Bonding Class #7

OB: defining the three kinds of Intermolecular Bonding, the weak attractions between molecules, much weaker than ionic or covalent bonds, but they are important and have a real effect on the compounds

Quick review….

Ionic bonds form between metals (that lose electrons) and nonmetals (that gain electrons). The transfer of these electrons result in the formation of neutral ionically bonded compounds, such as NaCl, MgO, or CuCl2

Covalent bonds form between 2 or more nonmetals (no metals ever) by sharing electrons. The molecules that form will have single, double or triple bonds, and atoms follow the octet rule. Examples are water, CH4, and CO2.

These bonds are all inside the compound.

There are three kinds of INTERMOLECULAR BONDS, bonds formed by the molecules with each other. These are all MUCH WEAKER that inside the compound bonds, but they are important.

Weakest to strongest they are: electron dispersion force, dipole interaction, and hydrogen bonding.

When I was in college there were only 2 kinds, electron dispersion and dipole interaction. Hydrogen bonding is very similar to dipole interaction, and we’ll see how they work today.

The weakest is the electron dispersion force. It’s created by the movement of electrons.

You need paper to draw now, don’t be cheap with your ink, or paper.

Electron Dispersion forces.

Each of these F2 molecules has a 2-7 doubled electron configuration. Each atom has 9 electrons, the molecules have 18 electrons.

When these electrons all “move” to one side, for a nanosecond, there will be a temporary dipole created, a positive side, and a negative side of the molecule.

This allows for the weakest of temporary attractions to exist.

F2 is a gas at STP, because the kinetic energy at 273 Kelvin exceeds the attractive force of the electron dispersion forces, so it’s a

GAS.

Example one: fluorine F2

Electron Dispersion forces. Each of these Cl2 molecules has a 2-8-7 doubled electron configuration. Each atom has 17 electrons, the molecules have 34 electrons.

When these electrons all “move” to one side, for a nanosecond, there will again be temporary dipoles. This happens more often than with fluorine, but not often enough to make a difference at 273 Kelvin.

This allows Cl2 to be a gas at STP, because the kinetic energy at 273 Kelvin exceeds the attractive forceof the electron dispersionforces, so chlorine is also a

GAS.

Example two: Chlorine Cl2

Electron Dispersion forces. Each of these Br2 molecules has a 2-8-18-7 doubled electron configuration. Each atom has 35 electrons, the molecules have 70 electrons.

These electrons all “move” to one side, so many times per second that there will be a dipole created, a positive side, and a negative side of the molecule. This happens often enough that these attractive forces make Br2 a liquid!

The weak but constant intermolecular attractions accumulate.

The 273 Kelvin kinetic energy cannot overcome the intermolecular attractions, so bromine becomes a

liquid.

Example 3: Bromine Br2

Electron Dispersion forces. Each of these I2 molecules has a 2-8-18-8-7 doubled electron configuration. Each atom has 53 electrons, the molecules have 106 electrons.

The electrons move so much, that a near constant dipole exists due to these electron dispersions.

This allows for the weakest of temporary attractions to exist at all times, which makes I2 a solid at STP.

The kinetic energy at 273 KelvinDOES NOT exceed the attractive force of the electron dispersionforces, so iodine is a

SOLID

Example 4: Iodine I2

The halogens clearly show how electron dispersion forces accumulate and then affect the molecules.

When there are dipoles, that means a positive and a negative side to a molecule (or a bond). Here, there are near permanent dipoles created by polar bonds but ONLY IN POLAR MOLECULES.

H

H C H

H

S

Cl Cl

In SCl2, the bonds are single polar covalent. The molecule itself is polar because it does not have radial symmetry. So, the sulfur will become positively charged most of the time, and the chlorine atoms will be negative most of the time.

Methane, which has polar bonds too, has radial symmetry.

This offsets that polarity, and the molecule is nonpolar. SCl2 will be liquid at room temp whilemethane would be a gas. Why???

H

H C H

H

S

Cl Cl

S

Cl Cl

S

Cl Cl

S

Cl Cl

S

Cl ClS

Cl Cl

All the positive sulfur atoms are nearly permanently attracted to the negative chlorine atoms. The EN difference in a polar molecule can create intermolecular bonds called dipole attractions.

H

H C H

H

H

H C H

HH

H C H

H

H

H C H

H

These methane molecules (nonpolar) have nearly no attraction to each other, so they will be gas at room temperature. Dipole attraction is way less powerful than ionic or even covalent bonding, but it can affect the phase of the compound. Nonpolar molecules are hardly attractive to each other.

Is there ANY attraction here between molecules?

Hydrogen bonding is exactly the same as dipole attraction, but, and it’s a SMALL but, hydrogen has to be included in the molecule.

H has a much smaller EN value than most other atoms, so when it’s included, like with water, the dipole it creates is usually much stronger than when it’s something like SCl2.

S

Cl Cl

O

H H

The EN difference between chlorine and sulfur is 3.2 – 2.6 = 0.6

The EN difference between oxygen and hydrogen is 3.4 – 2.2 = 1.2

This greater difference creates a “stronger” dipole. Strong enough that we now have to give it a new name. Instead of just calling it a strong dipole attraction, we call it hydrogen bonding.

O

H H

O

H H

O

H H

O

H H

O

H H

O

H H

All of the negative oxygen are magnetically attracted to the positive hydrogen atoms in nearby molecules. This is an intermolecular attraction. Hydrogen bonding is the strongest of the 3 intermolecular attractions.

Give an example molecule (or formula unit) for each type of bond:

Ionic

Single nonpolar covalent

Single polar covalent

Double nonpolar covalent

Double polar covalent

Triple non polar covalent

Triple polar covalent

Coordinate covalent

Resonant

Ionic + Covalent at the same time

Breaks the octet rule (more than 8e-)

Breaks the octet rule (less than 8e-)

Give an example molecule (or formula unit) for each type of bond:

Ionic……………………………………………………………………………………..NaCl

Single nonpolar covalent……………………………………………………F-F

Single polar covalent………………………………………………………….H-Cl

Double nonpolar covalent………………………………………………….O=O

Double polar covalent………………………………………………………..O=C=O (both)

Triple non polar covalent…………….…………………………………….NΞN

Triple polar covalent………………………………………………………….NΞC-H

Coordinate covalent…………………………………………………………..carbon monoxide

Resonant……………………………………………………………………………….ozone O3

Ionic + Covalent at the same time………………………………….CuSO4·5H2O*

Breaks the octet rule (more than 8e-)………………………….PCl5

Breaks the octet rule (less than 8e-)…………………….………H-H (too small)

* Also has hydrogen bonding as well. (wow!)