Attraction forces

Forces of
Attraction

Multiple Choice

State of matter with the least spacing between particles

• solid

• liquid

• gas

Multiple Choice

Which of the following states of matter can easily be compressed (made to fit into a small volume)?

• solid

• liquid

• gas

Why are some materials gases while others are liquids or
solids at room temperature even though all particles of matter
at room temperature have the same kinetic energy?

It is all about the attractive forces within and between the particles.

Why are some materials gases while others are liquids or
solids at room temperature even though all particles of matter at room temperature have the same kinetic energy?

� In a solid, the attractive forces keep the particles together tightly enough so that the particles do not move past each other.
Their vibration is related to their kinetic energy.

� In a liquid, The attractive forces between particles are strong enough to hold a specific volume.

� In a gas, The kinetic energy of the molecule is greater than the
attractive force between them, thus they are much farther apart and move freely of each other.

Intramolecular forces

The attractive forces that hold particles together in ionic, covalent, and metallic bonds.

The prefix intra- means within and the term molecular
refers to atoms, ions, or molecules.

Intermolecular forces

The prefix inter- means between or among.

They are forces of attraction which act between neighboring particles (atoms, molecules or ions).

Identical particles such as water molecules in a glass of water.

Two different types of particles, such
as carbon atoms in graphite and the
cellulose particles in paper

Multiple Choice

Intermolecular forces tend to be _____________ than intramolecular forces

• stronger

• weaker

Multiple Choice

In a ___________, intermolecular forces hold the individual particles in place

• solid

• liquid

• gas

Intermolecular Forces

Dispersion Forces

Dipole-dipole

Hydrogen Bond

When things are different at each end, we call them
polar. Some molecules have positive and negative ends too, and when they do, we call them polar. If they
don't, we call them non-polar. Things that are polar can attract and repel each other (opposite charges attract, alike charges repel).

1) Dispersion Forces:

The force of attraction
between the nonpolar
molecules is called a
dispersion force, and
sometimes called London forces after the German- American physicist Fritz London.

Types of intermolecular forces:

Weak forces that result from
temporary shifts in the density
of electrons in the electron
clouds (due to the constant
motion of the electrons).
When two molecules are in close
contact, the electron cloud of
one molecule repels the electron
cloud of the other molecule.
The electron density around each
nucleus is, for a moment, greater
in one region of each cloud, each
molecule then forms a temporary
dipole.

Dispersion Forces

Oxygen molecules are nonpolar because electrons are equally shared
between the equally electronegative oxygen atoms. However, oxygen
under the right conditions can be compressed into a liquid, for oxygen to condense and become liquid there must be a force of attraction between
its molecules.

The sign δ sign represents an area of partial charge on the molecule. When
temporary dipoles are close together, a weak dispersion force exists between oppositely charged regions of the dipole i.e. between δ-and δ+.

Factors Affecting the Strength of the Dispersion Force:

Molecular Size

•Larger and heavier molecules exhibit stronger dispersion forces than smaller and lighter ones.

the number of inner electrons increases

increasing the size of the of molecules.

creating a greater difference between the positive and negative regions of their temporary dipoles .

stronger dispersion forces.

This difference in dispersion forces explains
why fluorine and chlorine are gases at room temperature while bromine is a liquid and iodine is a solid at room temperature.

• Why is chlorine and fluorine gases at room temperature while

bromine is liquid and iodine is solid at room temperature?

• As we go down the halogens group, the atomic radius increases,

increasing the size of the molecules, thus increasing the strength of the dispersion forces holding the molecules.

1) Dipole-dipole
Polar molecules contain permanent dipoles (a region of the polar molecule is partially negative, and the other region is partially positive)
The attraction between oppositely charged regions of a polar molecules are called dipole- dipole forces.

Neighboring polar molecules align themselves so that
the oppositely charged regions align.
Hydrogen chloride gas, the partially positive hydrogen atom Hδ+ in one molecule is attracted to the partially negative chlorine atom Clδ-in another molecule.

Stronger than dispersion weaker than Hydrogen bond.

3) Hydrogen Bond

Special dipole-dipole
attractions that occur between molecules that contain a hydrogen atom bonded to a small, highly electronegative atom with at least one lone pair of electrons, typically fluorine, oxygen, or nitrogen.

In a water molecule, the
hydrogen atoms have a large partial positive charge Hδ+and the oxygen atom has a large partial negative charge Oδ-, when water molecules approach one another, a hydrogen atom in one molecule is attracted to the lone pair of electrons of the oxygen atom in the other molecule.
The strongest one of them.

• Hydrogen bonds: explain

why water is a liquid at
room temperature while compounds of
comparable mass are
gases

• Ammoniaand Water

both form hydrogen
bonds but oxygen is more electronegative than nitrogen making O-H bonds more polar and
thus stronger.

Why is water a liquid at room temperature, while compounds of comparable mass are gases?

The difference between methane and
water is easy to explain. The methane
molecule is a nonpolar molecule as it is a symmetric molecule and the partial
charges are balanced. The only force that holds methane molecules together are weak dispersion forces.

Both water and ammonia are polar molecules
due to their asymmetric shape that results
from the presence of lone pairs of electrons on both the nitrogen and oxygen atoms.

Why is water a liquid at room temperature, while compounds of comparable mass are gases?

The difference between ammonia and water
is quite difficult. Ammonia is gas at room
temperature which means that the attractive forces between ammonia molecules is not as
strong. The N – H bonds in ammonia are less
polar than the O – H bonds in water because
oxygen is more electronegative than
nitrogen. As result, the hydrogen bonds
between water molecules are stronger than
the hydrogen bonds between ammonia
molecules.

Although some
intermolecular forces are weaker than others, all intermolecular forces are weaker than the
intramolecular forces
involved in bonding.

Note:

Summary

Multiple Choice

The strength of the intermolecular attraction ______________ with increasing distance

• increase

• decrease

• stays the same

Multiple Choice

A substance with strong intermolecular forces will tend to have a _____________ boiling point than one with weak intermolecular forces

• higher

• lower

Multiple Choice

Which is the weakest intermolecular force?

• hydrogen bond

• dipole-dipole bond

• London dispersion bond

• covalent bond