To draw molecular structures, one should: 1. Draw the Lewis structures of components and make the most electronegative atoms outer atoms, except for hydrogen. 2. Ensure outer atoms have a full outer shell of 8 electrons using single, double, or triple bonds and charges. 3. Central atoms in period 2 can have up to 8 outer electrons, and period 3 atoms can have up to 18. The VSEPR theory then predicts 3D structure by minimizing electron pair repulsion between outer shell electron pairs and lone pairs.

1. In order to draw the molecular structure we should follow the
next steps:
• Draw the Lewis stucture of partecipants
• Put the more electronegative atoms (see the table in the following
page) as outer atoms, except when H is also present in the
molecule
• Make sure that the outer atoms have 8 electrons in thir outer shell
(except Hydrogen, which should have two).You can do it using
single, double and triple bond, but also + or – charges
• Remember: if the central atom is from the period two it should have
no more than 8 electrons in its outer shell.
If the central atom is from the period three it may have up to
18 electrons in its outer shell.

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3. Applying information obtained using the Lewis structure, we can
predict the tridimensional structure of molecules using the
Valence Shell Electron Pair Repulsion theory (VSEPR)
VSRPR : Pairs of electron in the valence (outher) shell of an atom
repel each other and will therefore take up positions in space to
minimise these repulsion, i.e. to be as far apart in the space as
possible.
Also lone pair must be considered

4. 1) Draw the Lewis structure
2) Count the number of negative centre in the mulecule:
bonds, double and triple bond counts as single lone pair,
and lone pair ( even if you can’t see it in the structure)
3) Look at the following table tp get the basic shape

5. These are the main molecular
strucures
and the relative angles. The
structures are ordered according to
the central atom’s group
membership.
Remember: we have to consider
also the presence of lone pairs!
Exercise: CH4, NH4 , CO2, BF3, NO2
http://learning.covcollege.ac.uk/content/Jorum/CHB_Molecule-shape-and-polarity_LM-
1.2-04FEB08/page25.htm
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See also table at page 109