Chemistry for Pharmacy Students : General, Organic and Natural ...

22 CH2 ATOMIC STRUCTURE AND BONDING
stable if they have a filled valence shell of electrons. Atoms transfer or share
electrons in such a way that they can attain a filled shell of electrons. This
stable configuration of electrons is called an octet. Except for hydrogen and
helium, a filled valence shell contains eight electrons.
Lewis structures help us to track the valence electrons and predict the
types of bond. The number of valence electrons present in each of the
elements is to be considered first. The number of valence electrons
determines the number of electrons needed to complete the octet of eight
electrons. Simple ions are atoms that have gained or lost electrons to satisfy
the octet rule. However, not all compounds follow the octet rule.
Elements in organic compounds are joined by covalent bonds, a sharing
of electrons, and each element contributes one electron to the bond. The
number of electrons necessary to complete the octet determines the number
of electrons that must be contributed and shared by a different element in a
bond. This analysis finally determines the number of bonds that each
element may enter into with other elements. In a single bond two atoms
share one pair of electrons and form a s bond. In a double bond they share
two pairs of electrons and form a s bond and a p bond. In a triple bond two
atoms share three pairs of electrons and form a s bond and two p bonds.
Sodium (Na) loses a single electron from its 3s orbital to attain a more
stable neon gas configuration (1s 2 2s 2 2p 6 ) with no electron in the outer
shell. An atom having a filled valence shell is said to have a closed shell
configuration. The total number of electrons in the valence shell of each
atom can be determined from its group number in the periodic table. The
shared electrons are called the bonding electrons and may be represented by
a line or lines between two atoms. The valence electrons that are not being
shared are the nonbonding electrons or lone pair electrons, and they are
shown in the Lewis structure by dots around the symbol of the atom. A
species that has an unpaired electron are called radicals. Usually they are
very reactive, and are believed to play significant roles in aging, cancer and
many other ailments.
In neutral organic compounds, C forms four bonds, N forms three bonds
(and a lone pair), O forms two bonds (and two lone pairs) and H forms one
bond. The number of bonds an atom normally forms is called the valence.
Lewis structure shows the connectivity between atoms in a molecule by a
number of dots equal to the number of electrons in the outer shell of an atom of
that molecule. A pair of electrons is represented by two dots, or a dash. When
drawing Lewis structures, it is essential to keep track of the number of electrons
available to form bonds and the location of the electrons. The number of
valence electrons of an atom can be obtained from the periodic table because it
is equal to the group number of the atom. For example, hydrogen (H) in Group
1A has one valence electron, carbon (C) in Group 4A has four valence
electrons, and fluorine (F) in Group 7A has seven valence electrons.