Molecular shapes and Dipoles

1. Molecular shapes and Dipoles Knowledge of shapes of a molecule are important for determining functions and activity of molecules. Example: proteins enzymes of a particular shapes serve a particular function and activity because an enzyme’s shape(key) is will only fit one other substrate( lock). General outcome: In this lesson, students will describe the roles of modelling and use theory of modelling in explaining and predicting the structure, bonding and properties of ionic and molecular substances. Knowledge of shapes of a molecule are important for determining functions and activity of molecules. Example: proteins enzymes of a particular shapes serve a particular function and activity because an enzyme’s shape(key) is will only fit one other substrate( lock)

2. Molecular shapes cont’d General learner outcomes Specific learner outcomes • 20–A2.1k recall principles for assigning names to molecular substances. • 20–A2.3k relate electron pairing to multiple and covalent bonds • 20–A2.2k explain why formulas for molecular substances refer to the number of atoms of each • 20–A2.4k draw electron dot diagrams of atoms and molecules, writing structural formulas for • molecular substances and using Lewis structures to predict bonding in simple molecules • 20–A2.5k apply VSEPR theory to predict molecular shapes for linear, angular (V-shaped, bent), • tetrahedral, trigonal pyramidal and trigonal planar molecules

3. Teaching Department

4. EDIT Course: Mark Weighting

5. Using VSEPR to predict shapes… • Valence-shell-electron-pair-replusion theory was designed by Linus Pauling in the late 1930s. The theory was based on the electron repulsion created by lone electrons and bonding electron that surround the central atom of a molecular structure. • According to VSEPR theory: • Only the valence e- of the central atom are important in determining the overall shape of a molecule. • Valence e- are paired in a molecule or polyatomic ion. • Bonded pairs of electrons and lone pairs of e- are treated equally. • Valence e- repel one another (very important) • Finally, the molecular shape is determined by the positions of the electron pairs when they are maximum distance apart

6. Drawing Lewis structures • First, consider all electronegative atoms and place the atom with lowest electro negativity in the center of structure. • Place atoms of high electro negativity around the central atom. • Count the total number of e- and subtract this from the e- already present in structure. • Finally, place all ‘left over’ electrons on the central atom.

7. VSEPR theory continued… • Key concepts to consider when drawing stuctures to determine shape. • 1. First draw the Lewis dot diagram and consider ALL arrangement. • 2. Draw any left over electron in a way that they are far apart from one another so that repel each other. • 3. Before determining shape look at the central atom (noted by letter A), then at other surrounding bonded atoms (noted by letter X), then finally lone pairs (noted by letter E)

8. VSEPR AX2- denotes as having 1 central atom, 2 surrounding bonding atoms and zero lone pairs. Shape: linear. Example: CO2 • AX3- 1 central atom, 3 bonding atoms, zero lone pairs. Shape: trigonal planar. Example: BH3 • AX4: 1 central atom, 4 bonding atoms, zero lone pairs. Shape: tetrahedral. Example: CH4, SiH4. • AX3E1: 1 central atom, 3 bonding atoms, 1 lone pair. Shape: Trigonal pyramidal. Example: NH3 • AX2E2- I central atom, 2 bonding electrons, 2 lone pairs. Shape: angular or bent-shaped. Example: H20 • AX3E1- I central atom, 1 bonding atom, 3 lone pairs. Shape: linear. Example: HCl

9. Guided practice • Example 1 – Draw sulfur dioxide. Include number of bonding atoms, lone pairs and VSEPR shape. • Example 2- Draw chloromethane. Include number of bonding atoms, lone pairs and VSEPR shape. • Example 3 – Draw oxygen. Include number of bonding atoms, lone pairs and VSEPR shape. • YouTube - Chemistry VSEPR Theory • http://www.youtube.com/watch?v=i3FCHVlSZc4