To obtain a correct dot and cross diagram or structural formula every time, a few set of rules can be followed rather than simply doing it by trial and error. I prefer to draw structural formulas because they contain all the information found in dot and cross diagrams and also includes the geometry of the molecule. The following examples will all be illustrated using structural formulas.
The central atom is usually the less electronegative atom.Write down the number of valence electrons of all the atoms like so:
The other atoms are known as the terminal atoms. To determine the number of bonds between the terminal atoms and the central atom, follow the following rules
If the terminal atom contains this number of valence electrons
- 6: double bond OR dative bond FROM central atom (dative bond only when octet rule is exceeded by drawing a double bond)
- 7: single bond
- 8: dative bond TO central atom
After drawing the bonds, check for lone pairs on the central atom and whether the octet rule is exceeded. If octet rule is exceeded and it is not a period 3 (and above) element, change the double bond to a dative bond FROM central atom.
Let’s try the rules out on the above molecules.
Next, we will examine how we can explain for difference in melting/boiling points using SBE rules.