Objective: Students will be able to interpret and draw Lewis dot diagrams for individual atoms and both covalent and ionic compounds.
Evaluation: Use the activity sheets from Dot diagrams in-class to gives students practice interpreting Lewis dot diagrams. Use same types of problems following week as a basis for the formative assessment.
Rationale: We know that atoms may come together to form molecules. But why do they do under some circumstances, but not others? How come atoms join in certain ratios, not others? Lewis dot structures let us understand why atoms come together in the way that they do.
Atoms bond by sharing electrons ( e- )
Atoms bonding usually follows “the rule of 8”
If the # of e- around each atom adds up to 8,
and if they share a pair of e-, then they bond.
They may share 1, 2 or 3 pairs of e-
a set of 8 e- around an atom is called an octet
shared e- are a pair that count for 2 different atoms
Example bonding C and O
Carbon has 4 valence e-. Oxygen has 6 valence e-.
Neither has a full outer energy level (stable octet)
When we put many of these atoms together then they exchange e-
Each atom feels itself surrounded by 8 e-, by sharing some of them
How many e- does an atom have available for sharing?
Consider the element Lithium.
It has 3 p, 3 e-
How are the e- arranged?
1st energy level : 2 e-
2nd energy level: 1 e-
So only 1 e- is in the outer energy level, and 1 e- is available for sharing
Consider the element Sodium. 11 p, 11 e-
How are the e- arranged?
1st energy level : 2 e-
2nd energy level: 8 e-
3rd energy level: 1 e-
So only 1 e- is in the outer energy level -> 1 e- is available for sharing
This will be a pattern
All elements in a vertical column (group, family) have the same # of e- available to share.
(This is only a general pattern, not a law of physics)
The same info can be represented in different ways
(For now we’re skipping transition elements. Their pattern is more complicated)
What is a dot diagram?
Why do we draw dot diagrams?
How do we draw dot diagrams?
We draw e- as dots •
There are many names for this
dot diagrams / dot structures
Lewis diagrams / Lewis structures
Named after Gilbert Lewis, who introduced it in his 1916 article The Atom and the Molecule
While most electrons are tightly bound to their parent atom,
the electrons in the outermost energy level are more loosely bound,
and can be ripped off and shared. These are valence electrons.
_ _ _ ____
Electron-dot symbols are derived by placing
valence e- (represented by dots) to the right, left, top,
and bottom of the element’s symbol.
Starting on any side, we place one dot at a time until there are
up to four unpaired e- around the symbol.
If there are more than four valence e- for an atom,
the remaining e- are added – one by one –
to the unpaired e- to form up to four pairs.
There is no set convention for the placement.
For example, chlorine atoms could be
Noble gases have an octet (except helium, which has only 2 e- total),
and they are so stable that they rarely form chemical bonds
with other atoms.
When atoms (other than the noble gas) form bonds,
they often have eight electrons around them in total.
Example: the unpaired e- of a chlorine atom often pairs
with an unpaired e- of another atom to form one covalent bond.
This gives Cl an octet. 2 e- from the two‑electron covalent bond,
and 6 from its three lone pairs.
This is why Cl gas exists as Cl2 molecules.
Note that each chlorine atom in Cl2 has an octet of electrons.
Apparently, the formation of an octet of electrons leads to stability.
This way of depicting a molecule—using the elements’
symbols to represent atoms and using dots to represent
valence electrons—is called a Lewis structure.
Covalent bonds are usually represented by lines in
Lewis structures, so the Lewis structure of a Cl2 molecule
can have either of two forms:
Some atoms do not form octets.
Hydrogen atoms form one bond, achieving 2 e- around them.
Atoms of helium – a noble gas – have 2 e-.
When H atoms form one covalent bond, they get 2 e- around them, like He atoms.
Knowing that hydrogen atoms form one covalent bond and that chlorine atoms form one bond and have three lone pairs helps us to build the Lewis structure for a hydrogen chloride molecule, HCl:
Like chlorine, the other elements in group 7A also have seven valence electrons, so their electron-dot symbols are similar to that of chlorine. The unpaired dot can be placed on any of the four sides of each symbol.
In order to obtain octets, atoms tend to form compounds in which they have one bond and three lone pairs.
Note how the structures of HF, HBr and HI resemble the structure of hydrogen chloride.
PCl3 is used to make pesticides and gasoline additives.
Carbon, in group 4A, has four unpaired electrons in its electron-dot symbol
from “An Introduction to Chemistry”, Chapter 3, by Mark Bishop. http://preparatorychemistry.com/
Limits of dot structures
When drawing Lewis structures, sometimes you will find that there are many ways to place double bonds and lone pairs about a given framework of atoms. How does we decide whether one or another placement is correct? Neither and both.
The actual arrangement is a weighted average of all the valid Lewis structure… The “real” molecule is said to be a resonance hybrid of all its contributing Lewis structures. .
The classical example of resonance is benzene, C6H6 .
2 good Lewis structures for benzene exist, that differ only in their placement of double bonds.
If either structure were correct, then benzene would consist of alternating long single bonds and short double bonds.
However, it has been determined experimentally that all 6 bonds are identical.
One interpretation is that the 3 double bonds are distributed evenly around the ring, so that each bond has a bond order of one and a half.
A double headed arrow is placed between resonance structures to denote them as such.
What does it really mean?
The beneze molecule doesn’t switch back-and-forth from one form to another.
Rather, Lewis diagrams simply fail for e- placement in many molecules.
As we see in this article, Lewis theory is really a simplification of a deeper, more complete theory (quantum mechanics) – Basic chemistry rules are actually magic number approximations
If we insist on using Lewis rules then we need to draw the molecule with 2 or 3 different e- arrangements, and then use arrows to imply that shift from one to the other.
But really the molecule exists in one form: an intermediate of the drawn structures.
So what do the e- “really” look like?
Represent Bonding with Lewis Dot Diagrams, American Chemical Society
2016 Massachusetts Science and Technology/Engineering Standards
HS-PS1-2. Use the periodic table model to predict and design simple reactions that result in two main classes of binary compounds, ionic and molecular. …Predictions of reactants and products can be represented using Lewis dot structures, chemical formulas, or physical models.