Drawing Lewis point structures (also known as Lewis structures or diagrams) can be confusing, especially for a novice chemistry student. If you're starting from scratch or just a refresher, here's the guide for you.
Steps
Method 1 of 3: Diatomic Covalent Molecules
Step 1. Determine the number of bonds between the two atoms
They can be single, double or triple bond. Generally, the bond will be such as to allow both atoms to complete a valence shell with eight electrons (or in the case of hydrogen, with two electrons). To find out how many electrons each atom will have, multiply the degree of bond by two (each bond involves two electrons) and add the number of unshared electrons.
Since both atoms must fill the outer shells, the covalent bonds between two atoms generally occur between atoms with the same number of valence electrons or between a hydrogen atom and a halogen
Step 2. Draw two atoms next to each other using their atomic symbols
Step 3. Draw as many lines connecting the two atoms as indicated by the degree of the bond
For example, nitrogen - N2 - has a triple bond that connects its two atoms. Thus, the bond will be represented in a Lewis diagram with three parallel lines.
Step 4. Draw the other electrons around each atom in the form of dots, making sure they are in pairs and evenly surround the atom
This refers to the unshared electronic doublets in each atom.
For example, diatomic oxygen - O2 - has two parallel lines connecting atoms, with two pairs of points on each atom.
Method 2 of 3: Covalent Molecules with Three or More Atoms
Step 1. Determine which atom is the central one
For the examples of this basic guide, let's assume we have a single molecule with a single central atom. This atom is usually less electronegative and is better able to form bonds with many other atoms. It is called the central atom because all other atoms are bonded to it.
Step 2. Study how the electron structure surrounds the central atom (including both unshared and bonding doublets)
As a general but not exclusive rule, atoms prefer to be surrounded by eight valence electrons - octet rule - which applies to fields of 2 - 4 electrons, depending on the number and types of bonds.
- For example, ammonia - NH3 - has three bond doublets (each hydrogen atom is bonded to nitrogen with a single covalent bond) and an extra unshared pair around the central atom, nitrogen. This results in a structure of four electrons and a single pair.
- The so-called carbon dioxide - CO2 - has two oxygen atoms in double covalent bond with the central atom, carbon. This creates a two-electron conformation and zero unshared doublets.
- The PCl atom5 or phosphorus pentachloride breaks the octet rule by having five bond doublets around the central atom. This molecule has five chlorine atoms in single covalent bond with the central atom, phosphorus.
Step 3. Write the symbol of your central atom
Step 4. Around the central atom, indicate the geometry of the electron
For each unshared pair, draw two small dots next to each other. For each individual bond, draw a line out of the atom. For double and triple bonds, instead of just one line, draw two or three, respectively.
Step 5. At the end of each line, write the symbol of the linked atom
Step 6. Now, draw the rest of the electrons around the rest of the atoms
By counting each bond as two electrons (doublets and triplets count as four and six electrons, respectively), add electron doublets so that the number of valence electrons around each atom comes to eight.
Of course, the exceptions include atoms that do not follow the octet rule and hydrogen, which has only zero or two valence electrons. When a hydrogen molecule is covalently bonded to another atom, there will be no other unshared electrons around it
Method 3 of 3: Ions
Step 1. To draw the Lewis point structure of the monatomic ion (one atom), first write the atom symbol
Then, it draws as many electrons around it as its original valence electrons are, roughly how many electrons it gained / lost during ionization.
- For example, lithium loses its one and only valence electron during ionization. Thus, its Lewis structure would be Li only, with no dots around it.
- The chloride gains one electron during ionization, giving it a full shell of eight electrons. Thus, its Lewis structure would be Cl with four pairs of points around it.
Step 2. Draw brackets around the atom and outside the closing one, top right, note the charge of the ion
For example, the magnesium ion would have a hollow outer shell and would be written as [Mg]2+
Step 3. In the case of polyatomic ions, such as NO3- or SO42-, follow the instructions of the "Covalent Molecules with Three or More Atoms" method above, but add the extra electrons for each negative charge where they fit best, in order to fill the valence shells of each atom.
Around the structure, put the brackets once again and indicate the charge of the ion: [NO3]- or [SO4]2-.
