer e-’s Share pairs e-’s

Type of Element Metal + nonmetal 2 Nonmetal (possible metalloid)

Physical State Solid S, L, or G

Melting Point High (>300°C) Low (< 300°C)

Solubility in Water High High to Low

Electrical Conductivity as aqueous soln

Good Poor to none

Attractions Between Molecules

16-3 Part II

Attractions between Molecules• van der Waals forces = weakest

attraction (ionic + covalent are stronger).–Three types:

1. Dispersion forces2. Dipole interactions3. Hydrogen bonds.

1) Dispersion forces = weakest of all molecular interactions; caused by motion of electrons.• Increases as the number of electrons

increases.• Halogen diatomic molecules (F2, Cl2, Br2, I2)• Fluorine + Chlorine have weak dispersion

forces (less electrons); thus are gases at STP.• Bromine (more electrons) is a liquid at STP,

and Iodine (most electrons) is a solid at STP.

2) Dipole Interactions = occurs when polar molecules are attracted to one another.– The slightly

negative region of a polar molecule is attracted to the slightly positive region of another polar molecule

– When placed in an electric field, dipole molecules become oriented with respect to (-) and (+) charge

3) Hydrogen bonds = attractive forces in which a hydrogen covalently bonded to a very electronegative atoms is also weakly bonded to an unshared electron pair of another electronegative atom.-Strongest of intermolecular (van der Waals) forces

Network Solids• Most molecules are easy to break; however, a few molecular solids are

very stable.• Network Solids = solids in which all atoms

are covalently bonded to each other.– Solid does not “melt” until 1000°C or higher,

in which it vaporizes without melting at all.– Ex: Diamond; made of carbon, each carbon

bonded to 4 other carbons. Quartz (SiO2)

C – C – C – C

C – C – C – C

Diamond +Silicon carbide (SiC)