CHEMICAL BONDINGCh. 6 – Part 1: Covalent Bonding

What is a chemical bond? mutual electrical attraction between the nuclei and

valence electrons of different atoms that bind the atoms together

Why don’t noble gases do this?Already have filled s and p orbitals stable octet: 8 valence e- (or 2 if, you’re helium)

Atoms that don’t have a stable octet are more reactive

Key Point #1: By forming bonds with each other, most atoms reduce their potential energy, becoming more stable.

Because forming bonds involves the rearrangement of atoms to recreate new substances, this is considered a chemical change. All chemical changes involve energy changes.

Types of Bonds What type of bonds can be formed?

Ionic bond Covalent bond

○ Nonpolar covalent○ Polar covalent

Ionic bonding: bonds that result from electrical attractions between cations and anions

1 atom losses electrons 1 atom gains electrons

Covalent bonding: sharing of electrons between 2 or more atoms

Key Point 2: Rarely is bonding between atoms purely ionic or purely covalent. Instead, it usually falls somewhere between the two

extremes. Why?

Key Point 3: The extent of ionic or covalent bonding between two atoms can be estimated by calculating the difference in each elements’ electronegativity.

Covalent Bonding Large difference in E.N.: bond has more ionic

character Small difference in E.N: bond has more covalent

character

Types of Covalent Bonds Non-polar covalent bonding: both electrons equally

shared between atoms

Polar covalent bonding: unequal attraction for the shared electrons

6.1 Practice WorksheetPart 1

The property of electronegativity, which is the measure of an atom’s ability to attract electrons, can be used to predict the degree to which the bonding between atoms of two elements is ionic or covalent.

The greater the electronegativity difference, the more ionic the bonding is.

If the calculated electronegative difference is…

> 1.7 : ionic bond is formed> 0.3 , < 1.7 : polar-covalent bond0 – 0.3 : non-polar covalent bond

Increasing difference in electronegativity

NonpolarCovalentshare e-

Polar Covalent

partial transfer of e-

Ionic

transfer e-

Elements Electronegativity Electronegativity Difference

Bond Type

Element 1

Element 2

Mg to Cl

H to O

C to Cl

N to H

C to S

K to F

Na to Cl

H to H

1.2

2.5

3.0

2.5

.8

.9

2.1

2.1

3.0

3.5

3.0

2.1

2.5

4.0

3.0

2.1

1.8

1.4

.5

.9

0

3.2

2.1

0

Ionic

Ionic

Ionic

Polar covalent

Polar covalent

Polar covalent

Non-polar covalent

Non-polar covalent

Polyatomic Ions It is also possible if a compound contains polyatomic

ions, for both types of bonding to be present.

Monatomic Ions: Fe2+ , Na+, Cl-

Polyatomic Ions: PO43-, NH4

+ , NO-1

Groups of atoms are bonded covalent together, but because of few or more than expected valence electrons they have an overall charge (so they can also bond ionically with other ions)

Ex: Ca2+ and SO42- CaSO4 (metal & diff. nonmetals)

Classify the following as ionic, covalent, or both

1. CaCl2 = __________

(metal & nonmetal)

2. CO2 = __________

(nonmetal & nonmetal)

3. MgO = __________

(metal & nonmetal)

4. HCl = ___________

(nonmetal & nonmetal)

5. BaSO4 = ___________

(metal & diff. nonmetals)

6. H2O = ____________

(nonmetal & nonmetal)

7. SO3 = ___________

(nonmetal & nonmetal)

8. AlPO4 = ___________

(metal & diff. nonmetals)

Covalent

Covalent

Covalent

Ionic

Ionic

Both

Covalent

Both

Covalent BondingSection 6.2

What is a molecule? Neutral group of atoms that are held together by

covalent bonds.

Chemical formula: indicates the relative numbers of atoms of each kind in a chemical compound by using atomic symbols and numerical subscripts.

Formation of Covalent Bonds The electrons of one atom and protons of the other

atom attract each another.

The two nuclei and two electrons repel each other.

These two forces cancel out to

form a covalent bond at a length

where the potential energy is

at a minimum.

Bond Length vs. Bond Energy Bond length (pm): distance between two bonded

atoms at their minimum potential energy

Bond energy (KJ/mol): energy required to break a chemical bond and form neutral isolated atoms.○ Breaking bonds: absorbs (requires) energy○ Forming bonds: releases energy

A few general principles – Bond length increases with atomic size Bond length and bond energy are inverses

○ As you increase the number of bonds (between 2 atoms), bond energy increases while bond length decreases

Bond Energies & Bond Lengths

Octet Rule Octet Rule: chemical compounds tend to form so that each

atom has an octet of e-’s in its highest occupied energy level

Exceptions to the octet rule: Atoms that cannot fit eight electrons Atoms that can fit more than eight electrons

Hydrogen forms bonds surrounded by only two electrons.

Boron has just three valence electrons, so it tends to form bonds in which it is surrounded by six electrons.

Phosphorus, Sulfur, & Xenon can form bonds with expanded valence, involving more than eight electrons.

Writing Lewis Structures Lewis Structures: formulas in which atomic symbols

represent nuclei and inner shells, dot-pairs/dashes represent shared electron pairs, and dots adjacent to only one atomic symbol represent unshared pairs of electrons.

Structural formulas: represent kind, number, arrangement, and bonds in a molecule but not does not include unshared pairs of electrons.