Lesson 8: Molecular Lewis Diagrams (Part 1)

When atoms of different nonmetals combine, the nature of covalent bonding - sharing electrons - often dictates that a certain number of atoms of each of the different elements combine with one another in order for the bonding to work out just right. This establishes a certain ratio of atoms, which in turn establishes a certain value for the composition of the material, which makes it a compound. In short, the nature of covalent bonding dictates that atoms of nonmetal elements combine with one another to form compounds .

In this section we will use Lewis diagrams to look at how they are put together. We will use Lewis diagrams in several ways related to covalent bonding. The most basic is this: covalent bonding involves sharing electrons. We can illustrate the covalent bonding that occurs between atoms by combining the electron dot diagrams of those atoms in a way that shows which electrons are being shared. This is one way that the electron dot diagrams can be put to use.

Another use of Lewis diagrams is for predicting the formulas of compounds. This is done by using the Lewis diagrams of the atoms. Often, you can figure out the formulas of covalent compounds just from considering how many electrons each atom needs to gain according to the octet rule. Keep in mind that different elements will need different numbers of electrons. What one atom needs must be provided by the others. In many cases, the ratio of the atoms contained in the compound is just the opposite of the ratio of the number of electrons needed by each atom. Again we can illustrate the bonding that occurs by combining the electron dot diagrams of the atoms.

We'll start by looking at four examples of how hydrogen bonds to other nonmetallic elements using Lewis diagrams to help us predict how the atoms will fit together. (These are also shown in example 2 in your workbook.) Study these examples as much as you need, then try your hand at the four in the next "Practice" section.

After that we'll move on to more complicated combinations.

Predicting Formulas: Simple Examples | More Complex Examples

Predicting Formulas: Simple Examples

Let's start with hydrogen and carbon. Hydrogen has one electron in its outer shell and it needs one more electron. Carbon has four electrons in its outer shell and it needs four more electrons. The "need" ratio of H:C is one-to-four so a reasonable atom ratio would be four-to-one. Each hydrogen can give the carbon atom one electron. Since the carbon atom needs four electrons, it bonds to four hydrogen atoms. That gives us the formula for a compound between carbon and hydrogen of CH₄.

Notice that the carbon starts with four electrons, and it shares each one of those four with a hydrogen, and each hydrogen shares its one electron with the carbon. Therefore, the carbon ends up with eight electrons around it and each hydrogen ends up with two electrons around it. Thus, the octet rule is satisfied for carbon because it has eight electrons surrounding it. Hydrogen is satisfied with two electrons because that is all it has room for in its outer shell.

H ·	has 1
	needs 1

	H	C
need ratio	1	4
atom ratio	4	1
formula	CH₄

· · C · ·	has 4
	needs 4

H
··
H : C : H
··
H

Next, let's consider hydrogen and nitrogen. Hydrogen has one electron and needs one more electron. Nitrogen has five valence electrons and needs three more electrons. Notice how three hydrogen atoms can move in and bond to the nitrogen to share electrons, form three covalent bonds and provide the electrons that nitrogen needs.

Since the nitrogen needs three electrons and each hydrogen has one, you end up with NH₃ as the formula for the compound. Notice that not all of nitrogen's electrons got bonded.

The nitrogen started with five electrons around it; two of them were paired up and three were not. Those three were shared with hydrogen; and each hydrogen, in turn, shared its one electron with the nitrogen. So the nitrogen ends up with eight electrons around it and each hydrogen ends up with two.

H ·	has 1
	needs 1

	H	N
need ratio	1	3
atom ratio	3	1
formula	NH₃

·· · N · ·	has 5
	needs 3

··
H : N : H
··
H

Next up is hydrogen and oxygen. The process for hydrogen and oxygen is the same but with slightly different numbers. The formula ends up as H₂O.

The Lewis diagram can be drawn in more than one way, and both are correct.

H ·	has 1
	needs 1

	H	O
need ratio	1	2
atom ratio	2	1
formula	H₂O

··
H : O : H
··

or

··
: O : H
··
H

·· · O · ··	has 6
	needs 2

(Remember that you can also write the bonding pairs as single lines if you prefer.)

The final example for these fairly simple compounds is the compound formed by hydrogen and chlorine.

The process for hydrogen and chlorine is also the same but again with different numbers. The formula ends up as HCl.

H ·	has 1
	needs 1

	H	Cl
need ratio	1	1
atom ratio	1	1
formula	HCl

··
H : Cl :
··

·· · Cl : ··	has 7
	needs 1

Practice Predicting Formulas for Very Simple Covalent Compounds

Try your hand at figuring out the formulas for the compounds formed by the following pairs of elements. Answers follow. (This is part e in Ex. 2 of the workbook.)

H and Si

H and P

H and S

H and Br

Answers

The correct formulas for the compounds are SiH₄, PH₃, H₂S and HBr.

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Predicting Formulas: More Complex Examples

Now we'll examine some slightly more complex examples, found in Ex. 3 of your workbook. Up first is the combination of oxygen and carbon.

Carbon needs four electrons; oxygen needs two electrons. The carbon to oxygen "need ratio" is 4:2, which reduces to 2:1. Therefore the carbon to oxygen atom ratio is 1:2; one carbon for two oxygens. Therefore, the formula for this compound is CO₂. Its name is carbon dioxide.

· · C · ·	has 4
	needs 4

	C	O
simplified need ratio	2	1
atom ratio	1	2
formula	CO₂
name	carbon dioxide

·· · O · ··	has 6
	needs 2

The electron dot diagram for CO₂ is a little bit more involved than the ones that we dealt with before, so think about what is involved. The carbon needs four electrons and it is going to get them from the oxygen atoms. Each oxygen atom needs two electrons and they will have to get them from the carbon. So the electron dots for the carbon and oxygen, must be arranged in a way that will allow them to accommodate the atoms they are going to bond to. Each of the oxygen atoms needs two electrons, so the dots around the carbon are placed in such a way that it will provide two electrons for each oxygen. The carbon, in return, needs four electrons so it will need two electrons from each of the oxygen atoms. So the six dots around each oxygen are placed so that two can bond to the carbon.

These are brought together to give us the electron dot diagram for CO₂ which shows the distribution of electrons within this particular molecule. This provides the carbon with a total of eight electrons and each oxygen has a total of eight electrons. The shared electrons are counted both for the oxygen and for the carbon.

Preliminary
·· O : ··	: C :	·· : O ··

Combined

·· ··
O : : C : : O
·· ··

Let's examine oxygen and nitrogen next. Oxygen needs two electrons. Nitrogen needs three electrons. By using the ratio, we can figure that we must have two nitrogen atoms and three oxygen atoms in order to have each element share the same number of electrons. Again the number of atoms of each element is just opposite the number of electrons needed by each element. Thus the formula for this compound is N₂O₃.

We won't try to figure out an electron dot diagram for this one because it does not have just one central atom that is bonded to all the other atoms. But if you want to try it, go ahead and then talk to the instructor about your creation.

· : N · ·	has 5
	needs 3

	N	O
need ratio	3	2
atom ratio	2	3
formula	N₂O₃
name	dinitrogen trioxide

·· · O · ··	has 6
	needs 2

Finally, we'll try oxygen and chlorine. The process for oxygen and chlorine is the same but with slightly different numbers.The formula ends up as Cl₂O.

·· · O · ··	has 6
	needs 2

	Cl	O
need ratio	1	2
atom ratio	2	1
formula	Cl₂O
name	dichlorine monoxide

Preliminary
·· : Cl · ··	·· · O · ··	·· · Cl : ··

·· : Cl · ··	has 7
	needs 1

Combined

·· ·· ··
: Cl : O : Cl :
·· ·· ··

Practice (Ex. 4)

Determine the formulas and draw the Lewis diagrams for the compounds formed by the following combinations of elements.

C and Cl, C and S, P and Br, N and I

Answers follow.