January 28, 2014 Introduction to Lewis Structures

January 28, 2014 You will be able to draw Lewis structures to show bonding – Today: Activity/Rules – Thursday: More Practice

Activity Identify what atoms and how many are present in a given molecule Take out an element symbol card for each atom you need Arrange (valence electrons) peas around each card See how you can arrange the cards and electrons so all atoms have a full shell (8 electrons) – Note: Hydrogen is happy with only TWO electrons Try to show bonding for: – Cl 2 CH 3 IPF 3 – CH 2 OCN-OH-

Lewis Structures Lewis structures are used to represent molecules with covalent bonds (in which electrons are shared.)

Lewis Structures: Used to represent molecules with covalent bonds (shared electrons) 1. Count the valence electrons from each atom 2. Add up the total number of valence electrons What if the compound has an overall charge? a)For anions (negative charge) add e- b)For cations (positive charge) take away e- Example: CN- C:4 e- N: 5 e- Charge: 1 e- Total: 10 e-

3. Arrange atoms and connect with single bonds. a)Each bond represents two electrons b)Nature likes symmetry! c)Halogens (F, Cl, Br, I) are not usually center atoms d)Hydrogen is NEVER center e)Carbon is always center, or the least electronegative atom if no carbon

4. Use “leftover” electrons and add unshared pairs (lone pairs) of electrons to atoms to give full shells a)This is called the octet rule because everything needs 8 electrons for a full shell b)Exception to the octet rule: i.Hydrogen has a full shell when it has two electrons c)Do not draw too many electrons!

5. If there are not enough electrons to give everything a full shell, try using multiple bonds. - Double bond: four shared electrons - Triple bond: six shared electrons

Check your work Have I accounted for all electrons? How many electrons does each atom have around it?

EXAMPLE: Cl 2 Valence electrons: – Cl – 7 – Total – 14 Two shared electrons in a single bond between the atoms Electrons not involved in bonding are shown around atoms in electron pairs Each atom has 8 electrons surrounding it, either in lone pairs or in bonds

EXAMPLE: CH 3 I Valence electrons: – C – 4 – H – 1 – I – 7 – Total: 14 Electrons not involved in bonding are shown around atoms in electron pairs Each atom has 8 electrons surrounding it, either in lone pairs or in bonds. Hydrogen only needs two electrons to have a full valence shell. Two shared electrons in a single bond between the atoms

EXAMPLE: PF 3 Valence electrons: – P – 5 – F – 7 – Total: 26 Electrons not involved in bonding are shown around atoms in electron pairs Two shared electrons in a single bond between the atoms Each atom has 8 electrons surrounding it, either in lone pairs or in bonds

EXAMPLE: CH 2 O Valence electrons: – C – 4 – H – 1 – O – 6 – Total: 12 Four shared electrons in a double bond between the atoms Each atom has 8 electrons surrounding it, either in lone pairs or in bonds. Hydrogen only needs two electrons to have a full valence shell. Electrons not involved in bonding are shown around atoms in electron pairs

CN - (Sorry it’s blurry) Valence electrons – C – 4 – N – 5 – Neg. charge – 1 – Total: 10 Electrons not involved in bonding are shown around atoms in electron pairs Each atom has 8 electrons surrounding it, either in lone pairs or in bonds Six shared electrons in a triple bond between the atoms Brackets and charge show that this is a charged molecule

EXAMPLE: OH - Valence electrons – O – 6 – H – 1 – Neg charge – 1 – Total: 8 Brackets and charge show that this is a charged molecule Electrons not involved in bonding are shown around atoms in electron pairs Each atom has 8 electrons surrounding it, either in lone pairs or in bonds. Hydrogen only needs two electrons to have a full valence shell. Two shared electrons in a single bond between the atoms