Octets and Lewis Structures
Central atoms in Lewis structures from the second row of the periodic table are small and may accommodate a maximum of four electron pairs (an octet).
Outer atoms other than H also have a maximum of an octet of electrons around them irrespective of their position in the periodic table.
However central atoms from the third row of the periodic table and beyond may have more than four electron pairs around them because they are larger.
For larger central atoms, what is the maximum number of electron pairs?
The number of electron pairs at the central atom in the most favourable Lewis structure depends on both
How do I count electrons that a central atom owns?
Central atoms in Lewis structures from the second row of the periodic table are small and may accommodate a maximum of four electron pairs (an octet).
Outer atoms other than H also have a maximum of an octet of electrons around them irrespective of their position in the periodic table.
For example outer halogen atoms Cl, Br and I from rows 2, 3 and 4 always have an octet of electrons.
However central atoms from the third row of the periodic table and beyond may have more than four electron pairs around them because they are larger.
For larger central atoms, what is the maximum number of electron pairs?
The number of electron pairs at the central atom in the most favourable Lewis structure depends on both
the nature of the electron pairs (bonding or non-bonding) AND
the group in which the central atom is found and
In the most favourable Lewis structure for a species, the central atom owns the same number of electrons that it has as an isolated atom
(this is the same number of electrons that it contributed to the pool and is equal to the last digit of the group number)
the group in which the central atom is found and
In the most favourable Lewis structure for a species, the central atom owns the same number of electrons that it has as an isolated atom
(this is the same number of electrons that it contributed to the pool and is equal to the last digit of the group number)
How do I count electrons that a central atom owns?
The central atom owns one electron of each of its bonding electron pairs.
The central atom owns all electrons of its non-bonding electron pairs.
How do I increase the electron count at the central atom when drawing a Lewis structure?The central atom owns all electrons of its non-bonding electron pairs.
Note that owning is not the same as sharing.
When assessing whether an octet is complete, the electron count includes all electrons shared with other atoms and non-bonding electron pairs.
When assessing whether an octet is complete, the electron count includes all electrons shared with other atoms and non-bonding electron pairs.
Shift non-bonding electrons pairs from outer atoms (1 pair from each) to create multiple bonds to the central atom.
Each pair shifted adds one to the electron count.
Example: Drawing the Lewis structure for the oxoanion SO42– and using this structure to determine the shape of this ion. This same procedure can be used for other oxoanions where the central atom from beyond row 2 in of the periodic tablle.Each pair shifted adds one to the electron count.
| Allocate available electrons | ||
Assume that the central atom is bonded to all oxygens.by single bonds. | Total valence electrons on SO42–: | 32 electrons |
| used in joining atoms: | –8 electrons | |
| used in complete octets at outer atoms | –24 electrons | |
| Electrons remaining | 0 electrons | |
| Count electrons owned by central atom. | ||
| (one-half its bonding electrons plus its non-bonding electrons) | Owned by central atom in structure: | 4 electrons |
| Compare with number on isolated atom | ||
| This is equal to the last digit of the group number. | On an isolated sulfur atom | 6 electrons |
| If necessary, adjust electron count at central atom | ||
| For each electron required at the central atom, move one non-bonding electron pair() from an outer oxygen to between central atom and oxygen). |  | |
| Final structure S has six bonds and owns six electrons.  | Charges Note that the charge on each of the singly-bonded oxygens is negative because these own one more electron (7) than on an isolated oxygen atom (6). | |
| Shape depends on number of electron-pair domains at central atom: | ||
| This equals the number of bonding (single or double count as one) domains plus the number of non-bonding electron pair domains. | For SO42– This is 4. Shape is tetrathedral. | |