Shapes+of+Molecules

=Shapes of Molecules = The Lewis diagrams above show the sharing of electrons but do not adequately represent the **three-dimensional shape** of the molecule. In any molecule the electron pairs mutually repel each other, and to reduce this repulsion the molecule adopts a shape which allows the bonding electron pairs to be as far apart as possible. (This is sometimes referred to as VSEPR - valence shell electron pair repulsion).

The shape of the molecule depends on  a) the number of atoms linked to the central atom
 * and ** b) the total number of regions of electron density (bonded and non-bonded electron pairs) around the central atom.

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 * NOTE: ** Multiple electron pairs (i.e. double and triple bonds) are considered to be **only one** region of electron density).

The shapes you need to know at Year 12 level are given below.
 * **Tetrahedral - ** occurs when there are 4 atoms linked to a central atom by 4 single covalent bonds (no additional non-bonding pairs on the central atom) e.g. CH4 shown as




 * Note: ** The bond angles are all 109.5o, the tetrahedral angle.

 Common for compounds of Group 15 atoms such as N and P.
 * 2. Trigonal pyramid (or triangular pyramid) - **occurs when the central atom is linked to only 3 other atoms and also has one non-bonding pair of electrons (i.e. a total of 4 electron pairs around the central atom but only 3 involving bonding with another atom).



<span style="font-family: Arial,sans-serif; font-size: 11pt;"> e.g. NH3
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">Note **<span style="font-family: Arial,sans-serif; font-size: 11pt;">: The bond angles are 107.8 <span style="font-family: Arial,sans-serif;">o <span style="font-family: Arial,sans-serif; font-size: 11pt;">, slightly different to the tetrahedral angle because the lone pair exerts a stronger repulsive force.


 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">3. Trigonal planar (or triangular planar) - **<span style="font-family: Arial,sans-serif; font-size: 11pt;">occurs when the central atom is linked to 3 other atoms and has no additional non-bonding electron pairs. Commonly found with compounds of carbon in which the carbon is linked by a double covalent bond to one atom and 2 single covalent bonds to 2 other atoms.



<span style="font-family: Arial,sans-serif; font-size: 11pt;"> e.g. H2CO
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">Note: **<span style="font-family: Arial,sans-serif; font-size: 11pt;"> The bond angles are 120 <span style="font-family: Arial,sans-serif;">o <span style="font-family: Arial,sans-serif; font-size: 11pt;">.

<span style="font-family: Arial,sans-serif; font-size: 11pt;">One unusual exception is BH3 which exists with 3 single bonds and no lone pairs i.e. it does not obey the “octet rule”.


 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">4. Bent - **<span style="font-family: Arial,sans-serif; font-size: 11pt;">occurs when the central atom is linked to only 2 other atoms but also contains **at least 1 other** non-bonding pair of electrons.

<span style="font-family: Arial,sans-serif; font-size: 11pt;">Commonly found in molecules of Group 16 atoms such as O and S

<span style="font-family: Arial,sans-serif; font-size: 11pt;"> e.g. H2S


 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">Note: **<span style="font-family: Arial,sans-serif; font-size: 11pt;"> Bond angles are much less than 109.5 because two lone pairs cause considerable repulsion. (eg. H2S has a bond angle of 104).

<span style="font-family: Arial,sans-serif; font-size: 15px;"> e.g. CO2. <span style="font-family: Arial,sans-serif; font-size: 11pt;"> **Note:** In a linear molecule the bond angle is 180o
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">5. Linear - **<span style="font-family: Arial,sans-serif; font-size: 11pt;">occurs when the central atom is linked to 2 other atoms but there are no additional non-bonding electrons. Commonly found with molecules having 2 double bonds (e.g. CO2) and is the only option for molecules containing 2 atoms only

<span style="font-family: Arial,sans-serif; font-size: 11pt;">Exercise
//<span style="font-family: Arial,sans-serif; font-size: 11pt;">Name the shape of each of the following molecules (Lewis structures drawn on page 44) // //<span style="font-family: Arial,sans-serif; font-size: 11pt;">CH4 NH3 H2S H2CO PCl3 CS2 CH3OH //