a double headed curly arrow indicates the movement of a pair of electrons

• this shows bond making and bond breaking (covalent and dative)

the arrow starts where the electrons come from, which could be:

• from a covalent bond

• from a lone pair

the arrow points to where the electrons are going, which could be:

• to another atom to become an extra lone pair

• between 2 atoms to become a bond between them

• to a single bond to form a double bond

electrons gained = -ve

electrons lost = +ve

+ve must gain electrons to be neutral

-ve must lose electrons to be neutral