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

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 (AX 4 ) 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. CH 4, CCL 4, CH 3 Cl, CH 3 -CH 3 Why is the last molecular structure The same as the 1 st ?

8 CH 3 CH H Carbon bonded to four other atoms (AX 4 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 Cl CCl Cl

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

10 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 degrees maximizes separation in these compounds :S = C = S: H-C=C-H Cl—Be –Cl

11 Trigonal Planar Structures Some compounds have bond angles of 120 degrees - These are know as trigonal planar structures Examples are BF 3, NO 3 -, SO 3 and H 2 CO F B F H C = O: H :O O: S :O:

12 Trigonal Pyrimidal Let’s look at ammonia (NH 3 ) next. Nitrogen has 5 valence electrons - three are used to form covalent bonds with hydrogen That leaves one non-bonding pair H – N – H H

13 How do we maximize the separation of electrons in NH 3 ? First, Recall that CH 4 has a tetrahedral structure HCHHHHCHHHH This Carbon compound has 4 C-H bonds each having an angle Of degrees Can we expect Ammonia to have a similar Structure? Why?

14 For Ammonia we should expect a similar structure In NH 3 the non-bonding pair of electrons push The bonding pair “downward” This is known as A trigonal pyrimidal structure Bond angles for ammonia are degrees The polyatomic H 3 O + 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