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Dot diagrams

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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.

Basic ideas

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)

C and O atoms Lewis

Enter a caption

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

atom1

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

Sodium energy levels

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

Gilbert Lewis Dot structure

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.

Lewis dot example placement

There is no set convention for the placement.
For example, chlorine atoms could be

Lewis Cl dot example placement

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.

Lewis Noble gas dot example placement

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.

Lewis Cl2 gas dot example placement

Note that each chlorine atom in Cl2 has an octet of electrons.
Apparently, the formation of an octet of electrons leads to stability.

Lewis Cl2 gas second

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:

Lewis Cl2 gas third

Some atoms do not form octets.

Hydrogen atoms form one bond, achieving 2 e- around them.

H2 atoms

Enter a caption

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:

HCl Lewis

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.

F Br I dot structures

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.

Hydrogen flouride Lewis

Lewis dot larger capture

from https://kaiserscience.files.wordpress.com/2014/09/bishop_book_3_ebook.pdf

PCl3 is used to make pesticides and gasoline additives.

Carbon, in group 4A, has four unpaired electrons in its electron-dot symbol

Carbon Lewis

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.

http://www.sparknotes.com/chemistry/organic1/covalentbonding/section2.rhtml

http://www.sparknotes.com/chemistry/organic1/covalentbonding/section2.rhtml

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?

 

More resources

Represent Bonding with Lewis Dot Diagrams, American Chemical Society

http://www.middleschoolchemistry.com/lessonplans/chapter4/lesson6

Overview: Dot structure and the periodic table (Click for the PDF file)

Dot structures “An Introduction to Chemistry” Mark Bishop.

11th grade: Molecular structure, Covalent bonds, Lewis structures, Resonance, Molecular Geometry

 

Learning Standards

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.

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