Unit 5-K: Chemical Bonding

Essential Questions

1. What types of elements combine to form ionic and covalent compounds?

2. What factors determine the shape of covalent molecules?

3. What is a polar molecule?

4. Can you identify the Lewis Dot Structures for ionic and covalent compounds?

5. How do you determine the type of bonding using electronegativity?

Vocabulary Review

• Ion-atom with a charge• Cation-Positive ion, lost electrons• Anion-Negative ion, gained electrons• Oxidation number -the charge that

represents the number of electrons lost or gained

• (New) Polyatomic ion -more than one element attached to the charge.

12311 Na

Chemical Bonding

• Bonds – the attractive force between atoms or ions in a compound.

• A bond depends upon:• The electron configuration (involves valence

electrons)• Electronegativity

• Why do elements bond?• To achieve a stable electron configuration (8

electrons; noble gas configuration)» Octet rule – atoms lose, gain or share

electrons to achieve a stable configuration of eight valence electrons

• To achieve the lowest possible energy state (lowest potential energy)

“NEW” LEWIS DOT STRUCTURES FAMILY SINGLE ELEMENT BONDED ELEMENT

IA X X

IIA X X

IIIA X X

IVA X X

VA through VIIIA the bonded element structure is the same as the single element

Types of Bonds

• Ionic – complete transfer of electrons (loses or gains); example: NaCl

• Nonpolar covalent (also called pure covalent) – equal sharing of electrons between atoms; example: O2

• Polar covalent – unequal sharing of electrons between atoms; example: H2O

• Electronegativity (EN) – indicates how strongly an atom wants to gain an electron (Decreases down, Increase Across)

Ionic Bonding

• Occurs when electrons are completely transferred from one atom to another.

• The strongest type of bond.

• Formed between a metal and a nonmetal.

Sodium Meets Chlorine

http://www.beyondbooks.com/psc92/3b.asp

Ionic Bonding - Metals

• Lose e- to form same stable configuration as Noble Gas in preceding period; forms ions.

Na Na+ + 1e-

1s22s22p63s1 1s22s22p6

Same Configuration as: Ne atom

1s22s22p6

Ionic Bonding - Nonmetals

• Gain e- to form same configuration as Noble Gas at end of the same period; forms ion.

Cl + 1e- Cl1-

1s22s22p53s23p5 1s22s22p63s23p6

Same Configuration as: Ar 1s22s22p63s23p6

Ionic Bonding

• Make sure you understand valence electrons and electron configurations

• Draw Dot structures of valence electrons• Know your oxidation numbers• Make sure that positive and negative

charges add up to zero!

http://wine1.sb.fsu.edu/chm1045/notes/Bonding/Ionic/Bond02.htm

Ionic BondsEquation showing an ionic bondMetals give up e- Non-metals gain e-Must get 8e- totalNa• + F Na+1 F -1

Na has no dots and F has all 8 dotsYou must show the charges (will equal 0)One Na+1 ion and one F-1 ion form a

formula unit (NaF)

Ionic Bonding

• The simplest ratio of the ions represented in an ionic compound is called a formula unit

• We use this because no single particle of an ionic compound exists

• Total # of e- gained by nonmetals atoms = total # of e- lost by the metal,so overall charge = 0

Ionic Bonding

• Examples:KBr Potassium bromide (1:1)

MgCl2 Magnesium chloride (1:2)

• Practice:Sodium phosphide, three sodium ions for every phosphide ion

Na3P

Ionic Bonding

Practice by writing the equations for the following ions:

1. Magnesium & Sulfur

2. Aluminum & Oxygen

3. Magnesium & Iodine

4. Copper & Bromine (note: Copper can

be +1 or +2)

Covalent Bonds

• The result of sharing valence electrons. The shared electrons are part of the complete outer shell of both atoms.– Occurs when elements are close together on

the periodic table– Between nonmetallic elements

• Molecule-formed when two or more atoms bond covalently.

• Nonpolar covalent (also called pure covalent) – equal sharing of electrons between atoms; example: O2

• Polar covalent – unequal sharing of electrons between atoms; example: H2O

Covalent Bonds

• Can exist as gases, liquids, or solids depending on molecular mass or polarity

• Usually have lower MP and BP than ionic compounds

• Do not usually dissolve in water

• Do not conduct electricity

TYPES OF COVALENT BONDSNONPOLAR COVALENT

e- are equally sharedNo difference in electronegativity

All diatomic molecules are nonpolar covalentH2,I2,O2,Cl2,N2,Br2,F2

TYPES OF COVALENT BONDSPOLAR COVALENT

unequal sharingE- spends more time with the more electronegative atomDifference in EN = 0.1-1.7regions of partial charges

also known as a dipole (two poles)

H Cl

+ -

pulled more bychlorine

Partial positive Partial negative

Covalent Bonds

• Two Hydrogen Atoms (H2)

http://web.jjay.cuny.edu/~acarpi/NSC/5-bonds.htm

Polar Covalent Bonds

• Unequal sharing of electrons• Have poles (dipoles) – regions that are

positive & regions that are negative• Electrons are pulling toward more

electronegative element• Symbols:

δ+ δ- : show regions of partial charge

: arrow points to more electronegative

element

Nonpolar Covalent

• Electrons are equally shared

• No difference in electronegativity (ex. diatomic molecules)

• Also, can be due to shape of molecule– Electrons pulled equally in all

directions, polar effect cancels

(ex. I Be I )

Predicting Bond Type

• Look at table of EN values and subtract the values for the 2 atoms involved in the bond

• take the absolute value• If EN difference is…

0 – 0.4 nonpolar covalent bond

0.5 – 1.6 polar covalent bond

>1.7 ionic bond

DIFFERENCE IN ELECTRONEGATIVITY 0 = NONPOLAR COVALENT>0 TO 1.6 = POLAR COVALENT1.7 TO 3.4 = IONIC

0 1.7 3.4

NONPOLAR

POLARCOVALENT

IONIC

To Determine Molecular Shape• Use VSEPR (valence shell electron pair

repulsion) rules:1) Draw the Lewis dot structure for the

molecule2) Identify the central atom3) Count total # of electron pairs around

the central atom4) Count # of bonding pairs of electrons

around the central atom5) Count # of lone pairs of electrons

around the central atom6) Look at summary chart, identify shape

Molecular Shape# of e- pairs around

central atom# of bonding

pairs of e-# of lone

pairs of e- Name Shape

2 2 0 linear

3 3 0trigonal planar

4 4 0 tetrahedral

4 3 1trigonal

pyramidal

4 2 2angular (bent)

4 1 3 linear

Practice

Determine the shape:

1. BCl3

2. CH4

Polarity

• TO DETERMINE THE POLARITY OF A MOLECULE, not a bond, you must know the type of bond and the shape.

• POLAR MOLECULES must meet 2 criteria:• Must have a polar covalent bond (EN

difference between 0.5 and 1.6) AND• Must have an asymmetrical shape: trigonal

pyramidal, angular, or 2 element linear• If both criteria are not met, it is not a polar

molecule, it is either a nonpolar molecule or an ionic compound.

Practice

Label the following MOLECULES as polar or nonpolar.

1. NH3

2. CH4

3. HCl

Multiple Covalent Bonds

• Atoms of some elements attain a noble-gas configuration by sharing more than one pair of electrons between two atoms

• When writing structural formulas a line can represent a pair of shared electrons

Multiple Covalent Bonds

• Double Bond: share 2 pairs of e-

O=O

• Triple Bond: share 3 pairs of e-

N N

Covalent Bonds

Exceptions to Octet Rule• Beryllium (2 valence e-, full with 4

valence e-) BeI2

• Aluminum (3 valence e-, full with 6 valence e-) AlCl3

• Boron (3 valence e-, full with 6 valence e-) BH3

More Exceptions to the Octet Rule

• Odd number of valence e-, cannot form an octet around each atom– NO2

• Compounds can form with fewer than 8 e- (rare)– BH3

• Expanded octet – can hold more than 8 valence e- due to empty d orbital– S and P most common elements

• SF6 or PCl5

Expanded Octets

• Some atoms bond so they have more than 8e- in the outer level

• Occur only around a central nonmetallic atom from period 3 or higher

PCl5 SF6

http://www.wou.edu/las/physci/poston/ch222/VSEPR-Geometries.htm

EXAMPLES FOR FAMILY VA

P PCl

Cl Cl ClCl Cl

ClCl

PCl5PCl3

EXAMPLES FOR FAMILY VIA

S SCl

Cl

SCl6SCl2Cl Cl

Cl

ClCl

Cl

Property Ionic Bonds Covalent BondsElectrons are: Transferred Shared

Difference in Electronegativity: ≥1.8

0-nonpolar>0 to 1.7-polar

Bond between: A metal and nonmetal

2 nonmetals

State at room temperature

Solid Usually gas (can be solid or liquid)

Particle Name: Formula Unit Molecule

Melting Point: High Low

Conducts Electricity? Yes No

Dissolves in water? Yes No (usually)

Flammable No Yes

Forms a more stable configuration?

Yes Yes

Examples: NaCl, MgI2 NH3, CHCl3

Metallic Bonds

• Metals often form lattices in the solid state similar to ionic crystals

• Even though metal atoms have at least one valence e- they do not share or lose electrons

• Electron sea model – all the metal atoms in a metallic solid contribute their valence e- to form a “sea” of e-

Metallic Bonds

• Positive ions in a “sea of electrons,” belongs to the crystal as a whole

• Force applied (left to right) highlighted cation unchanged.

• Explains ease of deformation of metals

http://cwx.prenhall.com/petrucci/medialib/media_portfolio/12.html