Polar and non-polar molecules
Chemical bonding is the result of either an atom sharing one or more outer orbit electrons with another atom or an atom taking outer orbit electrons from the atom with which it is bonding. Normally, an atom has an even distribution of electrons in the orbits or shells, but if more end up on one side that the other in a molecule, there can be a resulting electrical field in that area.
Water is a polar molecule because of the way the atoms bind in the molecule such that there are excess electrons on the Oxygen side and a lack or excess of positive charges on the Hydrogen side of the molecule.
Water is a polar molecule with positive charges
on one side and negative on the other.
Examples of polar molecules of materials that are gases under standard conditions are:
Sulfur Dioxide (SO2)
Hydrogen Sulfide (H2S).
Also, Ethanol is polar, since its oxygen molecule draws electrons towards it due to its high electro-negativity, causing a negative charge around itself.
A non-polar molecule is one that the electrons are distributed more symmetrically and thus does not have an abundance of charges at the opposite sides. The charges all cancel out each other.
The electrical charges in non-polar Carbon Dioxide are evenly distributed
Examples of non-polar liquids
Most hydrocarbon liquids are non-polar molecules. Examples include:
Alkynes are non-polar because they cannot be dissolved in water, as do polar molecules. However, alkynes but do dissolve in other non-polar substances. A rule is that like substances dissolve in like substances.
Examples of non-polar gases
Common examples of non-polar gases are the noble or inert gases, including:
Other non-polar gases include:
Carbon Dioxide (CO2)
Since Chloroform is more soluble in fats than in water, it is also classified as non-polar.
Rule for Solutions
The rule for determining if a mixture becomes a solution is that polar molecules will mix to form solutions and non-polar molecules will form solutions, but a polar and non-polar combination will not form a solution.
Water is a polar molecule and oil is a non-polar molecule. Thus they won’t form a solution. On the other hand, since alcohol is a polar molecule, it will form a solution with water.
From these examples, we note that three factors determine whether a molecule is polar and non-polar. These are
The polarity of the covalent bond
The number and position of lone pairs
The shape of the molecule
General Structures of Crystals
The external shape of a crystal is the result of the spatial arrangement of the particles that make up the crystal. The three-dimensional arrangement of the particles is called the crystal lattice. The crystal lattice may be considered to be made up of unit cells which are each the smallest portion of the crystal lattice which shows the complete pattern of the particles in their relative positions
There are three kinds of unit cells based on the cubic structure. These are as follow:
Simple cubic: These are structures in which the particles are placed one at each corner of the cube.
Face-centred cubic: These are structure in which there is a particle at each corner and one in the centre of each face of the cube
Body-centred cubic: These are structure in which there is a particle at each corner and one at the centre of the cube.
Covalent Compounds – Simple Molecules
Form between non-metal atoms. Each bond consists of a shared pair of electrons, and is very strong. Covalently bonded substances fall into two main types:
Simple molecules and
Giant covalent structures.
These contain only a few atoms held together by strong covalent bonds. An example is carbon dioxide (CO2), the molecules of which contain one atom of carbon bonded with two atoms of oxygen.
Properties of Simple Molecular Substances
Low melting and boiling points – This is because the weak intermolecular forces break down easily.
Non-conductive – Substances with a simple molecular structure do not conduct electricity. This is because they do not have any free electrons or an overall electric charge.
Hydrogen, ammonia, methane and water are also simple molecules with covalent bonds. All have very strong bonds between the atoms, but much weaker forces holding the molecules together. When one of these substances melts or boils, it is these weak ‘intermolecular forces’ that break, not the strong covalent bonds. Simple molecular substances are gases, liquids or solids with low melting and boiling points.
Covalent bonding – Giant Covalent structures
Giant covalent structures contain a lot of non-metal atoms, each joined to adjacent atoms by . The atoms are usually arranged into giant regular lattices – extremely strong structures because of the many bonds involved. The graphic shows the molecular structure of diamond and graphite: two allotropes of carbon, and of silica (silicon dioxide).
From left to right – graphite, diamond, silica
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