FORCES OF
ATTRACTION 2
OVERVIEW
1. VSEPR THEORY
2. INTERACTIONS BETWEEN
MOLECULES
3. COMPOUND CLASSIFICATION
2
VSEPR THEORY
3
SHAPES OF MOLECULES
VSEPR THEORY
• Valence Shell Electron Pair Repulsion Theory
• Used to predict the shape of a molecule
• Assumes that the geometry of a molecule depends
only upon electron-electron interaction
4
SHAPES OF
MOLECULES
5
SHAPES OF MOLECULES
• The shape of a molecule refers to how the atoms of a
molecule arrange themselves in space
• Affects physical and chemical properties of the
molecules
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
SHAPES OF MOLECULES
VSEPR THEORY: ASSUMPTIONS
1. Two electrons are needed to form a single bond
2. Some atoms in a molecule may have electrons not
involved in bonding (non bonding electrons/lone
pairs)
3. Bonding pairs (bp) and lone (lp) around a particular
atom will adopt a position which will allow them
minimum interaction with each other
4. Repulsion: lp - lp > lp - bp > bp - bp
22
SHAPES OF MOLECULES
VSEPR THEORY: RULES FOR PREDICTING SHAPES
1. Identify the central atom
2. Count its valence electrons
3. Add one electron for each bonding atom
4. Add or subtract electrons for charge
• Add for negative charge
• Subtract for positive charge
5. Divide the total of these by 2 to find the total number of electron
pairs
6. Separate into lone pairs and bonding pairs
7. Use this information to predict the shape
23
VSEPR THEORY
APPLIC ATION: PREDICTION OF SHAPE XeF2
5 electron pairs
Central Atom Xe
# Valence Electrons 8 2 b.p. 3 l.p.
Number of F atoms bonded 2
No Charge 0
Linear
# of Electrons 10
# of Electron Pairs 10÷2= 5
Hybridisation??? spppddddd
Hybridisation sp3d
24
INTERACTIONS
BETWEEN MOLECULES
INTERACTIONS BETWEEN MOLECULES
POL AR MOLECULES
• When a covalent bond occurs between two different
atoms, a polar covalent bond is formed
• Atoms are said to have acquired a partial charge
• There is an unequal distribution of electron density
INTERACTIONS BETWEEN MOLECULES
POL AR MOLECULES
INTERACTIONS BETWEEN MOLECULES
DIPOLES AND DIPOLE MOMENTS
•A dipole is a separation of electrical charges
• They can occur between two ions in an ionic bond or
between atoms in a covalent bond
•D i p o l e moments arise from differences in
electronegativity
INTERACTIONS BETWEEN MOLECULES
DIPOLES AND DIPOLE MOMENTS
• The dipole moment is a measure of the polarity of the molecule
•A dipole is defined qualitatively by its dipole moment,
If two charges of equal magnitude but opposite spin are
separated by a distance, r, then the system is said to have a dipole
moment
The dipole moment is given by,
= charge (q) x distance (r)
Unit of measurement - Debyes
INTERACTIONS BETWEEN MOLECULES
DIPOLES AND DIPOLE MOMENTS
• The larger the difference in electronegativity, the larger
the dipole moment
• The distance between the charge separation is also a
deciding factor into the size of the dipole moment
INTERACTIONS BETWEEN MOLECULES
DIPOLES AND DIPOLE MOMENTS
•A dipole is represented by an arrow with a plus sign that points
from the positive charge to the negative charge
• The overall dipole of a molecule is the sum of the individual
bond dipoles within the molecules
INTERACTIONS BETWEEN MOLECULES
DIPOLES AND DIPOLE MOMENTS
• The shapes of a molecule is important in deterring whether a
molecule will have a resulting dipole
• Molecules that are highly symmetric are non polar
INTERACTIONS BETWEEN MOLECULES
DIPOLES AND DIPOLE MOMENTS
Can a molecule have polar bonds but yet be non-polar (no
dipole moment) ?
• A molecule can possess polar bonds and still be non-
polar
• If the polar bonds are evenly (or symmetrically)
distributed, the bond dipoles cancel and do not create a
molecular dipole
INTERACTIONS BETWEEN MOLECULES
DIPOLES AND DIPOLE MOMENTS
Can a molecule have polar bonds but yet be non-polar (no dipole moment) ?
• For example, the three bonds in a
molecule of BF3 are significantly polar,
but they are symmetrically arranged
around the central boron atom
• No side of the molecule has more
negative or positive charge than
another side, so the net dipole
moment is zero
• Hence, the molecule is non-polar
INTERACTIONS BETWEEN MOLECULES
INTRAMOLECUL AR/INTERMOLECUL AR FORCES
• Intramolecular forces are the forces that hold atoms
together within a molecule (eg. ionic, covalent and metallic)
• Intermolecular forces are forces that exist between molecules
or functional groups of macromolecules
INTERACTIONS BETWEEN MOLECULES
INTERMOLECUL AR FORCES
• Intermolecular forces are much weaker than
intramolecular forces
• The strength of intermolecular forces determines
how high or low a substance melting and boiling
point is
• To melt or boil a molecular substance, the molecules
must be set free from one another
INTERACTIONS BETWEEN MOLECULES
INTERMOLECUL AR FORCES
• The strong covalent bonds within molecules remain
intact when a molecular substance melts or boils
• The stronger the intermolecular forces, the higher
the boiling point
INTERACTIONS BETWEEN MOLECULES
T YPES OF INTERMOLECUL AR FORCES
van der Waals forces of attraction:
1. Dipole-dipole interactions
2. Dipole-induced dipole interactions
3. London/dispersal forces
INTERACTIONS BETWEEN MOLECULES
T YPES OF INTERMOLECUL AR FORCES
Dipole-dipole interactions
• Forces that occur between two molecules with permanent
dipoles
• If a molecule has a dipole, then the positive end of the dipole of
one molecule will attract the negative end of the dipole of
another molecule
• The strength of the dipole moment depends on the
electronegativity differences of the atoms in the molecule
INTERACTIONS BETWEEN MOLECULES
T YPES OF INTERMOLECUL AR FORCES
Dipole- induced dipole interactions
• When a molecule with a permanent bond dipole and comes close to
one with no bond dipole, the electric field associated with the
permanent dipole can temporarily distort the electron distribution in
the non-polar molecule
• This induces a temporary dipole
INTERACTIONS BETWEEN MOLECULES
T YPES OF INTERMOLECUL AR FORCES
London/Dispersal Forces
• Present in all molecules; whether polar or non-polar
• Since the electrons are in constant motion, it is possible
that at some point most of the electrons will be on one
side of the atom
• At that moment one side of the atom will have partial
negative charge while the other will have a partial positive
charge
INTERACTIONS BETWEEN MOLECULES
T YPES OF INTERMOLECUL AR FORCES
London/Dispersal Forces
• The molecule is said to have an instantaneous
dipole; they attract each other
• The larger the molecule, the greater the volume of
space and the more electrons it has, there more likely
the instantaneous dipole will occur
INTERACTIONS BETWEEN MOLECULES
T YPES OF INTERMOLECUL AR FORCES
London/Dispersal Forces
INTERACTIONS BETWEEN MOLECULES
T YPES OF INTERMOLECUL AR FORCES
London/Dispersal Forces
The strength of the London forces increases as the relative molecular mass
(RMM) and the atomic number increases
• Larger and heavier atoms and molecules exhibit stronger dispersion
forces than smaller and lighter ones
• The valence electrons are farther from the nuclei than in a smaller
atom or molecule
• Hence, less tightly held and can more easily form temporary dipoles
INTERACTIONS BETWEEN MOLECULES
T YPES OF INTERMOLECUL AR FORCES
London/Dispersal Forces
Shape also affects strength of the London forces
• Long, skinny molecules (like n-pentane) pack together
more efficiently- stronger interaction
• Short fat ones pack less efficiently – Weaker interaction
• Hence, a branched molecule has less LF than a long-
chained molecule of the same RMM
INTERACTIONS BETWEEN MOLECULES
T YPES OF INTERMOLECUL AR FORCES
London/Dispersal Forces
• As you go down group 17, the halogens go from gas to
liquid to solid
• Florine and Chlorine (gases)
• Bromine (liquid)
• Iodine (solid)
• As you go down the homologous series, the boiling point
increases
INTERACTIONS BETWEEN MOLECULES
T YPES OF INTERMOLECUL AR FORCES
Hydrogen Bonding
• A unique case of dipole-dipole interaction
• Strongest intermolecular force known
• Hydrogen has one electron so when it is bonded to a highly
electronegative atom (e.g. O,N,F) that atom pulls the electron
density towards itself, leaving the backside of the hydrogen with a
large positive charge
• The large partial positive charge attracts the non-bonding
electrons on an atom of a neighbouring molecule
INTERACTIONS BETWEEN MOLECULES
T YPES OF INTERMOLECUL AR FORCES
Hydrogen Bonding
INTERACTIONS BETWEEN MOLECULES
T YPES OF INTERMOLECUL AR FORCES
Hydrogen Bonding
Hydrogen bonding explains why:
1. The boiling points of HF, NH3 and H2O are high
compared to the hydrides of their corresponding
groups
2. Ice floats on water. In ice each water molecule is
tetrahedrally surrounded by 4 others. The hydrogen
bond holds the molecules for apart causing it to have an
open structure, making it less dense than water
STRENGTH OF
INTERMOLECULAR
FORCES OF ATTRACTION
CLASSIFICATION
OF COMPOUNDS
CLASSIFICATION OF COMPOUNDS
1. Molecular (simple covalent)
2. Ionic Crystalline
3. Giant Covalent
4. Metallic
CLASSIFICATION OF COMPOUNDS
SIMPLE COVALENT
• Atoms are covalently bonded to each other forming a molecule
• Molecules are soft and have low melting points
• Molecules are held together by weak intermolecular forces
• Molecules do not conduct electricity (no free electrons or ions)
CLASSIFICATION OF COMPOUNDS
IONIC CRYS TALLINE
• Extensive array of ionic bonding throughout the structure
• Ions arrange themselves in crystal so as to maximise attraction and
minimise repulsion
• Electrostatic force holding the ions together are very strong
• High lattice energy ——> hard and brittle
• In solid form, ions are held in place ——> non conductor
• In molten form, ions are free to move ——> conductor
CLASSIFICATION OF COMPOUNDS
GIANT COVALENT COMPOUNDS
• Extensive network of covalent bonds between atoms that extend
through the entire structure
• Covalent bonds are strong
• Compounds are hard ——> high melting and boiling points
• Lack of ions and all electrons are held in place ——> non conductor
• Exhibit hardness; used in cutting tools
CLASSIFICATION OF COMPOUNDS
METALLIC
• Atoms are close packed i.e. they try to get as close together as possible
• Solid is held together by the electrostatic forces between the lattice of
positive ions and the sea of electrons that are constantly moving
• Since electrons are moving ——>very good conductor
• Melting point and hardness vary over wide ranges
• Some are soft e.g. Na
• Some are hard e.g. Fe
