Chpt. 5: Chemical Bonding – Chemical Formulas

1. Chpt. 5: Chemical Bonding – Chemical Formulas

2. This topic will be investigated under five main headings: • Chemical Compounds • Ionic Bonding • Covalent Bonding • Electronegativity • Shape of Molecules and Intermolecular forces

3. Chemical Compounds Table Salt Water Dry Ice In terms of elements can you think of a more scientific name for each of the above substances????

4. Chemical Compounds: • A compound is a substance that is made up of two or more different elements chemically combined. • 2H2(G) + O2 (G) → 2H20(L) • The atoms of the elements in a compound are held • together by attractive forces called chemical bonds • Compounds can be broken down into their elements. • If an electric current is passed through water • (electrolysis) the compound splits into its elements of • hydrogen and oxygen

5. *Note: • Valence electrons – are the outer electrons • Bonding involves only the valence electrons

6. The Octet Rule: The Octet Rule states that when bonding occurs, atoms tend to reach an electron arrangement with eight electrons in the outermost shell • 1916 - Gilbert Lewis & Irving • Langmuir proposed the • Octet Rule • Elements will try to loose, • gain or share electrons to • achieve eight electrons in • their outer shell • This outermost energy level • is also known as the valence • shell

7. Exceptions to the Octet Rule: Transition Metals - can have more OR less than eight electrons in their outermost energy level Elements near Helium e.g. Hydrogen, Lithium etc. tend to achieve electronic arrangement of Helium - 2 electrons in outer shell!!!!

8. Loss & Gain of electrons

9. Valency

10. Ions An ion is a charged atom or group of atoms Metals(left side of periodic table) - lose valence electrons - achieve a stable valence shell (usually 8 e-) - gains a positive charge, i.e. a positive ion.

11. Loss of electrons (formation of cations): • Na 2,8,1 -> Na+ + e- 2,8 • Li 2,1 -> Li+ + e- 2 • K 2,8,8,1 -> K+ + e- 2,8,8 • Mg 2,8,2 ->Mg2+ + 2e-  2,8 • Ca 2,8,8,2 -> Ca2+ + 2e- 2,8,8 • Al 2,8,3 -> Al3+ + 3e- 2,8.

12. Non-metals(right side of periodic table): • - gain valence electrons - achieve a stable valence shell (usually 8 e-) • - gains a negative charge i.e. a negative ion

13. Gain outer electrons (formation of anions): F 2,7 + e- -> F-2,8 Cl2,8,7 + e- -> Cl-2,8,8 Br 2,8,8,7 + e- -> Br-2,8,8,8 I 2,8,18,18,7 + e- -> I-2,8,18,18,8 O 2,6 + 2e- -> O2- 2,8 S 2,8,6 + 2e- -> S2-2,8,8 N 2,5 + 3e- -> N3-2,8 H 1 + e- -> H-2.

14. Uses of unreactive group – Noble Gases Helium is a much safer alternative to hydrogen as it is stable and is therefore used in weather balloons and blimps. Argon is used in electric light bulbs to prevent the tungsten filament from evaporating or reacting.

15. Lewis Symbols – ‘Dot and Cross Diagrams’ Lewis symbols show the valence electrons as dots arranged around the atomic symbol. hydrogen: sodium: Na chlorine: Cl H · · · · · · · · ·

16. Sodium & Chlorine

17. 2 Ionic Bonding • Ionic bonding is the force of attraction between oppositely charged ions in a compound. • Remember: Ions are elements which have a positive or negative charge e.g. • Na has 11 e- (E.C.= 2,8,1) when Na gives away this one e- it now has more protons than electrons so it has an overall positive charge. • Ionic bonds generally form between metals and non-metals.

18. Formation of Ionic Compounds Using Ionic Bonding Formation of Table Salt (Sodium Chloride): Bohr Type Circle Diagram Dot and Cross Diagram

19. Example 1: Formation of Sodium Chloride – Bohr-type diagram Step 1: Draw Bohr diagrams of each atom 11p 11n 18n 17p Na atom 2,8,1 Cl atom 2,8,7

20. Step 2: Determine ions formed 11p 11n AND 17p 18n Na+ ion 2,8 Cl- ion 2,8,8

21. Step 3: The formation of an Ionic Bond The attraction between the positive sodium ion and the negative chlorine ion results in the formation of an ionic bond to give the ionic compound Sodium Chloride:

22. Na · Example 1: Formation of Sodium Chloride – Dot-and-Cross Diagrams Step 1: Represent atoms using dot-and-cross diagram Step 2: Determine ions formed: xx xx + Cl x xx Na atom 2,8,1 Cl atom 2,8,7 __ xx + Na and · Cl xx x xx Na+ ion 2,8 Cl- ion 2,8,8

23. ® [ ] Na Na+ Cl · · Step 3: Formation of Ionic Bond: xx xx + Cl xx x x xx xx xx Sodium Chloride

24. Example 2: Show the formation of the ionic bond in magnesium fluoride, MgF2, by means of dot-and-cross diagrams Try: Show the formation of the ionic bond in magnesium chloride by means of dot-and-cross diagrams

25. Writing Formulas of Ionic Compounds A chemical formula is a way of representing a compound using symbols for the atoms present and numbers to show how many atoms of each element are present. You must know how to write the formulas of ionic compounds of the first 36 elements. When writing formula remember: an ionic compound has no net charge, overall it is neutral *Note: transition metals will be discussed separately

26. *Note: • A compound which contains just two elements always • ends in - ide • A compound which contains oxygen as well as two • other elements always ends in -ate

27. Working with Simple Ions: Write the formula of: i) Potassium bromide ii) Calcium Chloride iii) Sodium Sulphide iv) Aluminium Oxide H+, H- Na+ ,K+ Be2+, Mg2+, Ca2+ Al3+ O2- ,S2- F-, Cl-, Br-, I-,

28. Potassium Bromide 1- + K Br K Br “To get correct formula just swop the valency numbers” • *Note: • The potassium ion is K+. • The bromide ion is Br- • Same number of positive and negative charges

29. Calcium Chloride 1- Ca 2+ Cl Ca Cl 2 “To get correct formula just swop the valency numbers” • *Note: • The calcium ion is Ca2+. • The chloride ion is Cl- • In order to have the same number of negative • charges as positive must use two Cl- ions

30. Sodium Sulphide 2- + S Na Na2 S “To get correct formula just swop the valency numbers” • *Note: • The sodium ion is Na+. • The sulphide ion is S2- • In order to have the same number of positive • charges as negative must use two Na+ ions

31. Aluminium Oxide 3 + 2 - Al O O Al 2 3 • *Note: • Bring all charges up to their lowest common • denominator – 6 • To get six positive charges we need 2 Aluminium • ions • To get six negative charges we need 3 Oxide ions

32. Working With Complex Ions: Must learn off by heart!!!!

33. Write the formula of the following: Potassium Hydroxide Calcium Hydroxide Sodium Sulphate Ammonium Phosphate

34. Potassium Hydroxide + 1- K OH K OH • Note: • The potassium ion is K+. • The hydroxide ion is OH- • Same number of positive and negative charges

35. Calcium Hydroxide 2+ 1- OH Ca Ca (OH) 2 “When complex ions occur more than once brackets are needed” • *Note: • The calcium ion is Ca2+. • The hydroxide ion is OH- • In order to have the same number of negative charges • as positive we must use two OH- ions

36. Sodium Sulphate + SO42- Na Na SO4 2 • *Note: • The sodium ion is Na+. • The sulphate ion is SO42- • In order to have the same number of positive charges • as negative charges we must use two Na+ ions

37. Ammonium Phosphate 3- 1+ PO4 NH4 (NH4) PO4 3 • *Note: • The ammonium ion is NH4+. • The phosphate ion is PO43- • In order to have the same number of positive charges • as negative charges we must use three NH4+ ions

38. Valency of Transition Elements • Transition elements have a variable valency (they loose • electrons) • The valency depends on the other elements the • transition element is bonding to:

39. The reason for this variable valency is because there is • such a small energy difference between the 4s and 3d • sublevels • *Note: You do not need to remember the actual valancies of any of the transition metals but you should be able to interpret the names as shown in the following examples

40. Working with Transition Metals 1) Write the formula of: i) Iron (II) carbonate ii) Chromium (III) Chloride 2) Name the compound Cr2(SO4)3

41. Iron (II) Carbonate CO3 2+ 2- Fe Fe CO3 • *Note: • The Roman number (II) indicates Fe2+ • The carbonate ion is CO32- • Valencies are balanced

42. Chromium (III) Chloride Cl1- 3+ Cr Cr Cl3 • *Note: • The Roman number (III) indicates Cr3+. • The chloride ion is Cl- • In order to have the same number of negative charges • as positive charges we must use three Cl- ions

43. Name the compound Cr2(SO4)3 • One sulphate ion has a negative charge of 2- so • three sulphate ions together must have a total • negative charge of 6- • Since overall charge must be zero the two • chromium ions together must carry a charge of 6+ • Therefore each chromium ion must be Cr3+ • Compound Name: • Chromium (III) Sulphate

44. Ionic Compounds and their Crystal Lattice Structure Ionic bonds result in a three dimensional crystal lattice structure. Unit Cell Sodium Chloride has a cubic structure

45. Uses of Ionic Substances in Everyday Life • (Know at least two) • Too little salt in diet = muscle cramps, so many athletes • take salt tablets to replace salt lost in sweating. • Food preservative (Brine) • Salt used in manufacture of soap, leather, detergents • Salt is spread on roads during winter to help melt frost • and snow • Fluoridation of water supplies to prevent tooth • decay

47. D- block elements and transition elements All of the elements from scandium to zinc are the first row of the d-block All of the elements from titanium to copper incl. are referred to as transition elements

48. Transition elements have the following characteristics: • In d-block of table • Variable valency • Note: Sc only forms Sc3+ ions and Zn only forms Zn2+ • ions • Usually form coloured compounds • Note: Sc and Zn only form white compounds • Widely used as catalysts • Note: Sc and Zn show little catalytic activity • Have incomplete d sublevel • Note: Sc3+ has empty d-sublevel and Zn2+ has full • d-sublevel

49. A transition metal is one that forms at least one ion with a partly filled d-sublevel Lets take a closer look: Even though the 4s sublevel is filled before the 3d sublevel (as it is lower in energy) electrons are lost from the 4s first as location wise it is further from the nucleus and are therefore more easily removed: 4s electrons are lost before 3d electrons Investigate e.c. of Sc, Sc3+, Fe, Fe2+ , Ti and Ti3+

50. 3. Covalent Bonding • A covalent bond is the chemical bond formed by sharing a pair of electrons • Covalent bonds are typical of non-metal elements. • A single bond has 1 shared pair of electrons. • A double bond has 2 shared pairs of electrons. • A triple bond has 3 shared pairs of electrons. • E.G. H-H O=O N N • A molecule is a group of atoms joined together. It is the smallest particle of an element or compound that can exist independently.