Section 5.2—Drawing Molecules

Objectives: •use the periodic table to identify the number of valence electrons in an atom•draw Lewis Dot structures for ionic and covalent compounds•introduce the idea of isomers

Drawing Molecules on Paper

Lewis Structures (or Dot Structures) are one way we draw compounds on paper

Since paper is 2-D and compounds aren’t, it’s not a perfect way to represent how atoms bond…but it’s a good way to begin to visualize compounds

Drawing Ionic Compounds

How do we determine the number of valence electrons in an atom?

The number found with the “A” in the group number IS the number of valence electrons.

1 2 3 4 5 6 7 8

Remember:

Put electrons on each side of the element’s symbol.

Put one in each spot before doubling up!

O

Oxygen is in group VIA or 6A.

There are 6 valence electrons.

Example:Draw the

Lewis Structure for an oxygen

atom

Practice: Oxygen

To Draw Ionic Compounds

Draw each atomTransfer electrons from metal atoms to

non-metal atoms, keeping track of their new charge

Practice: KCl

ClK

Potassium has 1 electron

Chlorine has 7 electrons

Example:Draw the

Lewis Structure for

KCl

+1ClK

-1

Example:Draw the

Lewis Structure for

KCl

Add more atoms if needed

Example:Draw the

Lewis Structure the

ionic compound of

Barium fluoride

FBa

Barium has 2 electrons

Fluorine has 7 electrons

Example:Draw the

Lewis Structure the

ionic compound of

Barium fluoride

The fluorine is full, but the Barium isn’t!

FBa

Example:Draw the

Lewis Structure the

ionic compound of

Barium fluoride F

Another fluorine atom is needed!

FBa

Example:Draw the

Lewis Structure the

ionic compound of

Barium fluoride

F

+2 -1

-1

Now all have full valence shells and the charges are balanced, just as when you learned to write in BaF2 in Chapter 2!

A note about Ionic Dot Structures

The atoms are not sharing the electrons—make sure you clearly draw the atoms separate!

Practice Problems

Draw Lewis Dot Structures for compounds that form between the following atoms. What are the formulas for the compounds?

1.Al and Cl

2.Ba and O

3.Na and P

Drawing Covalent Compounds

Arrange the Atoms Atoms found in the middle of covalent

compounds are the atoms with the LEAST electron affinity.

Hydrogen & Halogens (F, Cl, Br, I) can only bond with one other atom—they can’t go in the middle of a molecules

Always put them around the outside

In general, write out the atoms in the same order as they appear in the chemical formula

Use the periodic table to decide how many electrons are around each atom

Write the electrons around each atom

Next:

H

HExample:Draw the

Lewis Structure for

CH4 C HH

Carbon is in the middle – the H’s are around it.Carbon has 4 electronsEach hydrogen has 1

Practice: CH4

H

H

Any electron that is being shared between two atoms gets to be counted by both atoms!

All atoms are “full” with 8 valence electrons (except H—it can only hold 2)

Each pair of electrons shared by two atoms forms a bond

A bond can also be represented by a single dash

C HH

Sometimes . . .

Sometimes, you don’t end up having a full valence shells for all atoms in the compound.

Example:Draw the

Lewis Structure for

CH2O

HC OH

•C has to go in the middle – the O and 2 H’s are around it.•C has 4 electrons, oxygen has 6 and each H has 1.

Practice: CH2O

Note: the carbon and oxygen only have 7 each However, they each have a single, unshared

electron. They could share those with each other!

Example:Draw the

Lewis Structure for

CH2O

HC OH

Example:Draw the

Lewis Structure for

CH2O

HC OH

Now the carbon and oxygen both have8 valence electrons! (And the H’s have 2!)

Double Bonds

Double bonds are when 2 pairs of electrons are shared between the same two atoms

Double Bond

Lone Pairs

Lone pairs are a pair of electrons not shared—only one atom “counts” them

Lone pair

And when a double bond isn’t enough…

Sometimes forming a double bond still isn’t enough to have all the valence shells full

Practice: C2H2

Both C’s go in the middle and 1 H goes on each side.

The C’s have 4 electrons each; the H’s have 1.

Example:Draw the

Lewis Structure for

C2H2

HC CH

Each carbon atom only has 7 electrons…not a full set of 8!

But they each have an un-paired electron left!

Example:Draw the

Lewis Structure for

C2H2

HC CH

Each carbon atom only has 7 electrons…not a full set of 8!

But they each have an un-paired electron left!

Example:Draw the

Lewis Structure for

C2H2

HC CH

Now they each have 8 electrons!

Triple Bonds

A triple bond occurs when two atoms share 3 pairs of electrons

Triple Bond

HC CH

Properties of multiple bonds

Single Bond

Double Bond

Triple Bond

Shorter bonds (atoms closer together)

Stronger bonds (takes more energy to break)

Practice Problems

Draw Lewis dot structures for:

1.H2O

2.O2

3.HCN

Special Notes

Polyatomic Ions

They are a group of atoms bonded together that have an overall charge

Polyatomic ions have a covalent bond within themselves…

But an ionic bond with other ions

Isomers

For many compounds, there is more than one correct way to draw a Lewis Dot Structure

HC CH CHH

HC CH CH

H

Chemical Formula: C3H4

Chemical Formula: C3H4

Contains 2 sets of double bonds between carbons

Contains 1 triple bond and 1 single bond between carbons

Both structures have full valence shells!

Both are “correct”

The chemical formula alone does not give you enough information to differentiate between the two structures

HC CH CHH

HC CH CH

H

Chemical Formula: C3H4

Isomers

Isomers: Structures with the same chemical formula but different chemical structure

Atoms can be bonded differently (multiple versus single bonds) or in a different order) but have the same overall chemical formula in isomeric structures.