Lewis Structures, Introduction and Common Bonding Patterns

Video by Janet Gray Coonce MS

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Now I’d like to show you how to draw Lewis dot structures, the preferred structure without having to use math.  Sound good?  I like it.  It’s just a visual representation of the valence electrons and how they are going to bond together.  So lets go over guidelines for writing Lewis dot structures.

First we know that HYDROGEN (H) only forms one bond so it is always going to be on the OUTSIDE of a molecule.  It has only 1 valence electron (which is the reason) it is in group 1A of the periodic table. 

The Lewis dot structure of the hydrogen atom is shown here (H-).  It is always going to want to form one bond so that this hydrogen atom will feel like it has 2 electrons (a pair) by sharing an electron with another atom, say a hydrogen atom H–H.  When two atoms of hydrogen share an electron in a bond to form a molecule of hydrogen gas, each hydrogen atom is happy .  By sharing its valence electron covalently with each other, each atom in the H₂molecule has its duet (a pair) of electrons in its valence shell.  This diatomic molecule of hydrogen gas (H₂) is sharing a pair of electrons in a covalent bond and is represented by the Lewis structure (H–H).  The H₂ molecule is isoelectronic to helium which is a noble gas.

CARBON has a valence of 4.  It always wants to form 4 bonds and we can remember this by looking at the Lewis dot structure for carbon.  Carbon has 4 valence electrons.  

Lewis dot structures are drawn in a fashion to illustrate the octet rule:  atoms of low (<20) atomic number tend to combine in such a way that they each have eight electrons in their valence shells, giving them the same electronic configuration as a noble gas.

If carbon picked up 4 more electrons by sharing in a bond then the outer shell would be filled, the octet rule would be satisfied and carbon would be happy .  So carbon wants to form 4 bonds.  It can be in the form of 4 single bonds; or a double bond and 2 single bonds; or a single bond and a triple bond.  The Triple bond is where a total of 6 electrons are shared between carbon and another atom.  When 2 more electrons are shared with another atom in a single bond, carbon’s outer shell is full (8 e^–) and carbon is happy .  When the octet rule is satisfied the carbon atom is isoelectronic with the noble gas neon.  The compounds you will find in nature that contain carbon are called organic compounds.

Most often carbon forms either 4 single bonds; or a double bond and 2 single bonds; or a single bond and a triple bond as illustrated in these Lewis dot structures.  In each situation illustrated, there are a total of 4 bonds around the carbon atom.  There are a total of 4 pairs of electrons (8 total, an octet) in its valence shell.  There are exceptions to this rule and they are called carbocations and carbanions but lets not talk about those now.  That’s for organic chemistry later.

The next element you need to know is OXYGEN.  What is its Lewis dot structure?  We know it has 6 valence electrons because it is in the periodic table Group 6A. 

The Lewis dot structure for oxygen has 6 electrons drawn around the oxygen symbol, 2 LONE PAIRS and 2 electrons available for bonding as illustrated.  Oxygen wants to pick up 2 electrons so when each lone electron is covalently bonded to another atom, the octet rule will be satisfied.  The 2 bonds may either be 2 single bonds or a double bond.  Either way when each of the lone electrons are shared, there will be 8 electrons in its outer shell.  The outer shell is full, isoelectronic with the noble gas neon, and this makes it happy .  Oxygen in our atmosphere exists as the diatomic molecule O₂.

In summary: Hydrogen has a valence of 1 and it will always want to form a bond outside the atom.  Hydrogen can only form 1 bond.

Carbon has 4 valence electrons in its outer shell and will want to form 4 bonds.

Oxygen has 6 valence electrons in its outer shell and will want to form 2 bonds and there will be 2 lone pairs.

Now lets look at the element NITROGEN. 

Nitrogen has the atomic symbol N.  It is in periodic table Group 5A so there are 5 valence electrons in its outer shell.  The Lewis dot structure has 5 electrons drawn around it.  It has one lone pair and 3 valence electrons available for bonding.  If it can pick up 3 electrons, it will be able to complete the octet rule which will make it stable. 

Nitrogen bonds are represented by 3 single bonds and a lone pair;  or a single bond, a double bond and a lone pair;  or a lone pair and a triple bond.  In each case, the octet rule is satisfied and nitrogen is isoelectric with the noble gas neon.  Like hydrogen and oxygen, elemental nitrogen gas exists as a diatomic molecule.  

Now lets look at the HALOGENS.  Halogens belong to Group 7A of the periodic table.  The halogens include fluorine (F), chlorine (C), bromine (B), and iodine (I).  They all have 7 valence electrons.  Therefore In a Lewis dot structure, any of the halogens can be represented by the letter X surrounded by 7 electrons represented by 3 lone pairs and a single electron available for bonding.  The halogen atom wants to pick up an electron.  It can do that through an ionic bond where the electron is completely transferred which will give the anion a negative charge, or it can share with another atom in a covalent bond.  When it shares a covalent bond it “feels” like it has the 8 valence electrons needed to complete the outer shell.

The halogen is represented by 3 lone pairs and one electron available for bonding.

Transcribed by James C. Gray, MD FACOG