7.2 Valence Electrons and Bonding Patterns
Only valence
electrons form
bonds
When a chemical bond forms, some valence electrons are either shared or transferred
between atoms. Only the unpaired valence electrons in an atom participate in chemical
bonds. For complex reasons, the fifth, sixth and seventh valence electrons pair up and
reduce the number of electrons available for bonding. The diagram below shows the
main group elements along with their paired and unpaired valence electrons.
Each unpaired
valence
electron can
form one
covalent bond
In a molecular compound, each unpaired valence electron can
form one covalent chemical bond. For example, both nitrogen
(N) and phosphorous (P) atoms each have three unpaired
valence electrons. In molecular compounds, these elements
both form three covalent bonds.
In a molecular compound, each unpaired valence
electron forms one covalent bond
Ion charge and
valence
electrons
When forming a positive ion, each valence electron can
contribute one unit of positive charge. This occurs when an
atom loses its valence electron(s) to another atom, leaving a charge of +1 for every
valence electron lost. For example, sodium (Na) and potassium (K) each have a single
valence electron and form singly charged ions (+1). Magnesium (Mg) and calcium (Ca)
have two valence electrons each. These elements form ions with a charge of +2.
In positive ions, each valence electron can contribute
one unit of positive charge
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Section 7.2 Valence Electrons and Bonding Patterns
The octet rule
Eight valence
electrons =
chemical
stability
In Chapter 6 you learned that the noble gases have eight valence
electrons (except for He, which has two). These elements form no
bonds because they are already at the most stable configuration of
electrons. From the noble gases we infer that elements with eight
valence electrons are chemically stable. The observed behavior of the
rest of the elements confirms that elements form chemical bonds to achieve the magic
configuration of eight valence electrons. This observation is known as the octet rule. The
octet rule states that elements transfer or share electrons in chemical bonds to reach a
stable configuration of eight valence electrons.
Elements form bonds to reach eight valence electrons
Two valence
electrons are
most stable for
H, Li, Be, and B
The elements hydrogen, lithium, beryllium, and boron have so few electrons that their
version of the octet rule is really based on helium as the closest noble gas. Helium fills
the first energy level with its two electrons. That means either zero or two valence
electrons are also “noble gas” electron configurations. The “octet rule” for hydrogen,
lithium, beryllium, and boron is more accurately the “duet rule” or “the rule of 2.” These
elements form chemical bonds to achieve two valence electrons.
H, Li, Be, and B form bonds to reach two valence electrons
Shared
electrons are
counted by
both atoms
In a covalent bond, a shared electron gets counted as a valence electron by both atoms.
For example, molecular hydrogen (H2) shares electrons to get two valence electrons, like
helium. In water (H2O) each of the two hydrogen atoms shares one electron with oxygen,
giving each hydrogen two valence electrons and oxygen eight valence electrons.
octet rule: rule that states that elements transfer or share electrons in chemical
bonds to reach a stable configuration of eight valence electrons; the light elements H,
Li, Be, and B have He as the closest noble gas, so the preferred state is two valence
electrons instead of eight.
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A NATURAL APPROACH TO CHEMISTRY
Valence electrons and ion formation
Ionic charge
and the
periodic table
There are two ways for elements to satisfy the octet rule: by sharing electrons in covalent
bonds or by transferring electrons in an ionic bond. When the difference in
electronegativity is larger than 2.1, the chemical bond is ionic. Most elements form some
ionic compounds. The table below shows the most common charges for ions of the main
group elements.
Ion patterns
You should see several patterns:
1. Elements on the left side tend to form positively charged ions, and elements on the
right side form negatively charged ions.
2. Elements in the middle sometimes form positive ions and sometimes form negative
ions. These are marked as “variable” charge.
3. The alkali metals (group 1) form +1 charged ions because they have one valence
electron.
4. The alkali earth metals (group 2) form +2 charged ions because they have two
valence electrons.
5. Boron-like elements (group 13) form +3 charged ions because they have three
valence electrons.
6. Oxygen-like elements (group 16) form –2 charged ions. These elements have six
valence electrons. Their easiest path to the octet rule is to gain two electrons.
7. The halogens (group 17) form –1 charged ions. These elements have seven valence
electrons. Their easiest path to the octet rule is to gain one electron.
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Section 7.2 Valence Electrons and Bonding Patterns
Electron configuration of ions
Why sodium
forms a +1 ion
Sodium is an alkali metal with one valence
electron. Sodium usually loses its one valence
electron to become Na+ to satisfy the octet
rule and have an electron configuration like
that of a noble gas. Theoretically, sodium
could also gain seven electrons to become Na–
7, but losing one is so much more likely that
we never see Na–7 ions in nature. The electron
configurations of Na, Na+, and Ne are shown
to the right. Note that Na+ has the same
electron configuration as neon. Sodium tends
to form Na+ ions because this is the lowest
energy path by which sodium can satisfy the
octet rule and reach a noble gas electron
configuration.
Why oxygen
forms a –2 ion
Oxygen atoms have six valence electrons. To
have an electron configuration like that of a
noble gas, oxygen could either gain two
electrons to have the same electron
configuration as neon or lose six electrons to
have the same electron configuration as
helium. Given that oxygen has a high
ionization energy, it is not likely to lose six
electrons. Also, oxygen has a high electronegativity, so it has the ability to grab
electrons from other atoms. It makes sense
that oxygen will gain two electrons rather than
lose six.
Write the electron configuration for a magnesium ion (Mg2+).
Asked:
Given:
Electron configuration of Mg2+
Mg, atomic number of 12, charge of +2
Relationships: The electron configuration of magnesium is 1s22s22p63s2.
Solve:
Answer:
210
Mg must lose two electrons to become Mg2+. Therefore it loses the
pair of 3s2 electrons.
The electron configuration of Mg2+ is 1s22s22p6, which is
identical to neon.
A NATURAL APPROACH TO CHEMISTRY
Simple ionic formulas
Ionic crystals
Ionic substances typically form crystals, which are
large groups of oppositely charged ions arranged
in a regular pattern. The calcium chloride (CaCl2)
crystal in the diagram is a good example. Calcium
chloride is often used to melt ice on roads because
it is better for the environment than sodium
chloride (also used to melt ice). Ionic crystals are
neutral even though they are formed through the
attractions of trillions of charged ions. You can
have any number of ions in the crystal as long as
the positive charges exactly balance the negative
charges.
Why calcium
chloride has
the formula
CaCl2
To determine the formula of an ionic compound, you need to balance the positive and
negative charges. For example, calcium makes a Ca2+ ion. Chlorine makes a Cl– ion.
Each calcium atom loses two electrons and each chloride ion gains only one. This means
the compound requires two chloride ions to have the same amount of negative charge as
one calcium ion’s positive charge. This makes the ratio of calcium to chlorine 1:2, and
the formula is therefore CaCl2.
What is the correct formula for calcium oxide, a compound used in making
paper, and pottery and adjusting the pH of soils?
Asked:
Given:
The formula for the ionic compound calcium oxide
Calcium oxide is made from calcium and oxygen ions. Calcium
forms +2 ions and oxygen forms –2 ions.
Relationships: Ca2+ and O2– must combine in a ratio that will balance out the
positive and negative charges.
Solve:
A NATURAL APPROACH TO CHEMISTRY
The charge on one Ca2+ ion will balance out with the charge on one
O2– ion. Therefore the ratio is 1:1 and the formula is CaO.
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Section 7.2 Valence Electrons and Bonding Patterns
Covalent bonds
Covalent bond
formation
In covalent bonds, electrons are shared between atoms, not transferred. The number of
covalent bonds is equal to the number of unpaired valence electrons. The diagram below
shows the number of covalent bonds by the main group elements.
Covalent bond
patterns
1. Only nonmetals and hydrogen are commonly found as covalently bonded parts of
molecules.
2. Carbon-like elements (group 14) form four covalent bonds.
3. Nitrogen-like elements (group 15) form three covalent bonds.
4. Oxygen-like elements (group 16) form two covalent bonds.
5. Halogens (group 17) form one covalent bond.
6. The number of covalent bonds is equal to the number of unpaired valence electrons.
Paired and
unpaired
electrons
Going from left to right across the periodic table, notice
that nitrogen has three unpaired electrons and one pair.
The eight electrons in s and p orbitals act like eight
strangers filling up four bench seats on a bus. Everyone
prefers his or her own seat, so at first each person sits
solo. The fifth person must pair up with someone. The
same is true of electrons. The fifth valence electron pairs
up and is no longer available for bonding! For reasons of
quantum mechanics, only unpaired electrons form
bonds.
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Bonds and unpaired electrons
Every unpaired
electron forms
a bond
Take a look at the molecule of vitamin C to the
right. This molecule is made from several
carbon, oxygen, and hydrogen atoms. Take a
close look at each carbon atom. You will see
that every carbon atom has four covalent bonds
to other atoms. The oxygen atoms each form
two covalent bonds, and the hydrogen atoms
each form one covalent bond.
Covalent
bonds and
noble gases
Carbon has four valence electrons, and they are all unpaired. Therefore carbon forms
four chemical bonds. Oxygen has six valence electrons. Four of these electrons are paired
and two are unpaired. Oxygen always forms two covalent bonds because it has two
unpaired electrons. Hydrogen has one valence electron, and its closest noble gas is
helium with two valence electrons. Hydrogen needs one more electron to be like helium.
It gets this by forming one covalent bond with another atom.
Unpaired
electrons
create very
reactive atoms
or molecules
One quick way to tell how many covalent bonds an atom will form is to look at its Lewis
dot structure. Atoms will form one covalent bond for each unpaired valence electron.
Atoms or molecules that have unpaired electrons are highly reactive and are known as
free radicals. These are the kind of molecules that can be responsible for aging and
diseases like cancer. Sometimes free radicals are created as part of our own natural
metabolism, and sometimes they are caused by outside sources such as ultraviolet
radiation from the Sun. Antioxidants are considered an important part of a person’s diet
because of their role in preventing free radicals from reacting with and damaging DNA.
free radical: a molecule or atom that is highly reactive owing to having one or more
unpaired valence electrons.
antioxidant: a molecule that reacts easily with free radicals, such as found in brightly
colored fruits and vegetables, vitamin E, and chocolate.
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