EquilibriumAcids and BasesRedoxOrganic

 Bonding

  • Introduction
  • Ionic Bonding
  • Covalent Bonding
  • Metallic Bonding

Noble gases are stable and unreactive because they have a full energy shells. All other elements are seeking this property too. They all want to have full outer most energy shells; this is why reactions take place. For example if an atom has 6 electrons in its outer most shell, it tries to gain two electrons from other atoms to complete the shell with 8 electrons. There are three types of bondings:

Ionic Bonding
Covalent Bonding
Metallic Bonding


Ionic bonding is based on the electrostatic force of attraction between the ions in the compound. For example, when sodium, which is in group one and has one electron in its outer most shell, reacts with chlorine, which is in group 7 and has 7 electrons in its outer most shell, the sodium gets rid of the only electron in its outer shell, thus the sodium atom will have its second most outer shell which is full become its most outer shell forming a positive ion. The electron which is lost by the sodium atom is gained by the chlorine atom to 8 electrons, thus filling its outer most shell and becoming a negative ion. This electron transfer causes the electrostatic force of attraction which holds the oppositely charged ions together in a compound. When an atom becomes an ion, it gets the properties of the noble gas which is nearest to it in the periodic table.

magnesium Oxide

 

Properties of Ionic Compounds:

1. Hard solids at room temperature.
2. High melting and boiling points because of strong attraction forces.
3 . When solid they are electrical insulators but conduct electricity when molten or aqueous

 

 


Covalent bonding occurs between non-metals. In order to obtain a full outer most energy shell, the atoms tend to share the electrons of their outer most energy shell, some or all of them.

 

 

Giant Molecular Structure:

They are also known as macromolecular structures. One molecule contains hundreds of thousands of atoms. They have extremely strong bonds between the atoms (intermolecular bonds).


In the graphite structure, each carbon atom is strongly bonded covalently to three other carbon atoms forming layers of linked hexagons. Each layer acts as a molecule, the intermolecular forces between the layers is very weak allowing layers to slide over each other. This makes graphite a good lubricant

The physical properties of graphite:

Has a high melting point, similar to that of diamond. In order to melt graphite, it isn't enough to loosen one sheet from another. You have to break the covalent bonding throughout the whole structure.

Has a soft, slippery feel, and is used in pencils and as a dry lubricant for things like locks. You can think of graphite rather like a pack of cards - each card is strong, but the cards will slide over each other, or even fall off the pack altogether. When you use a pencil, sheets are rubbed off and stick to the paper.

Has a lower density than diamond. This is because of the relatively large amount of space that is "wasted" between the sheets.

Is insoluble in water and organic solvents - for the same reason that diamond is insoluble. Attractions between solvent molecules and carbon atoms will never be strong enough to overcome the strong covalent bonds in graphite.

Conducts electricity. The delocalised electrons are free to move throughout the sheets. If a piece of graphite is connected into a circuit, electrons can fall off one end of the sheet and be replaced with new ones

 

 

graphite

 

In diamond’s structure, each carbon atom is covalently bonded to four other carbon atoms by very strong bonds forming a 3D tetrahedral shape.

The physical properties of diamond:

Has a very high melting point (almost 4000°C). Very strong carbon-carbon covalent bonds have to be broken throughout the structure before melting occurs.

Is very hard. This is again due to the need to break very strong covalent bonds operating in 3-dimensions.

Doesn't conduct electricity. All the electrons are held tightly between the atoms, and aren't free to move.

Is insoluble in water and organic solvents. There are no possible attractions which could occur between solvent molecules and carbon atoms which could outweigh the attractions between the covalently bound carbon atoms.




diamond

Properties of Covalent Compounds:

Simple molecular structures are usually gases or liquids and sometimes solids with low melting points; this is because of weak forces of attraction between the molecules which can be broken easily.

Giant molecular structures have very high melting points because the whole structure is held together with very strong covalent bonds.

Most of them do not conduct electricity except Graphite.

Most of them are insoluble in water.

 

Metals form metallic lattice.
The crystal lattice of metals consists of ions surrounded by a 'sea of electrons' forming another type of giant lattice.
The outer electrons (-) from the original metal atoms are free to move around between the positive metal ions formed (+).
These free or 'delocalised' electrons are the 'electronic glue' holding the particles together.
There is a strong electrical force of attraction between these free and mobile electrons (-) and the 'immobile' positive metal ions (+) and this is the metallic bond.
Metallic bonding acts in every direction about the fixed (immobile) metal ions.

metallic lattice

This strong bonding generally results in dense, strong materials with high melting and boiling points.

Usually a relatively large amount of energy is needed to melt or boil metals. Energy changes for the physical changes of state of melting and boiling for a range of differently bonded substances are compared in a section of the Energetics Notes.

Metals are good conductors of electricity because these 'free' electrons carry the charge of an electric current when a potential difference (voltage!) is applied across a piece of metal.

Metals are also good conductors of heat. This is also due to the free moving electrons. Non-metallic solids conduct heat energy by hotter more strongly vibrating atoms, knocking against cooler less strongly vibrating atoms to pass the particle kinetic energy on. In metals, as well as this effect, the 'hot' high kinetic energy electrons move around freely to transfer the particle kinetic energy more efficiently to 'cooler' atoms.

Metals also have a silvery surface but it may be easily tarnished by corrosive oxidation in air and water.

Alloys:
An alloy is a mixture of metals or metals and non-metals. Sometimes, and alloy is better than a metal because they have better properties. They are harder, more resistant to corrosion and have a more attractive appearance than the metals they are formed of.
Alloys are harder than metals because they have different sized atoms which prevent the layers from sliding over each other.
And alloy is made by heating the metals or metals and non-metals together until they all melt, and leaving them to cool mixed.
Examples of alloys and their content:
Brass: Copper-Zinc,
Bronze: Copper-Tin,
Steel: Iron-Carbon,
Stainless Steel: Iron-Carbon-Chromium-Nickel.