Lewis Diagrams

Lewis Symbols

For example, as seen above Calcium(Ca) has 2 valence electrons so its associated Lewis Structure has two valence electrons. This is the principle way to draw out a lewis symbol for an atom/ion. You write its periodic table abbreviation in the center and surround it with dots, which represent valence electrons.

Lewis Structures are also used to represent the formation of ions from neutral atoms, like above.

Lastly, lewis symbols can also represent ion formations like above. When you draw an ion for a lewis symbol or structure, you take the structure and surround it with a bracket and the sign of the electrical charge of the ion.

Lewis Structures

Lewis Structures are drawings that consist of lewis symbols with lines that represent covalent bonds. They can represent molecules and polyatomic ions themselves. To indicate a bond, you can either use a line or just draw the shared valence electrons between the atoms that create the bonds.

When two atoms share just one pair of electrons, it is known as a single bond.

Octet Rule

The Octet Rule is the tendency of atoms(that aren't transition elements or noble gases) to obtain eight valence electrons to be stable. This makes sense because noble gases get their name from the fact that they have eight valence electrons, making them stable, or noble(don't react much with anything else).

Note: Hydrogen is an exception to the Octet Rule as it only needs 2 valence electrons to be stable.

Using the octet rule, one can figure out how many bonds are required for the atoms in a compound to become stable. Take carbon tetrachloride above, also known as CCl₄. All 4 chlorine atoms have 7 valence electrons each and carbon has 4. Since carbon has 4 valence electrons and needs 4 more, it forms 4 singular bonds with the four chlorine atoms to become stable and have 8. Similarly, the four chlorine atoms need one more valence electron each and they get that from their singular bonds with the carbon atom in the center.

Note, that the same principle is in effect for silane, formulaically written as SiH₄. However, H only needs two valence electrons so the plan shifts a bit. However, Si only needs 4 valence electrons like carbon so it makes four single bonds with the four hydrogens to get its octet. Similarly, the single bonds that each H forms with the central Si atom gives each H atom 2 valence electrons.

It is worth noting that sometimes, in order to achieve a full octet, atoms need to form double or triple bonds with other atoms. This is the limit to the number of electron pairs two atoms can share in a bond.

Procedure for Drawing Lewis Structures

1)Count up the total number of valence electrons of all the atoms in the molecule.

2) First draw out the skeleton of the molecule, with all the involved atoms.

3) Connect every atom with the ones adjacent to them with single bonds.

4) Take the number of single bonds you made and double it. Take the result you get from the last sentence and subtract it from the total valence electrons you found in step 1. The result of this subtraction is how many electrons you can use to place lone pairs(pairs of 2 valence electrons, denoted as dots, that don't partake in bonding).

5) Take the result from Step 4 and distribute it as lone pairs across the outer atoms(except Hydrogen) first until all of them have full octets. Then, if you have any electrons remaining, apply them to the central atom.

6) If any atom isn't a full octet, remove a lone pair and make any single bonds a double bond if they need to be. You may have to even remove two lone pairs and make triple bonds if the molecule requires it.

This same procedure is found for polyatomic ions except in Step 1, make sure to calculate the valence electrons and subtract for positive charges and add for negative charges of the ion.

Note that not every molecule can be drawn using these steps as there will be molecules that are "electron-deficient". However, the large majority of molecules can be drawn using the above steps(assuming the steps themselves are adequate).

Resonance

Notice that NO₂^- has two possible structures where you can form a double bond, and both work equally well. How do we figure out which one has the right distribution of electrons? A double bond is shorter and stronger than a single bond between two atoms(triple is stronger and shorter than double, too). However, it is empirically proven by scientists that the bond lengths in NO₂^- are equal. How is this possible? It has to do with a concept known as resonance. If you can draw out two arrangements for a molecule or ion that work, then the actual distribution of electrons in the molecule's bonds are the average of the two structures. This means that for molecules that exhibit resonance, you must draw out each possible structure type.

Formal Charge

Sometimes, you can also draw out Lewis Structures such that the lone pairs and bonding pairs can be drawn in several ways. Note that this isn't the same as resonance because resonance deals with what happens if you can apply a double or triple bond in multiple places. Formal charge deals with what happens if the structures themselves are entirely different but still obey the Octet Rule.