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molecular dot diagrams and determine shape,bond angle and molecule polarity

What are the steps/rules to drawing molecular dot diagrams? How do you determine the molecule shape, bond angle, and molecule polarity from this?
Examples: H2, HCl, H2O, CO2, NH3, CH4.... etc

Thank you!

kingchemist Mon, 12/20/2010 - 04:42

Look at the number of electrons in the outer shell and decide how many more electrons are needed to fill it. eg H has 1 electron and needs 1 more to fill the first shell
It shares 1 electron with another H atom = H:H

HCl - H has 1 electron, Cl has 7 - it needs 1 more electron to fill its outer (3rd) shell. Of the 7 electrons around Cl, 6 of these are in 3 orbitals called non-bonding pairs (or lone pairs). The other one occupies an orbital on its own
    ..
  : Cl .
    ..

This shares with H
    ..
  : Cl : H so the single electron becomes a bonded pair of electrons
    ..

As far as shape is concerned, the orbitals move as far from each other as possible. Repulsion is strongest between non-bonded pair, intermediate between a bonded and a non-bonded pair and weakest between bonded pairs
With the previous 3 egs. the molecules have only 2 atoms and are linear.

H2O - O has 6 outer electrons of which 4 electrons occupy 2 non-bonded pairs and the other two electrons occupy separate 1/2 filled bonding orbitals. These share with one electron each from H

        ..
  H : O : H
        ..
However, the extra repulsion of the non-bonded pair causes extra repulsion on the bonded pairs and the HOH angles becomes about 105degrees. The molecule is non-linear or bent.

In NH3, there are 5 electrons around a N atoms and 3more electrons are shared with H making 1 lone pair and 3 bonded pairs. The lone pair pushes the 3 bonded pairs slightly closer together and the HNH angle is 107degress.
CH4 - there are 4 bonded pairs around C which repel equally. This gives a tetrahedral shape with HCH angle of 109.5 degrees

As far as polarity is involved you need to look at electronegativity difference between the atoms. In H2 there is no difference (same atoms) so non-polar. HCl, H2O and NH3 are polar because in each case, H is the lower electronegativity atom and carries a slight positive charge and the other atom is delta negative.

CH4  - the C-H bonds are virtually non-polar
CO2 has double bonds between C and each O atom  (O=C=O). The molecule is linear. Although O is more electronegative than C, the shape of the molecule makes it behave as a non-polar molecule