ChemStandard2e

Today I am going to talk about ChemSatandard 2c, which talks about the Lewis Dot Structure.

Lewis structures, also called Lewis dot diagrams, electron-dot diagrams or electron-dot structures, are diagrams that show the bonding between atoms of a molecule, and the lone pairs of electrons that may exist in the molecule. A Lewis structure can be drawn for any covalently bondedmolecule, as well as coordination compounds. The Lewis structure was named after Gilbert N. Lewis, who introduced it in his 1916 article The Atom and the Molecule. They are similar to electron dot diagrams in that the valence electrons in lone pairs are represented as dots, but they also contain lines to represent shared pairs in a chemical bond (single, double, triple, etc.).
Lewis structures show each atom and its position in the structure of the molecule using its chemical symbol. Lines are drawn between atoms that are bonded to one another (pairs of dots can be used instead of lines). Excess electrons that form lone pairs are represented as pairs of dots, and are placed next to the atoms.
Although many of the elements react by gaining, losing or sharing electrons until they have achieved a valence shell electron configuration with a full octet (8) of electrons, there are many noteworthy exceptions to the 'octet rule'. One example is hydrogen (H), which has only a single valence electron and tends to react to attain either 0 or 2 valence electrons. When H has two electrons in its valence shell, it could be said to obey a 'duet rule', and achieves a valence shell electron configuration equivalent to helium (He).

Once the total number of available electrons has been determined, electrons must be placed into the structure. They should be placed initially as lone pairs: one pair of dots for each pair of electrons available. Lone pairs should initially be placed on outer atoms (other than hydrogen) until each outer atom has eight electrons in bonding pairs and lone pairs; extra lone pairs may then be placed on the central atom. Here is a video describing the dot structure.

1) Add up the total number of valence electrons in the molecule by totalling the valence electrons on each atom in the molecule or polyatomic ion.
If you are drawing a Lewis Structure for a polyatomic ion:
Negative Ion: Add the number of electrons equal to the negative charge on the ion.
Positive Ion: Subtract the number of electrons equal to the positive charge on the ion.

2) Draw the skeleton structure of the molecule or polyatomic ion in which the covalent bonds between the atoms are drawn as single lines. Each bond equals two valence electrons. If the molecule has more than two atoms, the atom with the lowest electronegativity is generally the central atom and is written in the middle.

3)Distribute valence electrons around the outer atoms as nonbonding electrons to until each atom has a complete outer shell (i.e. 8 electrons except for H which has only 2 valence electrons).
4)Add the remaining valence electrons to the central atom.
BONDING ELECTRONS + NONBONDING ELECTRONS = TOTAL VALENCE ELECTRONS

5)Check the central atom.
If the central atom has eight electrons surrounding it, the Lewis Structure is complete.
If the central atom has less than eight electrons, remove a nonbonding electron pair from one of the outer atoms and for a double bond between that atom and the central atom. If needed, continue to remove nonbonding electron pairs from the outer atoms until the central atom has a complete octet.
If the central atom has more than eight electrons, then this means that the central atom has expanded its valence shell to hold more than eight electrons. This is allowed for atoms with valence shells in the third energy level or higher.