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The Chemical Bond

The Chemical Bond. What is a chemical bond?. It is a force of attraction between atoms, ions or molecules. How are bonds formed ?. By electron loss, gain or sharing. Which elements lose electrons?. Metals Cations are formed. Why metals lose electrons ?.

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The Chemical Bond

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  1. The Chemical Bond

  2. What is a chemical bond? • It is a force of attraction between atoms, ions or molecules.

  3. How are bonds formed ? • By electron loss, gain or sharing.

  4. Which elements lose electrons? • Metals • Cations are formed

  5. Why metals lose electrons ? • They usually have one, two or three electrons. • By losing these electrons they can achieve a full outer shell and become more stable. • It is in energy terms easier to lose electrons than gain electrons

  6. Which elements gain electrons? • Non-metals • Anions are formed

  7. Why non-metals gain electrons? • They usually have five, six or seven electrons. • By gaining three, two or one electron, they can achieve a full outer shell and become more stable. • In energy terms it is easier to gain these electrons than lose the outer electrons.

  8. Bonds • Covalent • Polar covalent • Dative covalent • Giant covalent • Ionic • Metallic

  9. How are covalent bonds formed? By sharing electrons between non-metals.

  10. Dot and cross diagram-Covalent bond • Each H atom contributes one electron to the shared pair. • Each H atom has two electrons in its outer shell. • Each H atom now has a full outer shell H : H

  11. What is electronegativity? • It is the ability of an atom to attract the bonding electrons in a bond.

  12. Electronegativity values • Hydrogen 2.1 Lithium 1.0 • Beryllium 1.5 Boron 2.0 • Carbon 2.5 Nitrogen 3.0 • Oxygen 3.5 Fluorine 4.0

  13. Which element is most electronegative? • Fluorine • The electronegativity is 4.0

  14. How does the presence of a more electronegative atom affect a covalent bond? • The more electronegative atom attracts the shared electrons more towards itself. • The more electronegative atom acquires a partial negative charge. • The less electronegative atom a partial positive charge. • The bond becomes a polar covalent bond.

  15. Calculate the electronegativity difference for the following • C – H . Li - F • O – H • H – F • N – H • C –N • C – O • C - C

  16. Which bond has the highest electronegativity difference? • Li F • Electronegativity differences greater than or equal 1.6 show that the compound is ionic • This compound is ionic!

  17. Ionic or polar covalent ? • This depends on the electronegativity difference • When the difference is relatively small, the bonding is polar covalent. • When the difference is large, the bonding becomes ionic. (1.6 or more)

  18. How are ionic bonds formed ? • By exchanging electrons. • Metals lose electrons and become positive ions. • Non-metals gain electrons and become negative ions. • The oppositely charged ions attract each other. This is ionic bond.

  19. Ionic bonding

  20. Ionic bonding

  21. Giant Covalent Bond • Structure of diamond

  22. Giant Covalent Bond • Structure of diamond

  23. Giant Covalent Bond • Structure of Graphite

  24. How are metallic bonds formed? • In metals , the outer electrons are free. • These delocalised electrons form a ‘sea’ of electrons. • The metal atoms exist as positive ions immersed in this ‘sea’ of electrons. • The attraction between the positive ions and the ‘sea’ of electrons is known as the metallic bond.

  25. Metallic bonding • Metallic bonding in potassium – the outermost electrons are ‘free’ and form a ‘sea’ of electrons which attracts the K+ions.

  26. Intermolecular Bonds . Van der Waals forces . Dipole dipole attraction . Hydrogen bonds

  27. Van der Waals Forces • Random electron movements creates dipoles. • Dipoles induces dipoles. • Process is repeated. • Oppositely charged dipoles attract.

  28. Van der Waals Forces

  29. Van der Waals Forces- Factors • More electrons, higher forces • Larger the area of contact, higher forces • Linear molecules, higher forces • Branched chains, lower forces • Spherical molecules, lower forces

  30. Dipole-dipole attraction . More electronegative atom in a covalent bond. . Permanent dipoles. . Attraction between dipoles.

  31. Dipole-dipole attraction-Factors • More the number of electronegative atoms, higher attraction • More electronegative atoms, higher attraction

  32. Hydrogen Bond . Hydrogen in a covalent bond with nitrogen, oxygen or fluorine. . Permanent dipoles. . Attraction between dipoles. . Strongest intermolecular force. . Intermolecular forces much weaker than normal covalent bonds

  33. Hydrogen bonding in water

  34. H-bond in H2O • But for its H-bonding water would be a gas at room temperature. • Due to H-bonding ice floats on water. • Water freezes with a lot of space in between due to H-bonding. This makes ice lighter. H O||||||||||| H H H O O|||||||| H H

  35. Hydrogen bonding in water

  36. Hydrogen bonding ….. • Ammonia can form H-bonds. • HF can also form H-bonds.

  37. Trend in the melting and boiling point of Group IV hydrides mp • As the elements get larger, there are more electrons. • Van der Waals force is larger. • MP and BP is larger from CH4 to PbH4. CH4 PbH4

  38. Anomalous melting and boiling points of hydrides • Why ammonia, the lightest hydride has the highest melting and boiling point than the corresponding hydrides in Group V? • Why water, the lightest hydride in GroupVI has the highest melting and boiling points? • Why hydrogen fluoride, the lightest hydride in Group VII has the highest melting and boiling points?

  39. What are these hydrides • Gr V: NH3, PH3, AsH3, SbH3,BiH3 • Gr VI: H2O, H2S, H2Se, H2Te, H2 Po • Gr VII: HF, HCl, HBr, HI, HAt

  40. Why the mp or bp of NH3 is unusually so high? • Nitrogen is electronegative. • NH bonds are polar • H-bonds formed between ammonia molecules. • No H-bonds in other molecules, only Van der Waals Forces.Partially covalent ionic bonds . High charge density of positive ions. . Positive ions polarising negative ions. . Resulting electron sharing – partial covalency.Sigma and pi-bonds . Sigma bond – direct overlap of orbitals . Pi- bond – sideways overlap of orbitals. N-H||||||| N -H |||| H-bond BP BiH3 NH3

  41. What are dative covalent bonds? • In a dative covalent bond, both the shared electrons come from the same atom. • The atom giving the electron pair is the donor. • The atom receiving the electron pair is the acceptor. • There is no difference in length and strength between a normal and a dative covalent bond.

  42. The End

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