Ionic Bonding

Today’s objectives In today’s class you should learn about What an ion is ( a cation and an anion) What an ionic bond is How ionic bonds are made in some simple compounds How to draw dot and cross diagrams To investigate the characterisitics of ionic compounds

How can reactive metal atoms become stable positive ions?

Definition

Sodium + Chlorine  Sodium Chloride - + Chlorine, Cl Sodium, Na Each sodium atom transfers one electron to each chlorine atom and an ionic electrostatic bond is formed

Formation of an ionic bond

Ionic lattices + - - + - - - + + + - + + - - - - + + + - - - + + + - - In an ionic compound, millions and millions of ions are packed together in a regular cubic arrangement, joined by ionic bonds. This forms a giant 3D structure called an ionic lattice. + - - + - - - + + + - + + - - - - + + + - - - + + + - - - + + + - - - + + +

The crystal structure of sodium chloride

These are crystals of sodium chloride Acknowledgement The image is a coloured scanning electron micrograph (SEM) of  common salt,  sodium chloride,  recrystallised from distilled water.  The salt crystal  is  built up  from  a cubic  lattice  of sodium and chloride ions.  In the  absence  of  impurities  the  exact cubic  crystal form is  produced.  This micrograph shows that in practice this basic cube is  usually disrupted  by  dislocations;   these  give rise to crystals with a variety of shapes,  although  they all    retain    the    basic    cubic   symmetry. Magnification: x280 at 5x7cm size. x975 at 10x8‘. Credit: Andrew Syred/Science Photo Library These are crystals of sodium chloride All ionic compounds form lattices and crystals when solid.

Calcium + Oxygen  Calcium Oxide 2+ 2- Oxygen, O Calcium, Ca Each calcium atom transfers two electrons to each oxygen atom and an ionic electrostatic bond is formed

Formation of an ionic bond

Calcium Oxide – A Regular Lattice

Dot and Cross Diagrams Sodium Chloride & Calcium Oxide - + X X X X Na + Cl X Na Cl X X X X X X X X X 2+ 2- X X X X Ca + O Ca X O X X X X X X X

The Characteristics of Ionic Compounds They are composed of ions which form a rigid structure (lattice) They are brittle They have high melting points and high boiling points When dissolved in solution or when melted they conduct electricity. They are usually soluble in water

Ionic compounds are brittle – they shatter when they are hit. force + - + - + - repulsion + - When the lattice is hit, a layer of ions is shifted so that ions with the same charges are lined up together. These like charges repel each other, thereby splitting the lattice.

High Melting and boiling points Ionic substances generally have high melting and boiling points because all the atoms are strongly bonded together to form a continuous 3D lattice. A large amount of energy is needed to break these bonds. + - strong ionic bonds holds ions together

How do properties differ?

Bonding and electrical conductivity

Conductivity – ionic substances Ionic substances cannot conduct electricity when solid because the ions are bonded together in the lattice. + - ions cannot move to carry a charge When liquid (molten) or dissolved, however, the ions are able to break free of the lattice. + - ions are able to move and carry a charge

Solubility Many ionic substances dissolve in water because water molecules have a slight electrical charge and can attract the ions away from the lattice. + -

Solubility experiment Points to note are that: when a solute dissolves, mass is conserved. when a solute dissolves, the solute and solvent particles intermingle. increasing the temperature increases the rate of dissolving.

Ionic Materials in Everyday Life Farmers add lime (calcium oxide) to soil to reduce the acidity of the soil Athletes take salt tablets to replace salt lost through sweating

Mandatory Experiment The Tests for Anions

Test for Chloride (Cl-) ions Add silver nitrate solution. A white precipitate is produced This white precipitate is soluble in ammonia solution

Test for Chloride (Cl-) ions

Test for nitrate (NO3-) ions Add cold iron(II) sulfate solution. Then add concentrated sulfuric acid down the sides of the test tube A brown ring forms.

Test for carbonate (CO32-) & hydrogencarbonate (HCO3-) ions Add dilute hydrochloric acid solution. Carbon dioxide gas is released which turns limewater milky Add magnesium sulfate solution to a fresh sample The carbonate produces a white precipitate. The hydrogencarbonate does not.

Test for sulfite (SO32-) and sulfate (SO42-) ions Add barium chloride solution. A white precipitate is produced Add hydrochloric acid solution The white precipitate dissolves  Sulfite The white precipitate remains  Sulfate

Tonight’ s homework Write up today’s experiments From the worksheet question 60 Learn all of the anion tests done so far off by heart