GCSE Bonding

Ionic Bonding

An ionic bond is an interaction between a cation and an anion; a metal that has lost at least one electron and a non-metal that has gained at least one electron. This occurs in order to satisfy the octet rule, which states that for an atom to be stable this should have eight electrons in the valence (outer) shell.

In summary, metals lose electrons from the valence shell whilst non-metals gain electrons to get up to eight electrons in the outer shell.

Examples:

Sodium, Na         2,8,1      loses 1 electron to get to               2,8

Calcium, Ca         2,8,8,2   loses 2 electrons to get to             2,8,8

Aluminium, Al    2,8,3      loses 3 electrons to get to             2,8

Nitrogen, N          2,5          gains 3 electrons to get to           2,8

Sulfur, S                2,8,6      gains 2 electrons to get to            2,8,8

Chlorine, Cl          2,8,7      gains 1 electron to get to              2,8,8

Drawing Ionic bonds:

NaCl

MgO

In both NaCl and MgO the number of electrons lost by the metal was the same exact number of electrons gained by the non-metal, and therefore the ratio between the metal and non-metal was exactly 1:1. This is not always the case and in some instances, more atoms of either the metal or the non-metal or both are needed.

Al₂O₃

Al                            O

2,8,3                      2,6

donates 3            needs 2

Cross multiply the electrons to get the number of atoms needed.

2Al                          3O

Properties

These are crystalline in structure.

• An ionic bond is not an interaction between 1 anion and one cation, but rather interactions between multiple anions and cations. For example, in NaCl, each Na⁺ is surrounded by 6 Cl^– and each Cl^– is surrounded by 6 Na⁺. The ratio for CsCl, where Cs⁺ is bigger than Na⁺, is 8:1. The overall formula will still be a 1 is to 1 ratio but the ions are shared amongst each other. This is called having a crystalline structure.

They have high melting points and boiling points.

• Because of this crystalline structure, it is very difficult to break the interactions between different ions, and therefore ionic compounds tend to have high melting points and boiling points.

Dissolve in water and other polar substances.

• The ions found in ionic compounds are charged, and this makes it easy for such compounds to dissolve in water, which is a polar molecule.

In solution they are conductive.

• Electricity travels as a charge, and whilst ionic compounds are neutral and no charge is available for electricity to travel through it, in solutions the ions do possess a charge and therefore ionic compounds are conductive in solution. This is also true for when such compounds are melted, as the ions would be free to move around.

Covalent Bonding

A covalent bond is the sharing of two electrons and in simple covalent compounds, these electrons will come from two different atoms.

Through covalent bonds, atoms can share electrons to fill the valence shell, and obtain a full octet. In order to know how many electrons an atom needs to obtain a full octet, the electronic structure needs to be noted together with the number of electrons needed. An electron can share the same number of electrons that it needs to get a full valence shell.

Example

Hydrogen, H      1              needs 1 electron to get to             2

Carbon, C            2,4           needs 4 electrons to get to           2,8

Oxygen, O           2, 6          needs 2 electrons to get to           2,8

Nitrogen, N         2,5           needs 3 electrons to get to           2,8

Sulfur, S               2,8,6        needs 2 electrons to get to           2,8,8

Chlorine, Cl          2,8,7       needs 1 electron to get to              2,8,8

HCl

For example, both Hydrogen and Chlorine require 1 electron to have a full outer shell, and thus 1 hydrogen and 1 chlorine can react to form HCl

H₂O

In water, oxygen needs two electrons whilst hydrogen can only share 1 electron. This means that oxygen will require 2 hydrogens to get a full outer valence shell.

CO₂

In carbon dioxide, carbon needs 4 electrons whilst oxygen needs 2. This means that for carbon to be able to share 4 electrons it will need 2 oxygens, each of which giving 2 electrons.

Properties

Compounds tend to be made up of small molecules.

• In covalent compounds, there are strong bonds between different atoms but this means that the molecule is only made up of up to a few atoms.

Compounds tend to have low melting points and boiling points.

• As opposed to ionic compounds, there are no charges in covalent compounds, and this means that there is very little interaction between different molecules. The end result is that covalent compounds have low melting points and boiling points as it would be easy to break the interaction between different molecules.

Inorganic covalent compounds tend to have low densities.

• The lack of interactions between different molecules highly influences the packing of such compounds. Very little interactions mean that there will be more spaces between molecules decreases the amount of compound that can fit in a specific volume, decreasing the overall density.

These are insoluble in water.

• The lack of charge in covalent compounds means that these compounds do not dissolve in water.

They do not conduct heat or electricity.

• The lack of charge in covalent compounds means that there are no charges to transfer electrons and therefore covalent compounds do not conduct electricity. They also do not conduct heat.

Metallic Bonding

Both Ionic bonding and Covalent, are mostly found in compounds, with the exception of the diatomic molecules were covalent bonds are found in elements. Most elemental metals have got metallic bonds, which in reality is a sea of electrons keeping metal ions together. This occurs due to the fact that metals would like to lose an electron, in order to have a noble gas configuration, and this is possible if an electron is free to jump from one atom to another.

Properties

They have high melting points and boiling points.

• Because of the charge separation, it is very difficult to break the interactions between different ions, and therefore metallic compounds tend to have high melting points and boiling points.

Insoluble in all solvents.

• The interactions between different particles are very difficult to break down, and therefore metallic compounds are insoluble in any solvent.

They are conductive to both heat and electricity.

• Electricity travels as a charge and as metallic compounds have a sea of electrons these are highly conductive to electricity. Metals also conduct electricity as the atoms are free to vibrate faster, transferring energy from one atom to another.

Metals are malleable and ductile

• The fact that in metals atoms only interact with each other and are not fully bonded means that these atoms can be repositioned in order to create different shapes. This allows for metals to either be flattened or to be drawn into wires.