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Recap – Covalent Bonding

Recap – Covalent Bonding. Electrons are shared , bonds are directional . Valence electrons are outer shell electrons. Valence of the element give No of bonds: 1 H, F, Cl, Br, I 2 O 3 N 4 C. Max no. of valence electrons. First shell holds a maximum of 2 electrons

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Recap – Covalent Bonding

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  1. Recap – Covalent Bonding • Electrons are shared, bonds are directional. • Valence electrons are outer shell electrons. • Valence of the element give No of bonds: • 1 H, F, Cl, Br, I • 2 O • 3 N • 4 C

  2. Max no. of valence electrons First shell holds a maximum of 2 electrons Hydrogen – max of 2 electrons in bond Second shell holds a maximum of 8 electrons C, N, O, F – max of 8 valence electrons / atom Third shell holds a maximum of 18 electrons P, S, Cl – may have 8, 10, 12 valence electrons / atom.

  3. Example - phosphorus

  4. Example - sulfur

  5. Polyatomic Ions • All compounds so far have been binary; all ions so far have consisted of single atoms with an electrical charge. • Some ions, however, have more than one type of atom bound together with covalent bonds – polyatomic ions.

  6. Polyatomic Ions Note: ‘octet’ of electrons

  7. Polyatomic Ions • or Note: ‘octet’ of electrons

  8. Polyatomic Ions and valence • Note the valence of an element tells us the number of bonds it forms in a neutral molecule. It is most useful when 8 electrons are involved (Period 2). What about polyatomic ions? • In general: • If the charge is negative the atom has one less bond than expected from its valence. • If the charge is positivethe atom has one more bond than expected from its valence.

  9. Polyatomic Ions – other examples • Sulfate: SO42- • Sulfite: SO32- • Carbonate: CO32- • Nitrate: NO3- • Phosphate: PO43-

  10. Naming See on-line module: http://firstyear.chem.usyd.edu.au/iChem/inorganic_nomenclature.shtml In brief: The element with the lower group no called by the element name The name of the other is derived from the element name and ends in …ide Use di…, tri…, tetra… as appropriate

  11. Polar Covalent Bonds • Covalent bonds between identical atoms have electrons shared equally. Cl — Cl H — H

  12. Polar Covalent Bonds If atoms are different, one atom may get the greater share of the electron pair. Example: HCl +- http://commons.wikimedia.org/wiki/File:Hydrogen-chloride-elpot-transparent-3D-balls.png

  13. Polar Covalent Bonds H — H - + Evenly distributed electrons Unevenly distributed electrons + - Cl — Cl

  14. Polar Covalent Bonds Example: H2O - http://commons.wikimedia.org/wiki/File:Water-elpot-transparent-3D-balls.png + The oxygen end of the molecule is slightly negative relative to the hydrogen ends. Overall water is a polar molecule

  15. Bond Types Cl Cl Na+ Cl-

  16. Learning Outcomes: By the end of this lecture, you should: recognise elements in Period 2 have 8 electrons when bonded elements in Period 3 have 8, 10 or 12 electrons when bonded know that polyatomic ions represent two or more covalently bonded atoms which have an overall charge recognise when a polar covalent bond occurs appreciate bonding represents a range of situations from even sharing of electrons to complete transfer of electrons be able to complete the worksheet (if you haven’t already done so…)

  17. Questions to complete for next lecture: • How do you name PCl3 and PCl5? (you may need to refer to the naming module http://firstyear.chem.usyd.edu.au/iChem/inorganic_nomenclature.shtml) • Chlorine and oxygen react to form the following polyatomic ions: ClO-, ClO2-,ClO3-, and ClO4-. Why can chlorine form ions with the same atoms but different formulas? • Draw the electron dot structure of the polyatomic ion H3O+ (Hint, review the formation of NH4+) • What condition is necessary for a bond to be polar? • Classify the bonds between the following pairs of molecules as ‘covalent’, ‘polar covalent’ or ‘ionic’: H and Br; O and O; Mg and Cl; P and O; I and I

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