Chapter 6 – Ionic Compounds

Properties of Ionic Compounds Week 7, Lesson 1 Properties of Ionic Compounds Ionic compounds have common properties: The have high melting and boiling temperatures The are hard but brittle The do not conduct electricity in the solid state The will conduct electricity if they are melted or dissolved in water.

What do these properties reveal about the nature of the particles in ionic compounds? To answer this questions, sodium chloride will be used as the example. Sodium chlorides crystals are cubic, this indicates that the particles have a regular arrangement. The physical properties of NaCl are very different to those of metals, so chemists have deduced that their arrangements in solid ionic compounds are different from those present in metals.

Properties and Structural Features of NaCl Property of NaCl What this tells you about the structure High Melting Temperature Forces between particles are strong Hard, brittle crystals Does not conduct electricity in solid state No free-moving charged particles present in the solid sodium chloride Conducts electricity in molten state Free-moving charged particles present in molten sodium chloride

Properties and particles cont… From these properties we can find that: The forces between the particles are strong There are no free moving electrons present in solid sodium chloride, unlike in metals There are charged particles present but in the solid state, they are not free to move. However, when the ionic compound melts the particles are free to move.

Ionic Bonding Model Chemists believe that when metallic and non metallic atoms react to form compounds, the following steps occur: Metal atoms lose electrons to non-metallic atoms and so become positively charged metal ions. Non-metal atoms gain electron to form negatively charged non-metal ions. Large numbers of positive and negative ions formed in this way then combine to form 3D lattices. The 3D lattice is held together strongly by electrostatic forces of attraction between the positive and negative ions. The electrostatic force of attraction is called ionic bonding

Ionic Bonding Model Two examples representing part of the crystal lattice of the ionic compound sodium chloride.

Properties of Ionic Compounds High melting temperature To melt an ionic solid, energy must be provided to allow the ions to break free and move. Hardness and Brittleness - There are strong electrostatic forces of attraction (ionic bonding) between ions, so a strong force is needed to disrupt the crystal lattice. A lattice cannot be scratched easily, however, if it is hit with a hammer and shatters it is said to be brittle. - This is because the layers of ions will move relative to each other due to the force of the blow. During this movement ions of like charge will move next to each and the resulting repulsion will cause the shattering.

Repulsion causing Shattering

Properties continued… Electrical Conductivity In the solid form, ions in sodium chloride are held in the crystal lattice and are not free to move, so they do not conduct electricity. When solids melt, ions are free to move. The movement of these charged particle to an electrode completes the circuit.

Properties continued… Reaction of Metal with Non-Metals When metallic atoms react with non-metallic atoms, the metallic atoms lose electrons to the non-metallic atoms. This mean that both atoms will often achieve the electronic configuration of the nearest noble gas.

Electron Transfer Diagrams Week 7, Lesson 2 Electron Transfer Diagrams When sodium and chlorine react together to form sodium chloride, the electron that a sodium atom loses to form a stable sodium ion is gained by a chlorine atom to form a stable chloride ion. In the diagram on the next slide, only the main electron shells are shown to demonstrate the electron transfer.

Sodium + Chlorine => Sodium Chloride

Other Representations… Electron transfer between sodium and chlorine can also be illustrated by the following equations: 11Na 1s22s22p63s1 11Na+ 1s22s22p6 e- 17Cl 1s22s22p63s23p5 17Cl- 1s22s22p63s23p6

Overview of the Formation of Sodium Chloride There are several stages in the reaction between sodium and chlorine to form solid sodium chloride. These include: Chlorine molecules splitting into separate chlorine atoms, Electrons being transferred from sodium atoms to chlorine atoms forming positively charged sodium ions and negatively charged chloride ions, Sodium and chloride ions combining to form a 3D ionic lattice.

Magnesium Chloride Magnesium chloride is often dissolved in natural mineral waters. If the ideas of electron transfer are applied to the formation of this compound then; A magnesium atom, electron configuration of 1s22s22p63s2, will have a stable outer shell if 2 electrons are removed. A Mg2+ ion is formed. A chlorine atom, electron configuration of 1s22s22p63s23p5, will have a stable outer shell if it gains 1 electron. A Cl- ion is formed.

Magnesium Chloride cont… This means magnesium donates two electrons to 2 different chlorine atoms. This means it has a chemical formula of MgCl2

Week 7, Lesson 3 Chemical Formulas Almost every compound in which a metal is combined with a non-metal displays ionic bonding. The formulas of simple ionic compounds, such as sodium chloride can be predicted from the electronic configuration of the atoms involved or from their location on the periodic table. Other cases are more complex so chemists find it useful to learn the charge on a number of common ions so that the chemical formula can be written quickly.

The Rules: Writing Formulas Simple Ions The positive ion is placed first in the formula and the negative second Positive and negative ions are combined so that the total number of positive charges is balanced by the total number of negative charges. When there are two or more of a particular ion in a compound, then in the chemical formula the number is written as a subscript after the chemical symbol.

Polyatomic Ions Some ions contain more than one atom. These are called polyatomic ions. They include nitrate, NO32-, carbonate, CO32-, hydroxide, OH- and ammonium NH4+. If more than one of these ions is required to balance the charge in a compound then it is placed in brackets with the required number written as subscript after the brackets.

Ions of Elements with Different Electrovalencies Some elements form ions with different charges. For example, iron sometimes forms a 2+ ion and sometimes a 3+ ion. In this situation you need to specify the electrovalency when naming the compound. This is done by placing an electrovalency of the ion immediately after the metal in the name of the compound, ie, Iron(II) chloride

Uses of Ionic Compounds Week 7, Lesson 4 Uses of Ionic Compounds Marble, calcium carbonate is an ionic compound used for its hardness and strength. It is almost insoluble in water so marble building and statues last a long time. Rocks and clays are much more complex materials, but they also owe useful properties to the presence of strong ionic bonding. Calcium phosphate is another ionic compound useful for its physical properties, it gives strength to our bones.

Uses cont… Many ionic compounds dissolve in water. In this process the ions separate and move freely in the solution. Such solutions will conduct electricity. Sodium hydrogen carbonate, or bicarbonate of soda, is used in baking. This compound decomposes when heated and produces the carbon dioxide which forms bubbles in cakes making them rise.

Uses cont… Sodium hydrogen carbonate is more commonly combined with another ionic compound, potassium hydrogen tartrate, or cream of tartar, in baking powder. It is also found in antacid products as well as some soluble pain killers. Sodium fluoride and calcium fluoride are added to domestic water supplies in some cities because the fluoride ion that is released reduces tooth decay. Fluoride toothpastes contain another ionic compound, sodium monofluorophosphate, as a source of fluoride ions.

Uses cont… Ammonium sulfate and ammonium nitrate are commonly found in fertilisers as they dissolve in water and release nitrogen containing ions, which can be absorbed by plants.