This tutorial will help you learn to draw Lewis structures which are also called electron-dot structures. An electron-dot structure represents an atom's valence electrons by dots, and indicates by the placement of the dots how the valence electrons are distributed in a molecule.

How to draw an electron-dot structure

1. The least electro-negative element will generally be the atom in the center. Hydrogen is an exception. The first shell (s subshell) of a molecule can only hold two electrons. This is known as the duet rule. Since hydrogen can only accept one more electron to fill its first shell, it can not be a central atom. The samples below show the central atom in red:

2. Place the remaining elements around the central atom. The second shell (p orbital) of a molecule can hold 8 electrons. This is known as the octect rule. The octect rule says a stable atom has 8 electrons in its outer valence. Elements in the second shell, C, O, N, and F, follow the octet rule. There are, of course, exceptions to the octet rule (it's a rule, not a law). The central atom expands its octet from the third shell out. For compounds that have expanded octets, they will involve s (2 electrons), p (6 electrons), and d (10 electrons) orbitals. If the octet rule is followed, only the s and p orbitals will be involved. Most often, H, C, O, N will be central atoms:

atom	# of bonds	# of electron pairs	"normal" bonding
H	1	n/a
O	2	2
N	3	1
C	4	0

3. An element's group number will give you the number of electrons available for bonding. Only the electrons in the outer valence are involved in bonding.

Covalent Bonding for Second-Row Elements
Group	Number of Valence Electrons	Number of Bonds	Example
3A	3	3	BH3
4A	4	4	CH4
5A	5	3	NH3
6A	6	2	H2O
7A	7	1	HF
8A	8	0	Ne

4. Write the formula of the molecule. ex: CH₄

5. Count the electrons in the valence shell, those available for bonding in each element. Carbon has 4, each hydrogen has 1 for a total of 8.

6. Draw the molecule with the central atom and all of the outer atoms.

7. If the central atom is not given to you, use the following guidelines to determine it:

• is not H or F
• is not O, Cl, Br, or I unless the only other atoms are H, O, or a halogen.
• is the least numerous in the molecule or ion
• is the largest in the molecule, (most toward the lower left side of periodic table)
• has the highest bonding capacity (most toward the lower left side of the periodic table)

In our example, C is the central atom.

8. Add enough electrons to hold the molecule together. A bond should be between a central atom and an outer atom or two central atoms, but not between two outer atoms.

9. Some atoms will now have octets (4 pairs). Use the remaining electrons to make octets around the remaining atoms, if necessary.

10. Check your math and make sure you have used all of the electrons. If all of the atoms are surrounded by octets, the electron-dot structure is complete. If not, try the following:

• rearrange the electron pairs, using double or triple bonds (do not make multiple bonds with H or F)
• if all atoms are surrounded by octets and electrons are left over, add the left over electrons to the central atom. You will have a central atom with more than 8 electrons and you will have an expanded octet. You will not have an expanded octet with N, C, or O as a central atom.