A shared pair of electrons is sometimes represented in Lewis structure by a straight line: The unshared electrons are called lone pairs and do not participate in the covalent bond between the two atoms.
Links
[1] https://hootsgo.org/?q=node/6462
[2] http://chempaths.chemeddl.org/services/chempaths/?q=book/General%20Chemistry%20Textbook/1337/further-aspects-covalent-bonding