Page 16 - Pharmacy College Admission Test Review Book

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mentioned, an orbital is not occupied by a pair of A B A + B Heterolysis
electrons until other orbitals of equal energy are
each occupied by one electron; the rest of three 10. (d) In the covalent bond, two atoms are joined
electrons will be occupied by each 2px, 2py and 2pz by sharing electrons. Both nuclei are held by the
orbitals. same electron cloud. However, in most cases the
two nuclei do not share the electrons equally. This
6.(d) The amount of energy that is given off when a happens when one atom has more electron
bond is formed is called the bond dissociation withdrawing power than the other atom. At this
energy. time the electron cloud is denser on one atom. This
causes one end of the bond being relatively
7. (b) The mixing of one s orbital and one p orbital negative and the other end of the bond being
results in a hybrid orbital known as an sp orbital. relatively positive. Such bond is said to be a polar
o
The bond angle between any two orbitals is 180 . In bond, and possesses polarity.
the same way, the mixing of one s and two p
orbitals results in the new hybrid orbital known as The bond possesses polarity when joins atoms have
2
sp orbital. The bond angle between any two different tendency to attract electrons. This
o
orbitals is 120 . Finally, the mixing of one s orbital property of the atom is called electronegativity.
and three p orbitals resulting in the hybrid orbital Out of the given choices, fluorine (F) possesses the
3
known as a sp orbital. The bond angle between any highest electronegativity.
o
3
two orbital in any sp hybridized molecule is 109.5 .
F > O > Cl, N > Br > C, H
8.(a) The amount of energy consumed or liberated
when a bond is broken or formed is defined as the 11.(d) The polarity of molecules is calculated by
bond dissociation energy. Fluorine has powerful the following equation:
electronegativity, which holds the hydrogen more
tightly than the other given choices (e.g. Cl, Br, I). µ = e x d
Thus it forms a very strong bond that requires more
energy to break it. Where,

Molecule Energy (kcal/mole) µ = dipole movement in Debye
e = magnitude of the charge in e.s.u
HF 136 d = the distance between centers of charge in cm
H 2 104
HCl 103 Molecules like H 2, O 2, N 2, Cl 2 and Br 2 have zero
HBr 88 dipole movement and are nonpolar. The two
HI 71 identical atoms of each of these molecules have the
same electronegativity and share electrons equally.
9.(a) Generally, a molecule breaks into an atom
or a group of atoms. Thus, of the two electrons Methane (CH 4) and Carbon tetrachloride (CCl 4) have
making up the covalent bond, one goes to each zero dipole movements since tetrahedral
fragment; such bond breaking is defined as arrangements of these molecules exactly cancel
homolysis. each other’s dipole movements.

When two electrons making up the covalent bond
go to the same fragment, it is defined as heterolysis.

A B A + B Homolysis

124