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Chemistry 100: Summary of Unit B.
This unit is where we start the “real” chemistry – putting atoms together to make molecules. We
start with how atoms stick in Unit B, then look at the shape that molecules take in Unit D. We
will grab a few sections out of Unit N so we can name the things we’re looking at. Oh, the
periodic table makes everything work!
Compare this handout to the Key Terms and Concepts section at the end of the chapter.
UNIT B: CHEMICAL BONDING

Atoms can stick together. When they do, the force that holds them is called a chemical
bond. There are 3 types – ionic and covalent, and metallic bonds. We will focus on ionic
and covalent.

A molecule is a pure substance that consists of two or more atoms held together by a
chemical bond. To represent the arrangement of atoms in a molecule we will use Lewis
structures that show each atom, chemical bond, and valence electron in the molecule. Atoms
are represented by the elemental symbol (Na, H, O…), chemical bonds are shown with solid
lines (  ) that connect the atoms, and electrons are shown with dots (  ) usually in
pairs ( : ). these structures are called Lewis Diagrams.

One thing to remember is that atoms (except Hydrogen) want to have a full valence which is
8 valence electrons, also called the octet. Atoms react in order to get their octet.

Important Topics
o Ions: How are they formed. What makes them stick to each other. Positive ions are
cations, negative ions are anions (definitions).
o Isoelectronic: Of course we have to use big words. This one just means: same electron
configuration (from unit Q) - “iso” means equal. An example is the anion F- (1s22s22p6 one extra electron) compared to neutral Ne (1s22s22p6). Both have 10 electrons which are
in the same orbitals.
o Ionic Bond: Occurs when one atom steals electrons from another. The bully gets extra
electrons and becomes an anion. The victim loses electrons and becomes a cation.
Opposite charges attract so the atoms stick together (ionic bond).
o Neutral Charge: When we make ionic compounds, it is very important to balance the
anions and cations so all of the charge balances out to zero. The amount of electricity
that would be stored in a 8 oz jar of +1 ions would push a train (ancient 80’s movie
reference)!
o Covalent Bond: If atoms can’t steal electrons from each other they will share. This
sharing becomes a covalent bond. We use the line (  ) to represent these bonds in
Lewis Diagrams.
o Lewis Diagrams: These pictures are used to quickly show the arrangement of atoms and
electrons in molecules. They work phenominally well and we use them a lot. Remember
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that each atom needs 8 electrons around it – coming from non-bonded electron pairs
(lone pairs), and shared electrons in the bonds. Each atom gets to cound all of its lone
pair electrons plus 2 electrons per bond.
o Double and Triple Bonds: Sometimes we get two atoms that are held together by extra
strong bonds that have more than one pair of electrons being shared. Double bonds (2)
have two shared electron pairs. Triple bonds (3) have three shared electron pairs.
o Polar Bonds: Some atoms will get a bigger share of the electrons in covalent bonds
resulting in a polar bond. One atom gets a slightly negative charge (labeled -) and the
other one becomes slightly positive (+). We determine which is which using
electronegativity.
o Pauling’s Electronegativity: Ever wonder how to tell what kind of bond you have? Use
electronegativity (EN for short). The numbers shown on Figure B.6 (pg B.9) will always
be available to you on the exam. The scale goes from 0.7 (weak) to 4.0 (strong) and
represents how grabby an atoms is to electrons. The difference between two atoms’
electronegativity EN can predict bond type. We will calculate and use EN all quarter.
Note: Scientist use the capital greek delta  to mean “difference.”
o Bond Type:
EN
<= 0.4
0.5 – 1.6
> 1.7
Bond Type
Example
Non-polar Covalent
C-H
2.5 – 2.1 = 0.4
NN
3.0 – 3.0 = 0.0
O-H
3.5 – 2.1 = 1.4
C-Cl
3.0 – 2.5 = 0.4
NaCl
3.0 – 0.9 = 2.1
KBr
2.8 – 0.8 = 2.0
Polar Covalent
Usually Ionic
EXCEPTION: Hydrogen is
never ionic!
o Big Molecules: This is section B.6. We use the “divide an conquer” technique to handle
big molecules – work with one bond at a time. Train yourself to break them down and
work with one item at a time. You can often save time by noticing similar bonds like the
following:
o Octet Exceptions: Like everything in chemistry, the octet rule doesn’t always work.
However, for Chem 100 we’re going to skip section B.7 and not worry about it.
o Metallic Bonds: Skip this section too.