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.