1. Chemical Bonding

2. Properties of Ionic and Molecular Compounds The 92 naturally occurring elements, as well as the few artificial elements, which combine to form thousands of compounds. Based on their physical properties, compounds can be classified into two groups: ionic compounds and molecular compounds.

3. Comparing Ionic and Molecular Compounds PropertyIonic CompoundMolecular Compound State at room temperature Crystalline solidLiquid, gas or solid Melting pointHighLow Electrical conductivity as a liquid YesNo Solubility in waterMost have high solubilityMost have low solubility Conducts electricity when dissolved in water YesNot usually

4. ThoughtLab Textbook P. 161 In pairs, you will try and work through this exercise.

5. ThoughtLab Hints: ▫Carbon tetrachloride is not soluble in water. ▫Consider what properties you know of water (melting point). ▫Transition metals can often give us coloured complexes or molecules. ▫Ethanol is a clear, colourless liquid.

6. Bond Formation Why do bonds form? Why are most substances found in nature as compounds and not pure elements? ▫The elements that are found pure in nature have a wide variety of properties. Even atoms that are found pure in nature are actually bonded together to other atoms of the same element.

7. Bond Formation In order for a substance to be solid, something must hold the atoms together. It is only the atoms of group 8 (noble gases) that are never found in nature bonded to any other atoms. You can find a very important clue about bonding by examining the atoms that do not form bonds.

8. Bond Formation What do atoms of the noble gases have in common that is unlike atoms of all other elements? ▫The outer energy level of all the noble gases is filled. ▫This indicates that a filled outer energy level makes atoms very stable and creates no tendency to form bonds with other atoms. ▫In fact, atoms of all other elements other than the noble gases form bonds in a way that will create a noble gas configuration.

9. Bond Formation The most stable arrangement of electrons, either within a single atom or when grouped in compounds, is to have an outer-shell of eight electrons, such as found with the gases of group 0.

10. Bond Formation This tendency is the basis of the octet rule, which states that when bonds form, atoms gain, lose or share electrons in a way that creates an octet or filled outer energy level for the atoms involved in the bonding. ▫The term octet does not apply to H and He for which the outer energy level is filled when 2 electrons are present.

11. Chemical Bonding There are three different groups into which strong chemical bonds can be classified, each according to the types of atoms involved: ▫Metal atoms combine with non-metal atoms to form ionic bonds. ▫Metal atoms combine with metal atoms to form metallic bonds. ▫Non-metal atoms combine with non-metal atoms to form covalent bonds.

12. Ionic Bonding Ionic bonding occurs as the result of a metal atom donating its valence electron(s) to a non- metal atom. As the metal atom loses electrons, it will gain an overall positive charge, becoming a positively charged ion (a cation). Similarly, the non-metal atom accepts electrons, becoming a negatively charged ion (an anion).

13. Ionic Bonding The positive and negative ions are attracted to each other by electrostatic attraction, producing a neutral compound. An ionic bond can be defined as the electrostatic attraction between positive and negative ions. The ionic compound formed is hard and brittle and is often described as a “salt.”

14. Ionic Bonding Atoms of all Group 1 elements have one electron in the valence level. If they lose this electron, the remaining outer level is filled and the resulting ion is stable. Similarly, all atoms of Group 7 elements have seven electrons in the outer level. If they gain an electron, the outer level becomes filled and the atoms are stable.

15. Ionic Bonding

16. Group 2 atoms can lose two electrons to form a full valence shell and Group 6 atoms can gain two electrons to form a full valence shell. Mg O Mg 2+ O 2-

17. Ionic Bonding How can atoms of fluorine, that can gain only one electron to form an octet, form ionic bonds with atoms such as calcium, that must lose two electrons in order to have a closed outer energy level?

18. Ionic Bonding Two atoms of fluorine accept one electron each from an atom of calcium. The resulting compound is represented by the symbol, CaF 2 to show that there are two fluoride ions for every one calcium ion.

19. Structure of Ionic Compounds We have seen that the positive charge on the cation must be balanced by the negative charge on the anion to produce a neutral compound. Ionic compounds do not exist in nature as separate units of, for example, one sodium ion and one chloride ion.

20. Structure of Ionic Compounds Ions that make up ionic compounds arrange themselves into a regular pattern, a lattice structure (an ionic lattice), containing many millions of ions that extend in all three dimensions.

21. Structure of Ionic Compounds No fixed number of ions are involved, but the ration of cations to anions is constant for a given compound and is shown in the empirical formula. The most stable arrangement of ions for any particular ionic compound will be the one in which the positively charged ions are packed as closely as possible to the negatively charged ions, and the ions with the same charge are as far apart as possible.

22. Structure of Ionic Compounds Since each Na+ ion is surrounded by six Cl- ion, it has a coordination number of six. The coordination number gives the number of ions of opposite charge that surround each ion in a crystal.