Bonding

Energy and Chemical Bonds

Chemical Bond: A force of attraction between atoms in a compound

All elements bond for one reason: to acquire an electron configuration of a noble gas (8 valence electrons =stable octet or 2 valence for He)

BARF

Bonds and Energy changes: As bonds are formed, energy is

released. This is an exothermic process.

As bonds are broken, energy is absorbed. This is an endothermic process.

Bonds and StabilityBonds form to make

elements/compounds stable.The greater the release of

energy, the stronger the bond.

Electronegativity (review)The measure of attraction an

atom has for electronsMetals have low

electronegativityNon metals have high

electronegativity

Ionic Bonding

Results from the complete transfer of electrons between metals (that lose electrons) and nonmetals(that gain electrons) Metals form positive ions called cations (memory hint: has a “t” in the name—

looks like a plus sign for positive) Non metals form negative ions called

anions

Example of Ionic Bonding

Na Cl 2-8-1 2-8-7

Result is: Na+1 Cl-1

2-8 2-8-8

Electronegativity Difference

If the difference between the electronegativities (higher minus lower) is 1.7 or more, the bond is ionic.

Exception: the bond is always ionic for a metal hydride: (group 1 and 2 + hydrogen is always ionic)

See examples

Covalent Bonding

Results from the sharing of electrons between two atoms

Polarity: Unequal sharing of electrons Each atom attracts electrons

by different amounts (like a tug of war)

Polar Covalent Bond

one atom has a slightly higher affinity for electrons

electronegativity difference (ED) is between 0.01 and 1.69

example of polar covalent bonds NH3 H2O

Nonpolar Covalent Bond

Atoms share the electrons equally Electronegativity difference (ED) is

0.0 Most common in diatomics or

‘triatomics’ Br2 I2 N2 Cl2 H2 O2 F2 and O3

Coordinate Covalent Bond

Atoms share the electrons, but one atom donates both electrons

usually the bond is shared with a proton (H+)

example NH3 + H+ NH4+

Metallic Bond These bonds are only found in metals Metals do not have a strong attraction

for electrons. The electrons are loosely held, so therefore, their electronegativity is very low.

These are often described as “positive ions surrounded by a sea of mobile electrons ”

the positive ions form a strong attraction for the electrons surrounding them causing strong bonds

Ionic Solids

contains ionic bonds Properties

crystalline in structure (regular geometric pattern)

relatively high melting and boiling points in the solid form, a poor conductor of

electricity in the liquid or aqueous form, a good

conductor of electricity ionic solids dissolve in water

Solids with Covalent Bonds

Molecular Solids Network Solids covalently bonded covalent in a 3-D network1. soft 1. hard, brittle2. poor conductors2. poor conductors3. low melting points 3. high melting pointsexamples: examples: NH3, HCl, H2O, CH4 C,SiC, and SiO2

(diamond, silicon carbide, silicon dioxide)

Metallic Solids

Contains metallic bonds Good conductors of heat and

electricity in due to“sea of mobile electrons”

examples: Cu, Ag, Au

Types of Molecules, Symmetry and Polarity

Polar Molecules (dipoles) This represents unbalanced charge

distribution along the bond Examples: H2O, NH3, NaCl

Nonpolar Molecules This represents balanced charge

distribution Examples:CO2 & CH4

All diatomics contain nonpolar molecules and nonpolar covalent bonds

Examples:O2 & F2

Molecular Attraction

These are forces between molecules, not to be confused with attractive forces between atoms which are bonds.

There are four types of molecular attractions:

Dipole – Dipole attraction

Neighboring polar molecules orient themselves so that oppositely charge regions line up.

Examples: HCl and H2O

Hydrogen “bonds”

This happens when hydrogen is bonded to a small highly electronegative element.

Only happens with F, O and N 3 substances are HF, H2O, and NH3

these attractive forces are SO strong they have been called bonds

hydrogen bonds are the reason that water has such a relatively high boiling point; this also gives insects the ability to “walk on water”

van der Waal’s Forces

weak intermolecular forces of attraction between individual molecules as molecular mass increases, van der

Waal’s forces increase as the distance between molecules

increases, van der Waal’s forces decrease.

The stronger the van der Waal’s forces, the higher the melting and boiling point.

Molecule-Ion Attraction

Ions are attracted to the negative and positive ends of water molecules or other polar solvents

Example: NaCl in water—sodium has a positive charge and is attracted to the negative or oxygen end of the water molecules

Multiple Covalent Bonds

Double covalent bond 2 pairs of electrons are shared Stronger than a single bond Shorter than a single bond More stable than a single bond

Triple Covalent Bond 3 pairs of electrons are shared stronger than a single or double

bond shorter than a single or double bond more stable than a single or double

bond

Summary of Bond Strength

Single < double < triple