AS Level Chemistry AQA Unit 1 Section 3: Bonding and Periodicity.

A breif overview of the key topics in Unit 1 Section 3.  Done using the CGP revision guide.

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Introduction

Atoms of different elements that are bonded together are refered to as compounds. There are many ways in which these atoms can bond together:

Ionic,

Covalent,

Metalic,

Coordinate (is actually a type of covalent bond)

Molecules are: 

"a group of like or different atoms held together by chemical forces."

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Ionic Bonding

The atoms in Ionic bonding are held together by electrostatic attraction and Ions are formed when electrons are transferred from one atom to another. This usually give one atom a positive charge and the other a negative charge due to the transfer of a negatively charged electron. 

The more simple ions have only lost or gained 1, 2 or 3 electrons such as:

Elements in groups 1 and 2 lose electrons as they have few electrons on their outer shell, such as:

Sodium forms Na(+) ions:    Na --> Na(+)  +   e(-)

Elements in groups 6 and 7 gain electrons as they only need a few electrons to fill their outer shell, such as:

Chlorine which forms Cl(-) Ions: Cl + e(-) --> Cl(-)

Electrostatic attraction will hold these oppositely charged Ions together strongly to form NaCl

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Ionic Compounds

NaCl as described on the previous card is an Ionic compound and can be displayed with a dot and cross diagram:

                            .   ----------------\

                         .     .                      \ >                      x   x

                   :      Na      :                                      x     Cl    x  

                          .     .                     /  >                     x    x

                            .   ----------------/

The transfer of electrons from the Na atom to the Cl atom produces 2 ions both with full outer shells. One atom will have a 2(+) charge (the Na) and the other will have a 2(-) charge (the Cl), this is due to the transfer of the negatively charged electrons.                   

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Ionic compounds continued.

The opposite charges that these ions posses will attract them together so that they form a Giant Ionic Lattice. A lattice has a regular structure. 

In the NaCl example the ions will be packed together to form a cube where every Cl ion is next to 4 Na Ions and every Na Ions is next to 4 Cl ions. 

This ionic structure explains the chemical properties of the chemical concerned. 

Ionic compounds only carry an electrical charge when molten as then the charged ions can move through the layers to carry a charge. They cannot move in a tightly packed solid. So they cant carry a charge when solid.

Ionic compounds have very high melting and boiling points as Ionic bonds are very strong and require a lot of energy to overcome.

Ionic compounds dissolve in water as the difference in electronegativity between the H and O atoms in water cause it to be polar and its delta charges are strong enough to pull apart the Ionic bonds between the Na(2+) an the Cl(2-) Ions.

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Covalent Bonding

Molecules are the smallest parts of compounds that can take part in chemical reactions and are formed when 2 or more atoms bond together. Molecules are held together with Covalent bonds.

In covalent bond two atoms share electrons so that they both have a full outer shell. The attraction comes from the difference in charge between the positive nucleus and the negative electrons. 

There are also double and triple covalent bonds when there are 2 or 3 pairs of electrons shared between the atoms. Such as N(2) where there is a triple bond between the N atoms.   

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Giant Covalent Structures

Giant covalent structures (or macromolecular structures) have a huge network of covalently bonded atoms. 

Carbon atoms can form this type of of structure as they can form 4 strong covalent bonds each. 

Diamond and Graphite are two giant covalent structures that you need to be able to discuss the physical and chemical properties of. 

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Diamond

Is the hardest know substance. Each carbon atom has made as many bonds as it can (4). This means that each carbon is covalently bonded to 4 other carbon atoms. The atoms arrange them selves in a tetrahedral shape- its a crystal lattice structure. 

Diamond has a very high melting point. and is very hard because of its covalent bonds.

It is also a good thermal conductor as vibrations travel easily through the lattice as it is very stiff.

It cant conduct electricity as all electrons are used in the bonds between the carbon atoms. 

It will not dissolve in any solvent as polar molecules can not separate the bonds between

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Graphite.

Consists of sheets of hexagons with delocalised electrons between the layers. Its properties are also down to its structure.

Graphite is used as a lubricant as the hexagons are in layers and the intermolecular forces are weak van der Waals forces that can slide over each other easily.

The delocalised electrons in graphite are free to move between the layers giving graphite the ability to carry an electrical charge.

 Graphite has a low density as there is a large gap between the layers.

It has a very high melting point and isn't soluble as the covalent bonds are strong.

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