Chemical Bonding

1. Chapter 6 Chemical Bonding

2. contents • You will learn about: • Ionic bonds – electron transfer • Covalent bonds – electron sharing • Properties of ionic and covalent compounds

3. The noble gases (Group 0 elements) are also known as inert gases. They are gases.

4. When the outer shell of an element is completely filledwith electrons, the element is very stable. Atoms of noble gases in Group 0 are very stable. Hence, they do not react with other elements. Noble gas configurations are very stable because they have fully filledouter/valence shells. Noble Gas Electronic Configuration

5. He Ne Ar • Noble gases (except for helium) have 8 electrons in their outer shells. Examples Arrangement of electrons in the outer shell 2 Duplet structure helium neon Octet structure 2.8 argon Octet structure 2.8.8

6. The tendency for atoms to surround themselves with a stable ‘octet’ of electrons is called the octet rule. Helium has only 2 electrons in its outer shell , but is stable as it too has a fully filled outer shell of electrons. The maximum number of electrons that can be accommodated in the 1st shell is 2. This is called the duplet rule.

7. Thus, they react with one another by losing, gaining or sharing their valency electrons to achieve the stable noble gas electronic configuration during a chemical reaction. ( valency electrons are electrons in the outer shell used in chemical bonding) Other elements do not have the stableelectronic configuration where the outer shell is fully filled with electrons forming either the duplet or octet structure.

8. Chemical bonding Covalent bonding Ionic bonding By sharing of electrons between atoms of non-metals By losing or gaining of electrons (i.e transferring of electrons) Takes place between atoms of metals and non-metals Covalent substances Ionic compounds

9. Formation of Cations (positive ions) and Anions (negative ions) An ion is a charged particle formed from an atom or a group of atoms by the loss or gain of electrons .It can be either positively or negatively charged. • atoms form ions by gaining or losing electrons to obtain the stable electronic structure of a noble gas - negative ion  add electrons more electrons than protons neutral atom electrons = protons + positive ion remove electrons more protons than electrons

10. Electronic configuration of Metals & Non-metals

11. Metalstend to lose their valence electrons to achieve the stable noble gas electronic configuration as they have fewvalence electrons. Non-metalstend to gain electrons to achieve the stable noble gas electronic configuration as they have a lot of electrons in their valence shell Note : only the valence (outer) electrons are involved in bonding; those in the complete inner shells do not take part. Electronic configuration of Metals & Non-metals

12. Formation of Cations Examples Li atom loses an electron readily and become a lithium ion, Li+

13. Na atom loses an electron and becomes a positive ion, sodium ion, Na+ which have the stable noble gas electronic structure as Neon (2,8). + loses 1 electron Na Na 2,8 Na+ ion Na atom 2,8,1 11 protons 10 electrons 12 neutrons 11 protons 11 electrons 12 neutrons Charged(+1) Neutral 11 protons (+11) and 10 electrons (-10)  overall charge = (+11 -10) = +1 [all +ve charges (protons) balanced by -ve charges (electrons)]

14. Al atom loses 3 electrons and becomes a positive ion, aluminium ion, Al3+ which have the stable noble gas electronic structure as Neon (2,8). 3+ loses 3 electrons Al Al 2,8 Al3+ ion Al atom 2,8,3 13 protons 10 electrons 14 neutrons 13 protons 13 electrons 14 neutrons Charged(+3) Neutral 13 protons (+13) and 10 electrons (-10)  overall charge = (+13 -10) = +3 [all +ve charges (protons) balanced by -ve charges (electrons)]

15. Draw diagrams to show the formation of (a) A potassium ion (b) A calcium ion  Now You Try!

16. Cl atom gains 1 electron from a metal atom and becomes a negative ion, named as chloride ion, Cl- which has the same noble gas electronic structure as Argon (2,8,8). ¯ x Cl Formation of Anions gains 1 electron Cl chlorideion chlorine atom 2,8,7 2,8,8 17 protons 17 electrons 18 neutrons Charged (-1) 17 protons (+17) and 18 electrons (-18)  overall charge = (+17-18) = -1 17 protons 18 electrons 18 neutrons Neutral

17. Draw diagrams to show the formation of a fluoride ion (b) an oxide ion (c) a nitride ion Now You Try ………

18. Potassium atom Magnesium atom Magnesiumion Potassium atom 39 24 24 39 Symbol Mg2+ K K+ Mg 12 19 19 12 No of protons 12 12 19 20 Number of electrons 10 19 18 12 Number of neutrons 12 20 20 12 Cations- more examples Note: A positive ion still has the same number of protons and neutrons as its atom, but, it will have less electrons than protons.

19. Sulphur atom Sulphide ion Oxygen atom Oxide ion No of protons 16 16 8 8 Number of neutrons 16 16 8 8 Anions – more examples Symbol S2- O O2- 32 16 S 32 16 16 8 16 8 Number of electrons 16 18 8 10 Note: A negative ion still has the same number of protons and neutrons as its atom, but, it will have more electrons than protons.

20. Ionic bonding – involves the transfer of electrons from one atom to another so that each can achieve the noble gas electronic configuration. • usually formed between atoms of metals and non-metals • positive and negative ions are formed after the transfer of electrons • The oppositely charged ions are attracted to each other by strong electrostatic force of attraction.

21. This strong electrostatic force of attraction between the oppositely charged ions is called ionic bond. Example

22. ionic bonds – electron transfer ionic bonds

24. chlorine atom, Cl 2.8.7 Diagrammatic Representation of Ionic Bonding Example 1 Formation of sodium chloride through ionic bonding electron transfer sodium atom, Na 2.8.1

25. An ionic compound, sodium chloride ( NaCl ) is formed. - + -electron of Na - electron of Cl Na Cl sodium ion, Na+ 2.8 chlorine ion, Cl- 2.8.8 has the same electronic structure as the noble gas, argon has the same electronic structure as the noble gas, neon ‘DOT and CROSS’ Diagram

26. electron transfer electron transfer ionic bonds – electron transfer Example 2 Formation of magnesium chloride through ionic bonding Mg Cl Cl magnesium atom, Mg 2.8.2 chlorine atom, Cl 2.8.7 chlorine atom, Cl 2.8.7

27. - - 2+ Cl Mg Cl chloride ion, Cl- 2.8.8 magnesium ion, Mg2+ 2.8 chloride ion, Cl- 2.8.8 has the same electronic structure as the noble gas, argon has the same electronic structure as the noble gas, neon has the same electronic structure as the noble gas, argon ionic bonds – electron transfer

28. ionic bonds Ionic bonds are formed between atoms of metals and non-metals in compounds. Examples include:

29. Structures of Ionic Compounds Ionic compounds have giant ionic lattice structure.

30. - - - + + - - + + + - - + - + - - + + + - - + - + Properties of ionic compounds I. Boiling points and Melting Points Have high melting points (above 250oC) and high boiling points (above 500oC) Reason: The ionic bonds (electrostatic force of attraction) between the ionsare very strong . A very large amount of heat energy is needed to overcome these strong bonds. • This also explains why all ionic compounds are solids at room temperature.

31. II. Solubility • Ionic compounds are usually soluble in water but insoluble in organic solvents. • Reason: water molecules can separate the positive ions from the negative ions, causing them to dissolve. • Exceptions: silver chloride, barium sulphate are ionic compounds which are insoluble in water. Organic solvents eg petrol, alcohol and turpentine

32. properties of ionic compounds III Electrical conductivity • Ionic compounds do not conduct electricity in the solid state because the ions are not freeto move about. • When the substance isin molten state (melted in liquid form)) or aqueous state ( when dissolved in water), it can conduct electricity. Reason : In these states, the ions are free to move. The moving ions conduct electricity.

33. What is covalent bond? Covalent bond is a bond formed by the sharingof electrons between atoms of non-metals. After bonding, each atom attains the stable noble gas electronic configuration. Why must atoms of non-metals share electrons while atoms of metals and non-metals form ions?

34. For elements with 4 valency electrons, gaining or losing 4 electrons to achieve a noble gas electronic configuration requires a large amount of energy. Thus, the non-metallic elements combined by sharing of electrons to form molecules. Why covalent bonds are formed between non-metal atoms? Valency electrons - are electrons in the outermost shell used to form bonds. Valency -is the number of electrons an atom uses to form bonds Valence electrons - are electrons in the outermost shell

36. Simple covalent moleculeslike H2, O2, H2O, CO2, NH3, CH4, HCl, N2, Cl2, etc or (ii)Giant covalent molecules (or macromolecules)( which is a three dimensional network of atoms bonded together by covalent bonds to form a giant molecule ) like diamond, graphite, silicon dioxide, etc The molecules formed can be

37. H H Formation of Covalent Bonds Single covalent bond + H H H atom H atom H H x o H2 molecule

38. Other ways to rep : or HO xx H x x xx O H H H O H

39. H Other ways torep : or H H C H H H C H H H x x x C H H x H

40. C O O O O C Other ways to rep : x xoox x orx O xoC ox O x xx xx

41. O O O O Double bond (2 pairs of electrons) Other ways to rep : x x o o or xo O xoO x x o o

42. N N N N Triplebond (3 pairs of electrons) Other ways to rep : x x oo or xo N xoN xo

43. Structure of Covalent Substances (a) Simple Molecular Structure Example 1 :Iodine is a simple diatomic covalent molecule. • It has a simple molecular structure. Reason : There exists weak intermolecular forces of attraction, between the iodine molecules, also known as weak Van der Waals’ forces. These weak forces of attraction requires only a small amount of heat enerrgy to overcome.

44. Example 2 Methane is also a simple covalent molecule. Four covalent bonds (C-H) are held together by strong forces of attraction. However, weak Van der Waals’ forces between methane molecules hold them together loosely. Therefore, methane exists as a gas at room temperature and pressure.

45. Properties of simple covalent compounds volatility • Covalent molecules have strong covalent bonds between atoms, but the Van der Waals’ forces which exist between separate molecules are weak. • During melting or boiling, the molecules do not break up into atoms, but merely move further apart. • Thus, simple covalent molecules are volatile, i.e., they have low melting points and boiling points, as not much heat energy is needed to overcome the Van der Waals’ forces. Molecules do not break up into atoms but merely move further apart during melting and boiling

46. Properties of simple covalent compounds solubility • Most covalent molecules are insoluble in water but are usually soluble in organic solvents. • Exceptions: Alcohol and sugar are covalent compounds which are soluble in water. • Some covalent molecules dissolve in water because of chemical reactions. E.g., chlorine. electrical conductivity • Covalent substances (elements or compounds) do not conduct electricity whether in the solid or molten state. This is because they do not contain ions or free electrons. • Exceptions: carbon, in the form of graphite, conducts electricity. Covalent compounds such as hydrogen chloride and hydrogen sulphide also conduct electricity when dissolved in water.

47. Differences between Ionic and Covalent Bonding Ionic Bond Covalent Bond • Formed between metals and non-metals. • Formed by transferring of electrons. • Formed between non-metals and non-metals. • Formed by sharing of electrons.

48. properties of ionic and covalent compounds • summary of main differences in properties between ionic compounds and covalent compounds

49. properties of ionic and covalent compounds