CHAPTER 12
CHEMICAL BONDING
Core electrons are found close to the nucleus, whereas valence electrons are
found in the most distant s and p energy subshells. The valence electrons are
responsible for holding two or more atoms together in a chemical bond.
Octet rule states that an atom tends to bond in such a way that it acquires
eight electrons in its outer shell. In order to obey this rule, atoms either
transfer electrons or share one or more pair of electrons.
Ionic bond is formed when a metal cation is attracted to a nonmetal anion
and are held together by electrostatic attraction, example, NaCl, MgO, AlN.
Covalent bond is formed when two nonmetal atoms share valence electrons.
Molecules are held together by covalent bond, example, NH3, H2O, CO2.
IONIC BOND
Ionic bond results from the attraction between a positively charged cation
and negatively charged anion. In the bonding process, energy is released.
Electrostatic attraction is similar to the attraction between opposite ends of
two magnets. When ionic bond created is strong, they create rigid crystalline
structure, for example NaCl.
FORMATION OF CATION AND ANION
When metal loses its valence electrons, it becomes positively charged,
example, Na (11 electrons) and Na+ (10 electrons). Main group metals
usually achieve a noble gas electron configuration after losing their valence
electrons.
When a nonmetal atom gains electrons, it becomes negatively charged.
Example, chlorine atom (17 electrons) gains 1 electron to become Cl- (17 +
1 =18 electrons)
IONIC RADII
The atomic radius of sodium atom (0.186nm) reduces once it becomes
sodium ion (0.095nm). The reason for this decrease in radius is because
sodium atom has lost its 3S energy sublevel.
In the case of chlorine atom, the atomic radius (0.099nm) increase when it
changes to chloride ion (0.181nm). The reason for this increase in radius is
that the additional electron repels the electron already present. In general, the
atomic radius of cations is smaller than anions.
COVALENT BOND
A covalent bond results from the sharing of electrons by nonmetal
atoms. The electrons being shared belong to both nonmetals. Each atom uses
these bonding electrons to complete its valence. Since a filled valence shell
is very stable, the resulting bond is also stable.
Let us consider the formation of hydrogen chloride, during bond
formation; hydrogen atom shares its one valence electron with the chlorine
atom. This additional electron gives chlorine eight electrons (stable octet Ar)
in its valence shell while this additional electron gives hydrogen two
electrons (stable octet of He) in its valence shell. The bonding electrons are
distributed over each atom, the electrons are free to move about the entire
molecule, thus the electrons are said to be DELOCALIZED.
BOND LENGTH
The distance between two nuclei is called bond length. From the
above example, there is overlap of 1S energy sublevel of the hydrogen atom
mixing with 3P sublevel of chlorine atom. The mixing of the sublevels
draws the two nuclei closer together. The radius of hydrogen atom is
0.037nm while that of chlorine atom is 0.099nm, the formed bond’s radius
should have been ( 0.037 + 0.099 = 0.136nm) but the experiment showed
that the bond length of HCl is 0.127. This explains the overlap of the orbital
during bond formation.
BOND ENERGY
Energy is released when two ions are attracted to one another and
form an ionic bond. For example, the attraction of Na+ and Cl- to form NaCl
released heat energy. In the same way, energy is released when two atoms
are attracted and form covalent bond.
Bond energy is the amount of energy required to break a covalent
bond between two atoms.
H(g) + Cl(g)
HCl(g) + heat
HCl(g) + heat
H(g) + Cl(g)
ELECTRON DOT FORMULAS OF MOLECULES
Guidelines for drawing Electron Dot:
 Calculate the number of valence electrons from all the atoms in the
molecule.
 Divide the valence electron total by two to find the number of the
electron pairs.
 The electron pairs (4 pairs) are placed around the central atom and
then the remaining atoms in the molecule so as to provide octet.
 To complete the octet sometimes, the nonbonding electrons are
utilized, in this case a double bond is formed.
One pair of electrons shared by two atoms forms a SINGLE bond. A
molecule can also contain two or three electron pairs between two atoms;
these are referred to as DOUBLE or TRIPLE bonds. Double and triple
bonds result from an insufficient number of valence electrons around
each atom in a molecule. To provide an octet, it may be necessary to
move nonbonding electrons between two atoms; these electrons then
become bonding electrons. In the structural formula of a molecule, a
single bond is shown as dash, a double bond is shown as two dashes, and
a triple bond as three dashes.
Electron dot formula for ammonium ion,
1. total number of valence electrons in this positive ion is 5 (valence
electrons from nitrogen) + 4( valence electrons from hydrogen) – 1 (
electron from the charge) = 8 valence electrons.
2. number of paired electrons = 8/2 = 4 paired electrons.
3. the central atom is nitrogen, 4 paired electrons are placed around the
central atom. Since there no more paired electrons, the other atoms
have no paired electrons on them.
4. the diagram is in the textbook.
POLAR COVALENT BOND.
Covalent bonds result from sharing of valence electrons. As we
discussed earlier, electrons are shared equally, in many instances, one of
the two atoms do not share electrons equally. When the electrons are
drawn more closely to one of the atoms, the bond is said to be
POLARIZED, also known as POLAR COVALENT BOND.
ELECTRONEGATIVITY TRENDS
Each element has an innate ability to attract valence electrons. This
ability is related to
 Nearness of the valence shell to the nucleus.
 Magnitude of the positive charge in the nucleus.
Linus Pauling, an American chemist, devised a method for measuring
the electronegativity values for each of the elements. He assigned
carbon a value of 2.5 and the determined the electronegativity of other
elements in relation to carbon. He found that fluorine is the most
electronegative element (4.0). Other highly electronegative elements
are oxygen (3.5), nitrogen (3.0) and chlorine (3.0). The most
electronegative elements are nonmetals. Electronegativity increases
from bottom to top also it increases from left to right on the periodic
table.
DELTA NOTATION FOR POLAR BONDS
Lets us consider the molecule H-Cl, the electro negativity of H
is 2.1 while Cl is 3.0. Since there is a difference in electro negativity
between these two elements (3.0 – 2.1 = 0.9), the bond in an H-Cl is
Polar. Since Cl is more electro negative, the bonding electrons are
attracted toward Cl atom and away from H atom. The Cl atom thus
becomes slightly negatively charged, whereas, the H atom becomes
slightly positively charged. The Greek letter delta ( δ) is used to
denote polar bond.
δ- is used to indicate atom having partially negative charge.
δ+ is used to indicate atom having partially positive charge.
These symbols are referred to as delta notation.
δ+
H- ClδIn practice, a bond between two atoms having an electro
negativity difference of 0.5 or less is usually considered a non polar
bond.
NONPOLAR COVALENT BOND
Polar bonds result when there is unequal sharing of bonding
electrons. In studying Pauling’s electro negativity value, we find out
that some elements have the same values, for example, N and Cl both
have 3.0 while C, S, and I all have 2.5.
When the electro negativity of each atom is the same, the bond
is not polarized. A covalent bond between two atoms with the same
electro negativity is referred to as nonpolar covalent bond or nonpolar
bond.
DIATOMIC NONPOLAR MOLECULES
A diatomic molecule consist of 2 nonmetal atoms joined by a
covalent bond. We recall the seven diatomic molecules, H2, N2,O2, F2,
Cl2, Br2, and I2. These molecules exhibit nonpolar covalent bond.
Electro negativity can also be said to occur when there is no
difference between the two bonded atoms.
COORDINATE COVALENT BOND
Covalent bond is formed when an electron pair is shared by 2
atoms. Each nonmetal is surrounded by non-bonding electron pairs to
complete its octet. A covalent bond resulting from one atom donating
an electron pair to another atom is called a coordinate covalent bond.
A good example is ozone molecule (O3 ) where an oxygen molecule
(O2 ) donates a nonbonding electron pair to an oxygen atom to
produce ozone.
SHAPE OF MOLECULES
The theory to explain the shapes of molecules was an extension
of electron dot formulas in which pairs of electron surround a central
atom. The theory states that the electron pairs surrounding an atom
tend to repel each other and the shape of the molecule is the result of
this electron pair repulsion. This model is referred to valence shell
electron pair repulsion (VSEPR)
VSEPR theory uses the term molecular geometry, or molecular
shape, to indicate the arrangement of atoms around the central atom as
a result of electron pair repulsion. Bond angle is the angle formed by
any two atoms bonded to the central atom.
TETRAHEDRAL MOLECULES
According to VSEPR, any element with four electron pairs
around the central atom has tetrahedral electron pair geometry. The
result of a molecule of CH4, the central C atom is surrounded by four
electron pairs that are repelled to the four corners of s 3- dimensional
figure called tetrahedron. The molecular shape of CH4 is tetrahedral,
the bond angle is 109.50
TRIAGONAL PYRAMIDAL MOLECULES
In a molecule of ammonia, the central N atom is surrounded by
three pairs of bonding electrons and one pair of nonbonding electron
pair. The electron pair geometry is tetrahedral and the bond angle
should have been 109.50 but experimentally, it was found to be 1070.
VSEPR explains the smaller bond angle by suggesting that the
nonbonding electron pair exerts a stronger repelling force than the
bonding pairs, thus, the H atoms are pushed closer together ( from
109.50 to 1070) The molecular shape formed is trigonal pyramidal.
BENT MOLECULES
Water molecule has the central O atom is surrounded by two
bonding electron pairs and two nonbonding electron pairs. The
predicted shape should be tetrahedral with 109.50, but experimentally,
it was found to be 104.50. The reason for this is again because the
nonbonding electron pairs exert a greater repelling force than the
bonding pairs. The resulting bond angle is smaller, allowing more
space for the unshared electron pairs. The molecular shape is said to
be bent of v-shaped.
LINEAR MOLECULES
In the molecule carbon dioxide, the central atom is bonded to
each oxygen atom by two bonding electron pairs; there is a double
bond to each O atom (O=C=O). Since the four electron pairs are on
opposite sides of the C, the three atoms lie in a straight line. The
electron pair geometry is linear and predicted bond angle is 1800.
SUMMARY OF VSEPR THEORY
Bonding/nonbonding Electron
Electron pair
pair
Geometry
4/0
Tetrahedral
3/1
Tetrahedral
2/2
4/0
3/0
Tetrahedral
Linear
Trigonal
planar
Molecular
shape
Bond
Angle
Example
Molecule
Tetrahedral
Trigonal
planar
Bent
Linear
Trigonal
planar
109.50
1070
CH4
NH3
104.50
1800
1200
H2O
CO2
CH2O
NONPOLAR MOLECULES WITH POLAR BONDS
Carbon tetrachloride is a nonpolar molecule even though it has polar
bonds because the attractive forces exerted by the four polar bonds cancel
each other. C-Cl is a polar bond, the four C-Cl bonds formed molecular
shape tetrahedral, indicating the four Cl atoms are located at the corners
of a tetrahedron.
Consequently, a molecule may contain polar bonds and yet be
nonpolar. In CO2, there are two polar bonds, while O=C=O is nonpolar
because the more electronegative oxygen atom pull equally in opposite
directions. On the other hand, formaldehyde is a polar molecule because
HC=O molecule has only one O atom. Oxygen atom pulls electrons away
from the central carbon atom and the two hydrogen atoms have little
effect on it. The molecular shape of formaldehyde is trigonal planar.