8.2 Electrons and Chemical Bonds
P165
Valence Electrons – electrons in the
highest unfilled energy level of an
atom. (farthest away from nucleus)
These electrons participate in
chemical bonds.
Most elements bond to reach 8
valence electrons.
All elements heavier than Boron
form chemical bonds to try and get
to a configuration with 8 valence
electrons.
8 electrons are a “complete” (filled)
energy level.
The noble gases already have 8
valence electrons, so they don’t
need to form chemical bonds.
For elements with atomic #’s
1,2,3,4,and 5 (H, He, Li, Be, B) form
bonds to reach 2 valence electrons
to completely fill first energy level.
Because it has only 1 electron,
Hydrogen is a very “friendly”
element and will bond with almost
any other element.
P166 Valence electrons and Periodic
Table
Going from left to right across a
period, each successive element has
1 more valence electron.
Be – 2 valence electrons
B – 3 valence electrons
C – 4 valence electrons
Bonding: oxygen has 6 valence
electrons. It wants to get to 8, so
oxygen will form bonds that give it 2
more electrons.
Examples: 1 oxygen will bond with 2
hydrogens or 1 oxygen with 1
beryllium
Double bonds SHARE 2 electrons.
Carbon has 4 valence electrons, so 2
oxygens can bond with 1 carbon. (it
wants 4 more electrons)
P167 Lewis Dot
Shows the element symbol
surrounded by outer valence
electrons
Each element forms bonds to reach
valence electrons of 2 or 8
Carbon has 4 dots and hydrogen has
1, so 1 carbon atom bonds with 4
hydrogen atoms.
P. 168 Oxidation #’s
Oxidation Number – indicates the
+/- charge of an atom when an
electron is lost, gained, or shared in
a chemical bond.
1+ means electron is lost
1- means electron is gained
Sodium atom always ionizes to
become Na+ when it combines with
other atoms to make a compound
When writing the oxidation #, the +
or – is written AFTER the number.
Oxidation numbers correspond to an
element group of the periodic table.
ALL of the alkali metals have
oxidation number of 1+, since they
all lose 1 electron.
ALL halogens have an oxidation
number of 1- because they prefer to
gain an electron.
P169 Predicting a chemical formula
When elements combine in
molecules and ionic compounds, the
TOTAL electric charge is zero.
Any electron donated is accepted by
another.
In CCl4, carbon is a 4+ and each
chlorine is a 1So, 4+ 1- 1-
1-
1-
=0
Many elements list multiple
oxidation numbers, but the most
common is listed in BOLD.
Nitrogen (element 7, with 5 valence
electrons) can be 5+, 4+, 3+, 2+, and
33- is most common because 5 + the
3 extra electrons is 8.
P170 Ionic and covalent bonds
Whether or not a compound has an
ionic bond or a covalent bond
depends on how much each
element needs an electron to get to
2 or 8.
Elements close to noble gases tend
to give or take electrons, NOT
SHARE them.
Alkali tends to give an electron;
halogen tens to take electron
Also, the farther apart on the
periodic table the elements are, the
more likely they are to have an ionic
compound.
Covalent compounds form when
elements are more equal in the
numbers of electrons they have.
Nonmetals that are close together
tend to form covalent bonds with
each other.
Compounds with Carbon, Silicon,
Nitrogen, and Oxygen are often
covalent.
Silica dioxide (SiO2) has a covalent
bond
Calcium Fluoride (CaF2) has an ionic
bond