Mrs. Schultz

Ch 8Covalent bonding

In nature only the noble gas elements in Group 18 (8A) exist as _______ atoms.

Considered to be MONATOMIC (single atoms)

Covalent Bonding – results from the sharing of electron pairs between two atoms (between nonmetals); forms molecules

A molecule is a neutral group of atoms that are held together by covalent bonds

A single molecule of a chemical compound is an individual unit capable of existing on its own.

A chemical compound whose simplest units are molecules is called a molecular compound.

ex/ O2 (oxygen gas); H2O (water)

Molecular Formula Chemical formula for a compound that has covalent bonds.

Recall: What does the word part “di-” mean?

Diatomic molecule – Two of the same atoms are bound covalently. (di = 2 ;Greek)

Example: 7 diatomic gases *starting with element 7 (N) – “makes a

seven on the Periodic Table”Br. HONClIFBr2 H2 O2 N2 Cl2 I2 F2

octet rule: Chemical compounds tend to form so that each

atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest occupied energy level.

atoms attain the electron configurations of noble gases.

Ex/ Draw the electron dot structure: F2

Ex/ Draw the e- dot structure, N2

Recall: What is the trend for electronegativity?

Draw the electron dot structures

1) NF3 *Note: Least EN atom is central

2) SBr2

Chapter 12

8

Electronegativity, ContinuedElectronegativity increases as you go left to

right across a period.

Electronegativity increases as you go from bottom to top in a family.

Polyatomic ions (many atoms w/charge + or -)

Ex/ PO43- phosphate ion and NH4

+

ammonium

Bond Dissociation Energies

BDE: energy required to break the bond between two covalently bonded atoms.

A large bond dissociation energy corresponds to a strong covalent bond.

Look at page 236 table 8.3Strong carbon-carbon bonds help explain the stability of

carbon compounds.Write down the 3 strongest covalent bonds and the 3

weakest see pg. 236 chart

Bond strengths

Strongest: C triple bond to O; C triple bond to C; C

double bond to O

Weakest:O single bond to O; N single bond to N, Cl

single bond to Cl

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

Symbol – (Greek) sigma (σ).

Molecular Orbitals – when atomic orbitals overlap to form orbitals that apply to the entire molecule

Sigma Bonds

s atomic orbital

s atomic orbital

Bond axis

Sigma-bonding molecular

orbital

represents the nucleus

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side-by-side overlap of p orbitals produces what are called pi molecular orbitals.

Molecular Orbitals

p atomic orbital

p atomic orbital

Pi-bonding molecular

orbital

represents the nucleus

Pi Bonds

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In a pi bond (Greek ), the bonding electrons are most likely to be found in sausage-shaped regions above and below the bond axis of the bonded atoms.

Because orbitals in pi bonding overlap less than in sigma bonding, pi bonds tend to be weaker than sigma bonds.

Molecular Orbitals

Pi Bonds

Polar covalent bond – bonded atoms have an unequal attraction for the shared electrons.

ex/ water H2O Nonpolar covalent bond – bonding

electrons are shared equally; balanced distribution of electrons

Ex/ hydrogen gas H2 Note : Polar is a word that means uneven

distribution of electrons.

Determination of Bond type?

Bond type: difference in electronegativity?Ionic : greater than or equal to 2.0Moderately Polar covalent : .4 – 1.0Very polar covalent: 1.0 – 2.0 Nonpolar covalent: 0 - .4 p. 181 Table 6.2

What type of bonding would be expected between the following pairs of elements?

a) Cl and K Ans. Ionic (EN diff = 3.0 - .8 = 2.2)

b) O and HAns. 3.5 – 2.1 = 1.4 very polar covalent

c) O and OAns. 3.5 – 3.5 = 0 nonpolar covalent

CW: Fill in the following chart with a partner

bonding between: EN diff.: Bond type: More EN atom: 1. Na and Br2. Mg and O3. N and N4. C and O5. Ba and O6. P and O7. Ca and Cl8. Be and F9. S and Cl10. N and O

Ch 8

VSEPR theory – stands for “valence-shell electron pair repulsion”

all sets of valence electrons will repel each other and will be oriented as far apart as possible.

This theory is used to determine shape of molecules (or MOLECULAR GEOMETRY)

Formula: e- dot model shape? polar?O2

H2O

CF4

CH2Cl2

NH3

BBr3

Chapter 12

21

Electronegativity, ContinuedElectronegativity increases as you go left to

right across a period.

Electronegativity increases as you go from bottom to top in a family.

Hybridization:

Mixing of two or more atomic orbitals of similar energies on the same atom to produce new hybrid atom orbitals of equal energy.

CH4

1s22s22p2 Draw orbital diagram:

The paired 2s orbital and the 2p mix to form 4 sp3 orbitals. (s,p,p,p)

This is why carbon forms 4 bonds (hybridization)

Intermolecular forces

Forces of attraction between molecules.Weaker than ionic or covalent bonds

Hydrogen bonding – “Not a true bond”The intermolecular force in which a hydrogen

atom that is bonded to a highly electronegative atom is attracted to an unshared pair of electrons of an electronegative atom in a nearby molecule.

Ex/ water molecules:

van der Waals forces (2 types)

Dipole interactions: Forces of attraction between polar molecules.

Ex/ammonia

Dispersion forces: forces of attraction between nonpolar atoms or molecules that are created by temporary dipoles.

ex/ Noble gases

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This table summarizes some of the characteristic differences between ionic and covalent (molecular) substances.Characteristics of Ionic and Molecular Compounds

Characteristic Ionic Compound Molecular Compound

Representative unit Formula unit Molecule

Bond formationTransfer of one or more electrons between atoms

Sharing of electron parts between atoms

Type of elements Metallic and nonmetallic Nonmetallic

Physical state Solid Solid, liquid, or gas

Melting point High (usually above 300°C) High (usually below 300°C)

Solubility in water Usually high High to low

Electrical conductivity of aqueous solution

Good conductor Poor to nonconducting

Intermolecular Attractions and Molecular Properties