Bonding and Structure

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  • Created by: Mary
  • Created on: 04-03-13 20:50

Ionic Bonding

Ionic bonding involves attraction between oppositely charged ions formed by electron transfer

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Dot and Cross Diagrams - Ionic Bonding

Metals lose electrons to form positive ions and non-metals gain electrons to form negative ions. The ions have a stable electronic structure, ie: 8 electrons in its outer shell. 

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Lattice Structure of NaCl

Sodium chloride is an ionic solid which has a lattice structure. Each sodium atom is surrounded by 6 chloride ions and vice versa. 

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Physical Properties of Ionic Compounds

Melting points: High due to strong electrostatic bonds between ions. 

Boiling points: High due to strong electrostatic bonding. 

Electrical conductivity: Do not conduct electricity when solid, but do conduct electricity when molten because the ions are free to move and carry charge. 

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Covalent Bonding

A covalent bond is defined as a shared pair of electrons. Some compounds have multiple bonds, eg: ethene (C2H4), nitrogen (N2) and carbon dioxide (CO2)

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Octet Rule

The octet rule says that an atom will form a bond, either ionic or covalent to gain a nobel gas configuration (ie: 8 electrons in its outer shell). 

However, there are exceptions such as beryllium dichloride (BeCl2) and boron trifluoride (BF3). 


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Physical Properties of Molecular Covalent Crystals

Many covalent compounds exist as molecular covalent crystals, one such example is iodine (I2).

Melting point: Low melting point due to weak bonds between molecules. 

Boiling point: Low due to weak bonds between molecules. 

Electrical conductivity: Cannot conduct electricity either when solid or when molten because they do not contain charged particles to carry charge.

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Coordinate Bonding

This is a special type of covalent bonding where both electrons come from the one atom and are shared between two atoms to form the covalent bond. They are indicated using an arrow. One such example is the ammonium ion (NH4+)

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Metallic Bonding

Involves a lattice of positive ions surrounded by delocalised electrons. 

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Properties of Metals

Melting point: High melting points due to strong bonding between positively charged ions and electrons. 

Boiling point: Generally high due to strong bonding between positively charged ions and electrons. 

Electrical conductivity: High, can conduct electricity both in the solid and the molten state, because the delocalised electrons can carry charge throughout the structure. 

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Graphite and Diamond

Both are allotropes of carbon and both have giant covalent structures. 

Graphite: contains hexagonal layers of carbon atoms. There are bonds between the layers called van der Waals. 

Diamond: tetrahedral structure, where each carbon atom is bonded to 4 others. 

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Physical Properties of Graphite and Diamond

Graphite: can conduct electricity (due to free electrons), not very hard (as the layers can pass over each other), high melting and boiling points because the bonds between carbon atoms are very strong and there are a lot of them.

Diamond: cannot conduct electricity, very hard, high melting and boiling points because the covalent bonds are very strong and there are a large number of them. 

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Electronegativity

Defined as the ability of an atom to attract electrons toward itself in a covalent bond

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Trends in Electronegativity

Across periods: the electronegativity generally increases across periods because the nuclear charge increase as you go across periods. Remember noble gases do not have a value for electronegativity because they exist as single atoms. 

Down groups: the electronegativity generally decreases down groups because the atoms get larger, hence the nucleus is further away from the electrons and cannot attract them as easily. 

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Bond Polarity

Differences in electronegativity causes some atoms to have a slightly positive charge and some atoms to have a slightly negative charge, sometimes within the same molecule. 

This means that the molecule becomes slightly polarised, ie: there is a slight separation of charge. 

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Polar Bonds

Bonds that have differences in electronegativity and have a separation of charge are said to be polar

However, the separation of charge that results can be either permanent or temporary - called permanent and temporary dipoles

In symmetrical molecules the dipoles cancel each other out, hence their separation of charge is only temporary and it is said to have a temporary dipole

Unsymmetrical molecules have dipoles which do not cancel each other out, hence they have permanent dipoles

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