Your guide to inter- and intra-

molecular forces

Different substances have different melting points, boiling

points, etc. • Why is this so?• What makes each substance different from each

other?• An obvious difference is the elements that make

up the substance.• The way these elements interact with each other

will help explain why different substances have different physical qualities such as MP & BP.

Let’s start with Intra-molecular attractions

• What are these?– The prefix intra means within, so these

attractions are the ones within a molecule, otherwise known as… BONDS

• We are familiar with two– Ionic (when one atom gives up an electron, the

other one takes it, then they are attracted due to opposite charges)

– Covalent (when two atoms share two electrons)

Which one is stronger?

• Ionic bonds are very strong, which explains why ionic compounds have very high melting points.

• NaCl has a MP of 801oC• Lithium Chloride has a MP of 614oC• So, covalent bonds are weak. BUT – there are

certain levels of strength in covalent bonds, and this depends on which elements are sharing the bond

We know that…• …when two atoms share electrons, one may be

greedier with the electrons than the other. This is due to…

• YES! Electronegativity, or an atom’s love for electrons.

• Since one atom will tend to pull the electrons towards itself more, there is an uneven sharing of electrons. The resulting bond is what we call…

• Right again! We call it POLAR.• But there are different degrees to the “polar-ness”

of a bond.Oh, how I

love electrons

…

F

How polar a bond is depends on its electronegativity value.

• Each element is assigned a value from 0 to 4 based on how much it LOVES electrons.

• Values for various elements are on page 405 of your book.

How to determine the degree of polarity

• Using the chart on p. 405, subtract the electronegativity values of the two elements sharing the bond, then take the absolute value of that difference.

• Then use the chart on p. 462 to determine the degree of polarity.

Determine the degree of polarity of a bond between…

• H and F?• Very polar

covalent• Na and O?• Ionic• C and S?• Non-polar

covalent• Se and I?• Non-polar

covalent• As and F?• Very polar

covalent

Now, the presence of polar bonds in a molecule does not mean that the

whole molecule is polar!

• If a molecule is symmetrical, then the polar bonds “cancel out” and the molecule becomes non-polar.

• On the other hand, if a molecule is asymmetrical and contains polar bonds, then the molecule is polar.

How does one determine symmetry? Let’s run through some examples…

•Let’s take Boron Trifluoride…

F

B

FF

A molecule is considered symmetrical if you can place a plane through the central atom and a surrounding atom, and there are mirror images on both sides of the plane.

Placing a plane through the Boron atom and a fluoride atom results in…

F

B

FF

So, BF3 would be non-polar since it is symmetrical

Let’s try another one…• We already know water is polar, but let’s

put our symmetry test to work…

H H

O

Placing a plane through the O and one of the H’s…

H H

O

Since the images on both sides of the plane are not the same, this molecule is asymmetrical, so it is POLAR!

Don’t worry – if it’s not obvious, I’ll tell you if a molecule is symmetrical or asymmetrical

This brings us to INTER-molecular attractions

• What does this term mean?• The prefix INTER means between (like

interstate highway – a highway that goes between states)

• INTER-molecular attractions depend on the type of INTRA-molecular attraction inside the molecule.

• Determining both of these will help explain the differences in MP’s and BP’s of different substances.

• Ready to go??

Due to different degrees of polarity, there are different degrees of inter-molecular

attractions.• When we have a polar molecule, we have one end of

the molecule being partially positive (+) and the other end being partially negative (-).

• Since one end is + and the other end is we call this a dipole (two poles!).

• The attractions between polar molecules are due to the attractions between the + and the - ends attracting each other. This is what we call dipole-dipole attractions.

• Let’s look at an animation to see how this is done… dipole-dipole animation

Let’s look at the BP’s of different covalent compounds

• What trend do you see?

• With the exception of the Carbon compounds, the first compound of the series has the highest BP.

Why does this trend occur?• The compounds that were listed first in the series were

HF, NH3 and H2O. These are all polar molecules.• The second compounds in each series were H2S, HCl

and PH3. • Calculate and compare the degree of polarity of each

bond for the first and second compound in each series• We see that the first compound has a higher degree of

polarity.• So the dipole-dipole attractions between the molecules

are unusually strong.• Since these are very strong, they go into a category of

their own – HYDROGEN BONDS!

Hydrogen Bonds• Most Hydrogen bonds will occur between

hydrogen and one of the following elements… Fluorine, Oxygen and Nitrogen

• FON! So, if the FON is ringing and Hydrogen is on the other end, you have a hydrogen bond.

• However, as long as the bond between two elements can be categorized as VERY POLAR COVALENT, you can assume the intermolecular attraction is strong (polar).

Hello? This is hydrogen, may I

speak to F, O or N?

London Dispersion forces

• This if the last type of intermolecular attractions.

• It occurs between non-polar molecules.• In a non-polar molecule, the electrons are

distributed evenly amongst the bonds and molecule. There are no partial positive or negative ends.

• So how do they attract each other?

How London Dispersion forces work

• We know that electrons move around.• Sometimes these electrons shift to one side of the

molecule, creating random, temporary polarity shifts.

• If one side of one molecule shifts, it will influence the neighboring molecules electrons to also shift.

• Temporary poles are made, and the molecules attract!

• Let’s watch an animation to see how this works. London Dispersion Animation

So the order of strength in intermolecular attractions…

• Ionic attractions

• Hydrogen bonds

• Dipole-dipole attractions

• London Dispersion forces

• So, that’s the end of our story, right?? Guess again…

BP’s and MP’s are not only affected by the type of inter- and intra-

molecular attractions…• But in their molecular mass as well.• The trend is… if a molecule has a higher mass, the

higher the BP and/or MP will be.• Molecules with higher masses will have higher amounts

of electrons. So, the increased interaction of electrons will also influence the BP and/or MP of a substance.

• Here’s some data to support this idea…• Various BP’s…

• CH4 – molar mass = 16.0 g, BP = 111.7oC• C3H8 – molar mass = 44.1 g, BP = 231.1 oC• C6H14 – molar mass = 86.2 g, BP = 341.91 oC

Now, we are done!!

Any questions???