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Formulae, Diagrams and Shapes

Formulae, Diagrams and Shapes. Type of Formulae Lewis Diagrams Shapes of Molecules. Types of Chemical Formulae. Molecular/Chemical Formula This is the ‘normal’ formula showing the elements and their numbers in a chemical i.e. H 2 O, HCl, Zn(NO 3 ) 2 Empirical Formula

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Formulae, Diagrams and Shapes

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  1. Formulae, Diagrams and Shapes Type of Formulae Lewis Diagrams Shapes of Molecules

  2. Types of Chemical Formulae Molecular/Chemical Formula • This is the ‘normal’ formula showing the elements and their numbers in a chemical i.e. H2O, HCl, Zn(NO3)2 Empirical Formula • This shows the proportion of each element in a compound – like a ratio. For example C2H4 becomes CH2 and C6H12O6 becomes CH2O Structural Formula • This type shows the linking inside covalent molecules. See the next page for details

  3. Structural Formulae - Details • There are actually two types of structural formulae: • Condensed – this does not show all of the bonds but it does show the structure. For example CH3CH2CH2CH3 • Expanded – this has all the sticks drawn in. We will learn how to create these in the next section. For example:

  4. Lewis Diagrams (Electron Dot Diagrams) • Lewis diagrams (also called electron dot diagrams) show the valence electrons of covalently bonded atoms. Here is how you draw them: • Work out how many valence electrons are present in the atoms you are working with, arrange them in pairs e.g. CH4 C has 4 electrons (in its outer shell) H has 1 electron • Arrange them so that all the valence shells are full (normally 8 except for hydrogen) The diagram shows how the carbon is sharing the electrons with the hydrogens to fill the shells of the carbon and the hydrogens

  5. Drawing Lewis Diagrams These are called lone pairs

  6. Shapes of Molecules Bonding pair – these can also be drawn as straight lines Lone pair • Molecules (covalent chemicals) form certain shapes depending on how many lone and bonding pairs of electrons it has. • Because the electron pairs repel each other we get certain shapes being formed. These are due to a certain rule called VSEPR (Valence Shell Electron Pair Repulsion)

  7. VSEPR – The Results

  8. 3D Shapes

  9. “Flat” Shapes

  10. Level 3 - Shapes

  11. Exam Practice - 2005 Can’t see the exam paper below? Go to the NCEA website and search for 90308 • Have a go at Questions: • One

  12. Exam Practice - 2006 Can’t see the exam paper below? Go to the NCEA website and search for 90308 • Have a go at Questions: • One (a) (b) and (c) • Four

  13. Exam Practice - 2007 Can’t see the exam paper below? Go to the NCEA website and search for 90308 • Have a go at Questions: • One (a) • One (b)

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