Part 1 – Electronegativity and Bonding

1. Part 1 – Electronegativity and Bonding

2. Curriculum Outcomes for Review • Understand the concept of electronegativity/ electropositivity • http://www.chemguide.co.uk/atoms/bonding/electroneg.html#top

3. Curriculum Outcomes • Understand the concept of electronegativity/ electropositivity • Understand the factors which affect electronegativity • Understand bond polarity, its causes and effects

4. Electronegativity • Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. The Pauling scale is the most commonly used. Fluorine = 4,0 Caesiumand Francium = 0,7 Linus Pauling

5. Electronegativity on the Periodic Table

6. Predicting Bond Types

7. Part 2 – Van Der Waals Forces

8. Curriculum Outcomes • Understand the causes and effects of dipole-dipole forces • Understand the cause and effects of van der Waals (induced dipole- dipole) forces

9. Two confusing terms Intermolecular Forces and Intramolecular Forces Notice that the words are very similar!

10. Intramolecular Forces • The forces within a molecule. • Ionic, Covalent and Metallic Bonding • We will avoid using this term and just refer to the type of intermolecular force.

11. Intermolecular Forces • Intermolecular forces are the forces between molecules. • Generally considered weaker than covalent or ionic bonds.

12. Review Questions • Aluminium (metal) and chlorine (non-metal) are bonded. What type of bond will this be? • Describe the most basic difference between intermolecular and intramolecularforces. • Considering these forces, why do crystaline substances have high melting and boiling points?

13. Two Types of Intermolecular Forces • Hydrogen Bonding • Van der Waals Forces

14. Each of these forces are forms of electrostatic attraction. The difference is only in their origin.

15. Activity – Modeling Dipoles

16. Dipoles A dipole is two separated but opposite electric charges. The symbol δ means slightly. So, the ends of this dipole have a slight positive or negative charge.

17. Atomic Dipoles • Electrons are mobile, and at any one instant they might find themselves towards one end of the molecule, making that end -. • The other end will be temporarily short of electrons and so becomes +. When the electrons for a helium atom are found on one side of the atom a dipole is created

18. Molecular Dipoles Molecules that are sharing electrons can also become polar through uneven electron distribution.

19. Activity – Induced Magnetic Dipoles

20. Induced Dipoles As one dipolar molecule comes close to another, one molecule can induce a dipole in the other.

21. Induced dipoles can occur through a whole matrix of molecules holding them together When the dipoles are induced the forces are called DISPERSION FORCES

22. Permanent Dipoles • Because of differences in electronegativity, some molecules will be permanent dipoles. • The permanent dipoles will be attracted to each other more strongly than the molecules with temporary dipoles.

23. Dipole-Dipole Forces • Where the force of attraction is caused by permanent dipoles the force is called a dipole-dipole force. • These dipole-dipole forces occur in addition to the dispersion forces caused by temporary fluctuating dipoles.

24. Effect on Boiling Points • The addition of forces means that for similar small molecules boiling points will be higher where there is a permanent dipole

25. Review Questions • Name two types of Van der Waals Forces • What is the difference in dipole-dipole forces and dispersion forces? • What type of bonding will be find in a polar molecule? • If two similar molecules are compared, one which is polar and the other non-polar, what might we observe when measuring the boiling points?

27. Resources and Further Reading • http://www.chemguide.co.uk/atoms/bonding/vdw.html#top • http://www.chemguide.co.uk/atoms/bonding/vdwstrengths.html#top

28. Part 3 – Hydrogen Bonding

29. Curriculum Outcomes • Understand the cause and effects of hydrogen bonding

30. Activity Worksheet 4 Comparing the melting point and boiling point of water with those of some other substances. Please graph the data values carefully and accurately.

31. Molecules that Hydrogen Bond • Ammonia – NH3 • Hydrogen Flouride – HF • Water H2O

32. Why? • The bond between the hydrogen and oxygen is highly electronegative. • Oxygen has a strong affinity for the electrons and causing the hydrogen to have a strong positive charge. • The oxygen has un-bonded pairs of electrons. • The positively charged hydrogen is strongly attracted to the un-bonded pairs of electrons.

33. Why?

34. The Perfect Design • Each water molecule can potentially form four hydrogen bonds with surrounding water molecules. • There are exactly the right numbers of +hydrogensand lone pairs so that every one of them can be involved in hydrogen bonding. • In hydrogen fluoride, the problem is a shortage of hydrogens. • In ammonia, the problem is a shortage of un-bonded electron pairs. • Water is the ‘perfect’ hydrogen bonding system

35. Review Questions • Name some common molecular forces and describe their relative strengths. • Why can water be described as a perfect hydrogen bonding system?

36. H-Bonding and DNA Hydrogen bonding joins the base pairs of the DNA down the centre of the molecule.

37. Part 4 - Coordinate Covalent Bonds

38. In the formation of a simple covalent bond, each atom supplies one electron to the bond • A co-ordinate bond (also called a dative covalent bond) is a covalent bond in which both electrons come from the same atom.

39. Further Reading • http://www.chemguide.co.uk/atoms/bonding/dative.html