SCH 102
Chemical Bonding
Dr. Solomon Derese
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Review of Chemical Bonding
SCH 102
To understand organic chemistry, it is necessary to
have some understanding of the chemical bond—the
forces that hold atoms together within molecules.
Bonding is the joining of two atoms in a stable
arrangement.
Bonding may occur between atoms of the same or
different elements.
Bonding is a favorable process because it always leads
to lowered energy and increased stability.
Dr. Solomon Derese
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One general rule governs the bonding process:
SCH 102
Through bonding, atoms attain a complete
outer shell of valence electrons.
Alternatively, because the noble gases in column 8A
of the periodic table are especially stable as atoms
having a filled shell of valence electrons, the general
rule can be restated as:
Through bonding, atoms attain a stable noble
gas configuration of electrons.
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SCH 102
A first-row element like hydrogen can accommodate
two electrons around it. This would make it like the
noble gas helium at the end of the same row.
Second-row element, such as C, N, O and F are most
stable with eight valence electrons around it like
neon.
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SCH 102
Elements that behave in this manner are said to
follow the octet rule.
There are two different kinds of bonding:
1. Ionic bonding and
2. Covalent bonding.
Ionic bonds result from the transfer of electrons from
one element to another.
Covalent bonds result from the sharing of electrons
between two nuclei.
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SCH 102
A chemical bond is the net result of attractive and
repulsive electrostatic forces. When bringing together
two atoms that are initially very far apart. Three types
of interaction occur, one attractive and two repulsive
force.
The nucleus-electron attractions are greater than the
nucleus-nucleus and electron-electron repulsions,
resulting in a net attractive force that holds the atoms
together in a molecule.
Dr. Solomon Derese
SCH 102
The type of bonding is determined by the location of
an element in the periodic table.
An ionic bond generally occurs when elements on the
far left side of the periodic table combine with
elements on the far right side, ignoring the noble
gases, which form bonds only rarely.
The resulting ions are held together by extremely
strong electrostatic interactions.
A positively charged cation formed from the element
on the left side attracts a negatively charged anion
formed from the element on the right side.
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SCH 102
Sodium chloride, NaCl, is an example of an ionic compound
Eight valence
Electrons (Like Ne)
One valence
electron
Eight valence
Electrons (Like Ar)
Ionic bond
Seven valence
electrons
Dr. Solomon Derese
Electrostatic attractions between Na+
and Cl- led to the formation of NaCl.
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• The element sodium, located in group 1A of the
periodic table, has just one valence electron in its
third shell. If this electron is lost, sodium forms the
cation Na+ having ten electrons in the second shell
with an electronic configuration like the noble gas
Neon.
• The element chlorine, located in group 7A of the
periodic table, has seven valence electrons. By
gaining one it forms the anion Cl–, which has a filled
valence shell (an octet of electrons), like Argon.
• Thus, Sodium chloride is a stable ionic compound.
Dr. Solomon Derese
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Dr. Solomon Derese
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SCH 102
The second type of bonding, covalent bonding, occurs
with elements like carbon in the middle of the
periodic table, which would otherwise have to gain or
lose several electrons to form an ion with a complete
valence shell.
Carbon is central to the study of organic chemistry
and has four electrons in its valence shell.
It can attain the electronic configurations of neon by
using each of these electrons, along with four
electrons from other atoms, to form covalent bonds.
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SCH 102
The simplest compound of carbon and hydrogen
results from the combination of four hydrogen atoms
with one carbon to produce methane, CH4.
The number of covalent bonds an atom forms
depends on how many additional valence electrons it
needs to reach a noble-gas configuration.
Hydrogen has one valence electron (1s) and needs
one more to reach the helium configuration (1s2), so it
forms one bond.
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Carbon has four valence electrons (2s2 2p2) and needs
four more to reach the neon configuration (2s2 2p6),
so it forms four bonds.
Nitrogen has five valence electrons (2s2 2p3), needs
three more, and forms three bonds; oxygen has six
valence electrons (2s2 2p4), needs two more, and
forms two bonds; and the halogens have seven
valence electrons, need one more, and form one
bond.
Notice that when second-row elements form fewer
than four bonds their octets consist of both bonding
(shared) electrons and nonbonding (unshared)
electrons, lone pairs.
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SCH 102
Number of bonds and lone pairs for H, C, N. O and the
halogens.
Two lone pairs
One lone pair
One
bond
Dr. Solomon Derese
Four
bonds
Three
bonds
Three lone pairs
Two
bonds
One
bond
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