Section 6.3 Valence
6.3 Valence
The meaning of
valence
In the last section we learned that only the electrons in the highest unfilled energy level
form chemical bonds. Think of the electrons in any completely filled energy levels as
“permanently grounded!” This greatly reduces the problem of understanding chemical
bonding. We don’t need to care about electrons in filled energy levels. We need only
concern ourself with the electrons in the highest unfilled level.
Sulfur has 6
valence
electrons
For example, Sulfur has 16 electrons. Ten of these are in the completely filled first and
second energy levels. That leaves 6 electrons in the unfilled third energy level. Sulfur’s
chemical properties come from the 6 electrons in the highest unfilled energy level.
Oxygen has 6
valence
electrons
Oxygen has 8 electrons. Two of these are in the completely filled first energy level. That
leaves 6 electrons in the partly filled second energy level. All of oxygen’s chemical
properties are due to having 6 electrons in the highest unfilled energy level.
Valence
Oxygen and sulfur have the same number of valence electrons. Valence electrons are
the special name given to electrons in the highest energy level. Because oxygen and
sulfur have similar valence electrons, they form similar chemical compounds.
valence electrons - electrons in highest unfilled energy level. These are the
electrons that make chemical bonds.
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Determining valence electrons
Identifying
valence
electrons
The electrons that are in the highest energy level are the valence electrons. To determine
how many there are you start with an element’s electron configuration.
1.
2.
3.
d orbitals do
not count
Write down the electron configuration
Count how many electrons are in the highest s and p orbitals. You should get a
number between 1 and 8.
These are the valence electrons .
For elements other than the transition metals, electrons in d orbitals do not count as
valence electrons. For a complicated reason of quantum theory, the s and p orbitals
become mixed when atoms bond with each other. This has the technical name
hybridization During bonding the separate s and p orbitals become a hybrid s-p orbital.
How many valence electrons does tin (Sn) have?
Given:
Valence electrons are the electrons found in the highest principle
energy level orbitals.
Relationships: Tin is in the 5th period (or row) of the periodic table so the highest
principle energy level electrons will be in level five.
Solve:
The electron configuration of tin is:
Sn = 1s22s22p63s23p64s23d104p65s24d105p2
or via the shorter notation
Sn = [Kr]5s24d105p2
There are a total of 4 electrons in level five s and p orbitals, so tin
has four valence electrons.
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Section 6.3 Valence
The main group elements
The main
group
elements
Valence is such an important concept that many chemists prefer to label the periodic table
groups according to valence electrons. In this scheme groups 1A through 8A are called
the main group elements. The transition metals are placed in a separate category
according to how the d-orbital electrons form bonds (groups 1B - 8B).
Valence of
main group
elements
All of the elements in the same column (group) in the main group elements have the
same number of valence electrons. For example, carbon, silicon, germanium, tin, and
lead have four valence electrons.
Valence of
transition
metals
The number of valence electrons for the transition metals do not follow a simple pattern.
Many act as if they have 2 valence electrons, like the group 2A elements. However, there
are more exceptions than rules, and it depends strongly on what other elements are in a
compound. For example, chromium makes bonds that involve 2, 3, 4, 5, or even 6
electrons.
How many valence electrons does magnesium (Mn) have?
Given:
Magnesium - atomic number is 12
Relationships: Mg is a group 2B element - all group 2B elements have 2 valence
electrons.
Answer:
Magnesium has 2 valence electrons.
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Introduction to Lewis dot notation
Lewis dots
show valence
electrons.
Because valence electrons are the most important electrons
involved in bonding, a special graphic notation called Lewis
dot diagrams was invented to represent them. A Lewis dot
diagrams shows each valence electron as a dot surrounding
the element symbol. There are up to 8 dots in Lewis dot
diagram. Electrons in filled energy levels don’t count because
they do not participate in bonding.
Draw individual
dots before
pairing them
What if there are more than four valence electrons? Where do they go? Once each side of
the atom symbol has one valence electron, you start to pair them up. Drawing the dots
this way, first drawing them individually, and then pairing them up when necessary, will
become important when we learn about covalent bonding in the next chapter. Below is a
part of the periodic table, showing you how many valence electrons each atom has.
Elements in the
same group
have the same
number of
valence
electrons.
Atoms in the same family (or column) have the same number of valence electrons. They
also react with oxygen in the same way, forming the same kind of chemical formula
when bonding with oxygen. By understanding the underlying structure of the electrons,
especially the valence electrons, we are now set up to better understand why atoms bond
together the way they do. The next chapter will take a deeper look at bonding.
Lewis dot diagrams - a diagram showing one dot for each valence electron an
atom has. These dots surround the element symbol for the atom.
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Section 6.3 Valence
More about valence electrons
Why valence
electrons bond
Valence electrons participate in bonding because they are the most loosely bound
electrons in an atom. The loosely bound electrons are the easiest to share. Electrons in
completely filled energy levels are tightly bound to an atom. That explains why the noble
gasses form no chemical bonds. There are no loosely bound electrons to bond with.
Valence
electrons and
Lewis dots
All the alkali metals have a single valence electron, like sodium. Their Lewis dot
diagrams have a single dot representing the single electron available for bonding. All the
alkali earth metals (group 2) have two valence electrons, like calcium. Elements in
boron’s group have three valence electrons. Carbon’s group has four valence electrons.
Nitrogen has five, oxygen has six and fluorine has seven. The second row ends with neon
which has 8 valance electrons.
Filled d orbitals
do not
contribute
valence
electrons
For elements in the main groups, electrons in the d orbitals do not count as valence
electrons. These elements have only completely filled d orbitals and the electrons in a
completely filled d orbital act like they are in filled energy levels. That is why selenium
(Se) has 8 valence electrons, putting it in the oxygen group. The electron configuration of
selenium shows a completely filled d orbital.
The transition
metals are
different
The situation is different with the transition metals. These elements have partially filled
d orbitals and therefore transition metals may have more than 8 valence electrons. This is
one reason that the bonding patterns among the transition metals are so complicated.
How many valance electrons there are depends partly on which other element or
elements are participating.
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Using the periodic table
Who uses the
periodic table?
So, why should we care about the periodic table and its guide to periodic properties?
Chemists, biologists, material scientists, engineers and physicists use the periodic table
every day to help design new drugs, create recyclable construction materials, utilize
sources of renewable energy, and invent the next cool electronic device. This is just a tiny
fraction of the applications in which the periodic table is a crucial guide.
A stronger type
of glass.
Take, for example, the making of strong glass. Standard
glass from which drinking bottles and window panes are
made from contains the element sodium. If you’ve ever
accidentally dropped a glass on the floor, you know that it
breaks pretty easily.
Common glass is primarily made from silicon and oxygen,
but it contains some sodium, calcium, and aluminum as
well. To make the glass stronger you could replace the
sodium atoms on the surface with another atom that is a
little bigger than sodium. However, the replacement atom
must have similar chemical properties as sodium in order to
interact with its neighboring atoms in a similar way and be incorporated into the glass. To
have similar chemical properties we would want to choose an element from the same
column (or family) as sodium, and the next largest atom would be potassium. If you were
a chemist working for a glass maker, choosing potassium would be a good option.
A pottery glaze is made from a mixture of several materials. One of those
materials is called a “flux”, which changes the melting point of the glaze. Here
are several fluxes that are commonly used: MgO, CaO, BaO, SrO. Why does it
make sense that if one of these compounds is a flux, the others might make
good fluxes too?
Asked:
Why does is it make sense that MgO, CaO, BaO, and SrO all act as
fluxes in pottery glazes if any one of them do?
Given:
A flux is a component of a pottery glaze. The compounds listed
above are all commonly used as fluxes.
Relationships: Mg, Ca, Ba, and Sr are all from the same family of the periodic table
- the alkaline earth metals.
Solve:
Elements in the same column have similar chemical properties and
follow either an increasing or decreasing trend in other properties.
If they come from the same column, then they can behave the same
way (with slight variations depending on their atomic level properties) in the glaze.
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