Showing electron movement using curly arrows
Organic chemists show movement of electron pairs in the steps of a mechanism using curly arrows.
Each movement of an electron pair is shown by one curly arrow with the tail of that arrow being at the source of the electrons.
If electrons at the tail of the arrow are nonbonding, and the atoms at the head and the tail of the arrow are not bonded to one another, the electron movement results in bond between two atoms (A and B).
In (1) shown above one bond is formed; therefore one arrow is required to show electron movement.
If formation of the new bond exceeds the valency (number of allowed bonds) at the atom at the head of the arrow, a bond to that atom is broken. A second arrow is used to show this.
In (2) one bond is formed and one bond is broken; therefore two arrows are required to show electron movement.
It is important to pay attention to the changes in charge that occur as a result of electron movement.
Each movement of an electron pair is shown by one curly arrow with the tail of that arrow being at the source of the electrons.
If electrons at the tail of the arrow are nonbonding, and the atoms at the head and the tail of the arrow are not bonded to one another, the electron movement results in bond between two atoms (A and B).
(1)
In (1) shown above one bond is formed; therefore one arrow is required to show electron movement.
(2) 
In (2) one bond is formed and one bond is broken; therefore two arrows are required to show electron movement.
- The charge on the atom at the tail of the arrow (or series of arrows) is one unit more positive in the product.
For both (1) and (2) above, A owns one fewer electrons in the product +AB because A is sharing its non-bonding electron pair with B.
- The charge on the atom at the head of the arrow (or series of arrows) is one unit more negative in the product.
In (1) B owns one more electron in +AB because it has a share of the electron pair in the bond.
In (2) X now owns one more electron in the product X– because the bonding pair it was sharing with B has become nonbonding at X.
- The charge on atoms that have the same number of bonds in the reactants and the products does not change.
In (2) B is involved in one bond in both the reactant BX and the product +AB.