Lewis structures are also known as electron dot structures. The diagrams are named for Gilbert N. Lewis, who described them in his 1916 article entitled The Atom and the Molecule. Lewis structures depict the bonds between atoms of a molecule as well as any unbonded electron pairs. You can draw a Lewis dot structure for any covalent molecule or coordination compound.
Lewis Structure Basics
A Lewis structure is a type of shorthand notation. Atoms are written using their element symbols. Lines are drawn between atoms to indicate chemical bonds. Single lines are single bonds. Double lines are double bonds. Triple lines are triple bonds. (Sometimes pairs of dots are used instead of lines, but this is uncommon.) Dots are drawn next to atoms to show unbonded electrons. A pair of dots is a pair of excess electrons.
Steps to Drawing a Lewis Structure
Pick a Central Atom
Start your structure by picking a central atom and writing its element symbol. This atom will be the one with the lowest electronegativity. Sometimes it's difficult to know which atom is the least electronegative, but you can use the periodic table trends to help you out. Electronegativity typically increases as you move from left to right across the periodic table and decreases as you move down the table, from top to bottom. You can consult a table of electronegativities, but be aware different tables may give you slightly different values, since electronegativity is calculated.
Once you have selected the central atom, write it down and connect the other atoms to it with a single bond. You may change these bonds to become double or triple bonds as you progress.
Lewis electron dot structures show the valence electrons for each atom. You don't need to worry about the total number of electrons, only those in the the outer shells. The octet rule states that atoms with 8 electrons in their outer shell are stable. This rule applies well up to period 4, when it takes 18 electrons to fill the outer orbitals. 32 electrons are required to fill the outer orbitals of electrons from period 6. However, most of the time you are asked to draw a Lewis structure, you can stick with the octet rule.
Place Electrons around Atoms
Once you have determined how many electrons to draw around each atom, start placing them on the structure. Start by placing one pair of dots for each pair of valence electrons. Once the lone pairs are placed, you may find some atoms, particularly the central atom, don't have a complete octet of electrons. This indicates there are double or possibly triple bonds. Remember, it takes a pair of electrons to form a bond.
Once the electrons have been placed, put brackets around the entire structure. If there is a charge on the molecule, write it as a superscript on the upper right, outside of the bracket.