CHEMICAL BONDING

Why do bonds form?

•To complete the valence or outer energy level

•Octet Rule – 8 is great!

•A bond forms when 2 atoms attract the same electron or electron pair

Bond Characteristics• Attractive Forces pull atoms together:

- (+ ) Nucleus of one atom attracts (–) electrons of another atom

- Opposite charges attract = ELECTROSTATIC ATTRACTION

• Bond Energy- When bonds form Energy is released = Exotherimc- When bonds break Energy is absorbed = Endothermic- When 2 atoms are held together by a chemical bond they

are at a LOWER energy condition than when they are separate!

Bond Characteristics (cont.)

• Bond Stability- Compounds with LOW energy are STABLE- Compounds with HIGH energy are UNSTABLE- Chemical Change = the rearrangement of atoms to form low

energy and stable compounds

• Bond Strength- Weak Bond – when a small amount of energy is released- Strong Bond – when a large amount of energy is released

In nature:

• Exothermic reactions are favored (bond formation)

UNSTABLE STABLEREACTIVE UNREACTIVEHIGH ENERGY LOW ENERGYWEAK BOND STRONG BOND

Electronegativity – Table S

•An atom’s ability to attract electrons•Highest EN value = 4.0 = Fluorine (non-metal)•Lowest EN value = 0.7 = Francium (metal)•The difference in EN values of two bonded atoms can tell you the bond type

ELECTRONEGATIVITY DIFFERENCE AND BOND TYPE

If the EN Difference is between:

4.0 – 1.7 1.6 – 0.2 0.1 – 0.0Ionic Bond Polar Covalent Non-Polar Covalent

Examples:BaO CH4 Cl2

BOND TYPES

• Ionic•Covalent:

- Polar Covalent - Non-Polar Covalent- Coordinate Covalent

•Metallic

Ionic Bonds

• Electrons are TRANSFERRED from one atom to another• Transfer from a metal to a nonmetal• Metals lose electrons to the nonmetals (gain e-)• High EN Diff. – the greater the difference, the stronger the ionic

character• Form ionic solids called Ionic Crystals (crystal lattice)

• Example: Which of the following has the strongest ionic bond?MgO NaCl LiF

Characteristics of Ionic Compounds

• High Melting Point• High Boiling Point• Conducts electricity when dissolved in solutions• Does not conduct electricity in the solid phase!• Soluble in water• Forms crystals in solid phase

Metal Hydrides

• Are ALWAYS IONIC• Occurs when Hydrogen bonds with a group 1 or 2 element• Examples: LiH NaH KH• H behaves as a nonmetal and is written second in the formula

Covalent bonds

• When 2 atoms SHARE electrons in the same orbital• Possible to have single, double, triple bonds (1, 2, or 3 shared e- pairs)• Occurs between 2 nonmetals• Includes the Diatomic Molecules (B O FINCH)• 2 types of Covalent Bonds

1. POLAR Covalent – sharing of e- is uneven2. NONPOLAR Covalent – sharing of e- is even

Polar Covalent Bonds

• There is an uneven distribution of + and – charges due an uneven sharing of electrons in the bond• EN Difference is between 0.2 – 1.6• Examples: HCl H2O

Non-Polar Covalent Bonds

• Even distribution of + and – charges due to even sharing of electrons• EN Diff = 0.0 – 0.1• ALL Diatomic molecules have nonpolar covalent bonds

Coordinate Covalent Bonds

• Occurs when only one atom contributes BOTH electrons in forming a bond• Occurs with the polyatomic ions (see Reference Table E)• Polyatomic Ions – 2 or more atoms that act as a single unit and have

one charge on the entire group• Ex. NH4

+ H3O+ NO3- SO4 -

Characteristics of Molecular Substances• Can exist as solids, liquids, or gases• SOFT in the solid phase• Poor conductors of heat and electricity *Good

insulators• Low melting and boiling points• Does not dissolve in water• Soluble in Benzene or Ether (organic molecules)

Network Solids

• In molecular solids, the the covalently bonded atoms are linked into a GIANT NETWORK throughout the entire solid• MACROMOLECULES• EXTENSIVE WEB OF COVALENT BONDS (3-D FISHNET)• Examples: Diamond, Silicon Dioxide, Asbestos• PROPERTIES:

- HIGH melting points- HARD solids- Poor conductors of heat and electricity

Metallic Bonds

• Bonds between metal atoms• Valence electrons drift from atom to atom• “SEA OF MOBILE ELECTRONS”• Properties:

- Very malleable- Good Conductors of Heat and electricity- Has Luster- Tenacity

Summary of Bond Types

Ionic Bonds• Metal – Nonmetal• Forms Crystal Compound• Exists as solids• EN Diff = 1.7 – 4.0

• High Melting/Boiling Pt• Conducts electricity when in solution• Soluble in water

Covalent Bonds• Nonmetal – Nonmetal• Forms Molecule• Exists as solids, liquids, gases• EN Diff = 0.2 – 1.6 Polar

0.1 – 0.0 NonPolar• Lo Melting/Boiling pt• Does not conduct electricity in solutions• Not soluble in water (dissolves in Benzene)

Summary of Bond Types

Metallic Bonds• Metal – Metal

• Sea of Mobile Electrons• Good Conductors of Heat & Elec• Malleable, Luster• High melting/boiling points

Network Solids• Covalently bonded clusters of

atoms• Macromolecule• Poor Conductors of Heat & Elect• Hard Solids• High melting point

ATTRACTIVE FORCES BETWEEN MOLECULESalso known as IMF’s – Intermolecular Forces• 2 TYPES OF MOLECULES

1. Polar Molecules – molecule has a (+) end and a (-) end- uneven or asymmetrical distributions of + -- Bond angle is less than 180 degrees

- also called DIPOLES Ex. H20, NH3, HCl

2. NonPolar Molecules – symmetrical, bond angle = 180 - All of the DIATOMIC MOLECULES - B O FINCH

*Important Note*

• All molecules with polar covalent bonds are dipoles EXCEPT:CF4 CCl4 CO2

• A molecule can have polar covalent bonds but be a NONPOLAR molecule

• Example: Draw Lewis Dot Diagram of CCl4 (Carbon Tetrafluoride), CO2, H2O

Attraction Between POLAR Molecules• Dipole-Dipole Attraction

- When + end of one molecule is attracted to the – end of another molecule- Very strong attraction gives a high boiling point due to energy needed to break the force of + - attraction

• Hydrogen Bond• NOT A BOND • Occurs when H is covalently bonded to an atom which has a

- Small atomic radius- Hi Electronegativity- H is bonded to N, O, F (NH3, H2O, HF)

Non-Polar Force of Attraction

• WEAK attractive forces• Van der Waals Forces OR London Dispersion Forces• Occurs between: Diatomic Molecules (B O FINCH)

NonPolar MoleculesNoble Gases

*Force increases as they get closed together:Distance btwn molecules decreases ORSize of molecules increases

Molecule – Ion Attraction

• EXAMPLE: NaCl dissolves in H2O• Hydration of the ions• Draw diagram