CHEMISTRY 4
CHAPTER 12
CHEMICAL BONDING
1 MONATOMIC IONS WITH NOBLE GAS ELECTRON CONFIGURATION
Noble gas electron configurations are generally the most stable.
Metal atoms can achieve a noble gas electron configuration by emptying their valence
orbitals.
Nonmetals can achieve the electron configuration of the next noble gas by receiving more
electrons on their emptied valence orbitals.
Hydrogen atom can achieve the electron configuration 1s2 of helium by receiving one
electron on its orbital 1s and thus becomes an anion H- .
Lithium atom, which has electron configuration 1s22s1 can achieve the electron
configuration of helium by losing one electron on orbital 2s, and thus becomes a cation
Li+.
Oxygen atom, whose electron configuration is 1s22s22p4, can achieve the electron
configuration 1s22s22p6of neon by receiving two more electrons on its 2p orbitals, and
becomes ion O2-.
Fluorine atom, whose electron configuration is 1s22s22p5 can achieve the electron
configuration of neon by accepting one electron on its 2p orbital, and becomes anion F-.
Sodium atom, whose electron configuration is 1s22s22p63s1, can achieve the electron
configuration of neon by giving away one electron on its 3s orbital, and becomes cation
Na+.
Magnesium atom whose electron configuration is 1s22s22p63s2, can achieve the electron
configuration of neon by giving away two electrons on its 3s orbital, and becomes cation
Mg2+.
Chlorine atom whose electron configuration is 1s22s22p63s23p5, can achieve the electron
configuration of argon by receiving one electron on its 3p orbital, and becomes ion Cl-.
Potassium atom (1s22s22p63s23p64s1) can achieve the electron configuration of argon by
losing one electron on its 4s orbital, and becomes ion K+.
Calcium atom (1s22s22p63s23p64s2) can achieve the electron configuration of argon by
losing two electrons on its 4s orbital, and becomes ion Ca2+.
There are some exceptions to the rules discussed here. For example tin forms both Sn2+
and Sn4+ , lead forms both Pb2+ and Pb4+ .
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2 IONIC BONDS
Ionic compounds are compounds made up of ions. The electrostatic forces that hold the
ions in fixed position in the crystal are called ionic bonds .Ionic bonds are very strong,
that is why nearly all ionic compounds have high temperature of fusion. Solid ionic
compounds are poor conductors of electricity, because the ions are locked in place in the
crystal. Only when ionic compounds are melted or dissolved that the ions are free to
move and able to carry electric current.
3 COVALENT BONDS
Hydrogen molecule H2. In the hydrogen molecule, the electrons reside primarily in the
space between the two nuclei. The electron cloud or charge density is concentrated in the
region between the nuclei. The simultaneous attraction of each electron by the two
protons generates a force that pulls the protons toward each other and balances the
repulsive force between protons and repulsive force between electrons.
The type of bonding in which electrons are shared by nuclei is called covalent bond.
Using Lewis symbols, the formation of hydrogen molecule can be represented as:
H
+
H ---->
H : H or H-H
The two dots or the straight line drawn between the two atoms represent the covalent
bond. By sharing electrons, each hydrogen atom in the molecule has two electrons , that
is each hydrogen atom has a filled valence shell. The pair of electrons shared by two
atoms is called bonding pair.
Fluorine molecule F2. Fluorine atom has the following electron configuration :
1s22s22p5
When two fluorine atoms form a molecule, they share their unpaired electrons on orbitals
2p, so that each atom has eight electrons (octet rule).
F.
+
.F
---->
F:F
or
F-F
or
F-F
Unshared electron pair are often called lone pairs.
Hydrogen fluoride. The formation of the covalent bond between one hydrogen atom and
one fluorine atom to form a HF molecule can be represented as:
H.
+
.F
--->
H:F
or H-F
or H-F
The hydrogen atom in HF has two electrons, the same as an atom of helium. The fluorine
atom has eight electrons, the same as the noble gas neon atom.
4 POLAR AND NONPOLAR COVALENT BONDS
A bond in which the distribution of bonding electron charge is symmetrical, or centered,
is said to be a nonpolar covalent bond. A bond between identical atoms, as in hydrogen
molecules or in fluorine molecules is always nonpolar.
A bond with an unsymmetrical distribution of bonding electron charge is a polar covalent
bond. In the hydrogen fluoride molecule HF , the fluorine atom has a stronger attraction
2
for the shared electrons than the hydrogen atom. The result is that the HF molecule has
the following charge distribution:
H F
The electronegativity of an element is the ability of its atom in a molecule to attract
shared electrons to itself The higher the electronegativity, the stronger is the attraction
of the atom for bonding electrons. The electronegativity is highest at the upper right
region of the periodic table and lowest at the lower left region.
The polarity of the bond increases as the difference in electronegativity increases. For
example the following variation in bond polarity is expected
H-H < S-H < Cl-H < O-H < F-H
Electronegativity difference
0
0.4
0.9
1.4
1.9
5 MULTIPLE BONDS
Covalent bond formed by the sharing of one pair of electrons is called single bond.
When two atoms are bonded by two pairs of electrons , it is called a double bond.
When two atoms are bonded by three pair of electrons, it is triple bond.
All four electrons in a double bond and all six electrons in a triple bond are counted as
valence electrons for each of the bounded atoms.
Triple bond in nitrogen can be represented as
N
+
N --->
N
N
or
N
N
The strong triple bond in N2 is one of the reason why elemental nitrogen is so stable and
unreactive.
6 ATOMS THAT ARE BONDED TO TWO OR MORE OTHER ATOMS
Water molecule. An oxygen atom which has two unpaired electrons, can form two bond
with two hydrogen atoms.
H
+
O
H
O
+
H
H
or
H
O
H
Ammonia molecule. A nitrogen atom has five valence electrons, three of which are
unpaired. Therefore it can form bonds with three hydrogen atoms to produce a molecule
of ammonia NH3
3
Methane molecule.
A carbon has four valence electrons, only two of which are
unpaired, but it can form four bond with hydrogen atoms to produce a methane molecule
CH4. Carbon in carbon dioxide is double bonded to two oxygen atoms. Ethylene C2H4
has a double bond between the two carbon atoms. There is a triple bond between carbon
atoms in acetylene C2H2.
7 EXCEPTIONS TO THE OCTET RULE
There are some exceptions to the octet rule.
Odd-electron molecules: NO and NO2 have odd number of electron
N
N
O
O
O
Central atom with fewer than 4 electron pairs.
Boron, for example , tends to form compounds in which the boron atom has fewer that
eight electrons. In boron trifluoride , boron has only six electrons around it.
Beryllium in beryllium fluoride has only 2 electron pairs
F
B
F
Be
F
F
F
Central atom with more than 4 electron pairs.
Some atoms like sulfur atoms, phosphorus atoms, exceed the octet rule. In SF6, sulfur
atom has 12 electrons around it. In PF5, phosphorus atom has ten electrons around it.
The Lewis diagram for oxygen molecule with double bond can not explain why oxygen is
paramagnetic (To be paramagnetic O2 must have unpaired electrons)
4
F
F
P
F
F
F
F
F
S
F
F
F
F
8 METALLIC BOND
Attractive force between positively-charged metal ions in a crystal and the negativelycharged electrons that move among them.
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