Types of

What is a chemical bond?

Bonds are forces that hold groups of atoms together and make them function as a unit

What do bonds tell us?

Responsible for physical properties like mp, bp, hardness, conductivity

Bond Energy- energy required to break a bond

Sodium Chloride

In sol’n it is Na+ and Cl-

Electrons are transferredWhen water evaporates, aggregate to form solid NaCl

Why??

The system can achieve the lowest possible energy by behaving this way!!

Ionic Bonding

Sodium chloride is very stable mp = 800oC

Ionic substances- when an atom that loses e- easily reacts with an atom with a high affinity for e-

Ionic Compound

Results when a metal reacts with a nonmetal

Coulomb’s Law

Can be used to calculate the energy of interaction between a pair of ions

E = 2.31 x 10-19Jnm(Q1Q2/r)

Q1 and Q2 = charge of ionsr = distance b/w ion centers

Such as...

Calculate the energy of interaction (kJ/mol) b/w Mg2+ and O2- if the distance b/w them is .205 nm (2.05 A)

Notice!

The answer is negative indicating that the pair is at lower energy

You can also find the repulsion b/w like ions, but the sign will be positive

Bonding b/w like atoms

Let’s look at H atoms2 unfavorable terms (e- e- repulsion and p+ p+ repulsion) and 1 favorable term (e- p+ attraction)

What conditions favor bond formation??

Whatever causes the H atoms to achieve the lowest possible total energy

Bond length = when energy is at a minimum

The Diagram

The energy terms involved are the net Ep that results from the attractions and repulsions among the charged particles and the EK is due to e- motion

The Diagram

The zero point of energy is defined with the atoms at infinite separation

The Diagram

At very short distances the energy rises steeply because of the importance of the repulsive forces when the atoms are very close together

The Diagram

The bond length is the distance at which the system has minimum energy

Covalent Bonding

The type of bonding encountered in H2 and any other molecules in which electrons are shared by nuclei

Two extremes

Ionic bonding transfers electrons

Covalent bonding atoms share electrons equally

The Middle Ground

Polar covalent bond- atoms unequally share electrons

Results in partial positive and partial negative charge

When will this happen?

Electronegativitythe ability of an atom in a molecule to attract shared electrons to itself

Linus Pauling (1901-95)

US scientistNoble Prizes for chem and peace

method for determining electronegativities

Electroneg Values

Have been determined for almost all elements

Range is 4.0 for F to 0.7 for Cs

Periodic Trend? You Bet!

Increases going left to right across a period

Increases going up a group

Put in order!

Based only on positions, place the following in order of increasing electronegativity

Sr, Cs, Se, O, Ba

Were you correct?

Least Most

Cs<Ba<Sr<Se<O

Bond type relationship

Electroneg diff

ZeroIntermediate

Large

Bond TypeCovalentPolar CovalIonic

Bond Polarity

Use tables of electroneg and calculate for each of the following bonds and then order from most covalent to most ionic

Try these...

Na-ClLi-HH-CH-FRb-O

The answers...

Na-Cl 2.1Li-H 1.1H-C 0.4H-F 1.9Rb-O 2.7

The Ranking...

CovalentIonic

H-C, Li-H, H-F, Na-Cl, Rb-O