Molecular Shapes

1. Molecular Shapes Mr. Shields Regents Chemistry U10 L04

2. Molecular Shapes When considering molecular shapes three things must be considered: 1) What kind of bonds are present? 2) How many bonds are there? 3) are there any lone pairs present? Recall that like charges repel AND all bonding and non-bonding pairs involve electrons. Electrons will distribute themselves around the molecule to minimize electron repulsion

3. H – O - H Bent Molecular Structures Notice that the hydrogen forms a bent structure with Oxygen But why can’t we instead write water as a linear structure?

4. This bent shape for water is the correct molecular Structure and the bond angle is always 104.5º Why do you think this bond angle is formed? Well … Recall like charges repel So … to understand what’s going On we need to note the location Of the lone pairs surrounding the oxygen and the two bonding pairs forming the covalent bonds

5. These non-bonding electron pairs “feel” a repulsive force between one another and with the bonding pair electrons.

6. WATER The bond angle of 104.5º MINIMIZES the repulsion between all non-bonding electron pairs and the Bonding pairs of the hydrogen atoms. Non-bonding e- cloud

7. The Tetrahedral shape (AX4) Let’s next look at the angle’s of the bonds formed by Carbon with 4 other atoms. Remember Carbon has 4 valence electrons so it can Form a max of 4 covalent bonds. ex. CH4, CCL4, CH3Cl, CH3-CH3 Why is the last molecular structure The same as the 1st ?

8. CH3 C H H H Cl C Cl Cl Cl Carbon bonded to four other atoms (AX4 compounds) form a structure known as the TETRAHEDRON The angle between the bonds in a tetrahedron are Always 109.5º . That’s the angle that maximizes the spacing Between electrons in all 4 bonds

9. The tetrahedral structure plays a Significant role in Organic and Biochemistry Notice where Carbon is located Relative to Hydrogen in the tetrahedron

10. H-C=C-H :S = C = S: Linear Molecules All 2 atom molecules, many DOUBLE/TRIPLE bonded compounds and Central atoms bonded to two other atoms (having no lone pairs) are Linear. - 180 degrees maximizes separation in these compounds Cl—Be –Cl

11. F B F F :O O: S :O: H C = O: H Trigonal Planar Structures Some compounds have bond angles of 120 degrees - These are know as trigonal planar structures Examples are BF3, NO3-, SO3 and H2CO

12. H – N – H H Trigonal Pyrimidal Let’s look at ammonia (NH3) next. Nitrogen has 5 valence electrons - three are used to form covalent bonds with hydrogen That leaves one non-bonding pair

13. H C H H H How do we maximize the separation of electrons in NH3? First, Recall that CH4 has a tetrahedral structure This Carbon compound has 4 C-H bonds each having an angle Of 109.5 degrees Can we expect Ammonia to have a similar Structure? Why?

14. For Ammonia we should expect a similar structure In NH3 the non-bonding pair of electrons push The bonding pair “downward” This is known as A trigonal pyrimidal structure Bond angles for ammonia are 107.5 degrees The polyatomic H3O+ also has this structure

15. Notice that as we decrease the bonding pairs and Increase the non-bonding pairs atoms are pushed Closer together 0 non-bonding Pairs 1 non-bonding Pair 2 non-bonding pairs