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Intermolecular forces

In the previous chapter, we discussed the different forces that exist between atoms (intramolecular forces). When atoms are joined to one another they form molecules, and these molecules in turn have forces that bind them together. These forces are known as intermolecular forces , and we are going to look at them in more detail in this next section.

Intermolecular forces

Intermolecular forces are forces that act between stable molecules.

You will also remember from the previous chapter, that we can describe molecules as being either polar or non-polar . A polar molecule is one in which there is a difference in electronegativity between the atoms in the molecule, such that the shared electron pair spends more time close to the atom that attracts it more strongly. The result is that one end of the molecule will have a slightly positive charge ( δ + ), and the other end will have a slightly negative charge ( δ + ). The molecule is said to be a dipole . However, it is important to remember that just because the bonds within a molecule are polar, the molecule itself may not necessarily be polar. The shape of the molecule may also affect its polarity. A few examples are shown in [link] to refresh your memory!

Polarity in molecules with different atomic bonds and molecular shapes
Molecule Chemical formula Bond between atoms Shape of molecule Polarity of molecule
Hydrogen H 2 Covalent
Non-polar
Hydrogen chloride HCl Polar covalent
Polar
Carbon tetrafluoromethane CF 4 Polar covalent
Non-polar

Types of intermolecular forces

It is important to be able to recognise whether the molecules in a substance are polar or non-polar because this will determine what type of inermolecular forces there are. This is important in explaining the properties of the substance.

  1. Van der Waals forces These intermolecular forces are named after a Dutch physicist called Johannes van der Waals (1837 -1923), who recognised that there were weak attractive and repulsive forces between the molecules of a gas, and that these forces caused gases to deviate from 'ideal gas' behaviour. Van der Waals forces are weak intermolecular forces, and can be divided into three types:
    1. Dipole-dipole forces [link] shows a simplified dipole molecule, with one end slightly positive and the other slightly negative.
      A simplified diagram of a dipole molecule
      When one dipole molecule comes into contact with another dipole molecule, the positive pole of the one molecule will be attracted to the negative pole of the other, and the molecules will be held together in this way ( [link] ). Examples of materials/substances that are held together by dipole-dipole forces are HCl, FeS, KBr, SO 2 and NO 2 .
      Two dipole molecules are held together by the attractive force between their oppositely charged poles
    2. Ion-dipole forces As the name suggests, this type of intermolecular force exists between an ion and a dipole molecule. You will remember that an ion is a charged atom, and this will be attracted to one of the charged ends of the polar molecule. A positive ion will be attracted to the negative pole of the polar molecule, while a negative ion will be attracted to the positive pole of the polar molecule. This can be seen when sodium chloride (NaCl) dissolves in water. The positive sodium ion (Na + ) will be attracted to the slightly negative oxygen atoms in the water molecule, while the negative chloride ion (Cl - ) is attracted to the slightly positive hydrogen atom. These intermolecular forces weaken the ionic bonds between the sodium and chloride ions so that the sodium chloride dissolves in the water ( [link] ).
      Ion-dipole forces in a sodium chloride solution
    3. London forces These intermolecular forces are also sometimes called 'dipole- induced dipole' or 'momentary dipole' forces. Not all molecules are polar, and yet we know that there are also intermolecular forces between non-polar molecules such as carbon dioxide. In non-polar molecules the electronic charge is evenly distributed but it is possible that at a particular moment in time, the electrons might not be evenly distributed. The molecule will have a temporary dipole . In other words, each end of the molecules has a slight charge, either positive or negative. When this happens, molecules that are next to each other attract each other very weakly. These London forces are found in the halogens (e.g. F 2 and I 2 ), the noble gases (e.g. Ne and Ar) and in other non-polar molecules such as carbon dioxide and methane.
  2. Hydrogen bonds As the name implies, this type of intermolecular bond involves a hydrogen atom. The hydrogen must be attached to another atom that is strongly electronegative, such as oxygen, nitrogen or fluorine. Water molecules for example, are held together by hydrogen bonds between the hydrogen atom of one molecule and the oxygen atom of another ( [link] ). Hydrogen bonds are stronger than van der Waals forces. It is important to note however, that both van der Waals forces and hydrogen bonds are weaker than the covalent and ionic bonds that exist between atoms .
    Two representations showing the hydrogen bonds between water molecules: space-filling model and structural formula.

Types of intermolecular forces

  1. Complete the following table by placing a tick to show which type of intermolecular force occurs in each substance:
    Formula Dipole-dipole Momentary dipole Ion-dipole hydrogen bond
    HCl
    CO 2
    I 2
    H 2 O
    KI(aq)
    NH 3
  2. In which of the substances above are the intermolecular forces...
    1. strongest
    2. weakest

Questions & Answers

if three forces F1.f2 .f3 act at a point on a Cartesian plane in the daigram .....so if the question says write down the x and y components ..... I really don't understand
Syamthanda Reply
hey , can you please explain oxidation reaction & redox ?
Boitumelo Reply
hey , can you please explain oxidation reaction and redox ?
Boitumelo
for grade 12 or grade 11?
Sibulele
the value of V1 and V2
Tumelo Reply
advantages of electrons in a circuit
Rethabile Reply
we're do you find electromagnetism past papers
Ntombifuthi
what a normal force
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it is the force or component of the force that the surface exert on an object incontact with it and which acts perpendicular to the surface
Sihle
what is physics?
Petrus Reply
what is the half reaction of Potassium and chlorine
Anna Reply
how to calculate coefficient of static friction
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how to calculate static friction
Lisa
How to calculate a current
Tumelo
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How to calculate force
Monambi
a structure of a thermocouple used to measure inner temperature
Anna Reply
a fixed gas of a mass is held at standard pressure temperature of 15 degrees Celsius .Calculate the temperature of the gas in Celsius if the pressure is changed to 2×10 to the power 4
Amahle Reply
How is energy being used in bonding?
Raymond Reply
what is acceleration
Syamthanda Reply
a rate of change in velocity of an object whith respect to time
Khuthadzo
how can we find the moment of torque of a circular object
Kidist
Acceleration is a rate of change in velocity.
Justice
t =r×f
Khuthadzo
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Precious Reply
hi
Shongi
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Leago
use fnet method. how many obects are being calculated ?
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Lungile Reply
you use Fnet equals ma , newtoms second law formula
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Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
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