Page 95 - Theory and Problems of BEGINNING CHEMISTRY

84 CHEMICAL BONDING [CHAP. 5

5.55. Draw an electron dot diagram for NH 4 HS.
Ans. +
H
–
H N H H S
H

5.56. Phosphorus forms two covalent compounds with chlorine, PCl 3 and PCl 5 . Discuss these compounds in terms of the
octet rule.
Ans. PCl 3 obeys the octet rule; PCl 5 does not. PCl 5 has to bond five chlorine atoms around the phosphorus atom,
each with a pair of electrons, for a total of 10 electrons around phosphorus.

5.57. Draw an electron dot diagram for NO. Explain why it cannot follow the octet rule.
Ans. N O

There are an odd number of electrons in NO; there is no way that there can be eight around each atom.
5.58. Distinguish between each of the following pairs: (a) an ion and a free atom; (b) an ion and an ionic bond; (c)a
covalent bond and an ionic bond; (d) a triple bond and three single bonds on the same atom; and (e) a polyatomic
molecule and a polyatomic ion.
Ans. (a) An ion is charged, and a free atom is uncharged.
(b) An ion is a charged atom or group of atoms; an ionic bond is the attraction between ions.
(c) A covalent bond involves sharing of electrons; an ionic bond involves electron transfer and as a result
the formation of ions.
(d) Although both involve three pairs of electrons, the triple bond has all three pairs of electrons between
two atoms, and three single bonds have each pair of electrons between different pairs of atoms.
N N H N H
H
Triple bond Three single bonds
(e) Both have more than one atom. The polyatomic ion is charged and is only part of a compound; the
polyatomic molecule is uncharged and represents a complete compound.

5.59. Write the formula for the compound formed by the combination of each of the following pairs of elements. State
whether the compound is ionic or covalent. (a) Mg and Br, (b) Si and F, (c) Ca and O, and (d) Br and Cl.
Ans. (a) MgBr 2 ionic (c) CaO ionic
(b) SiF 4 covalent (d) BrCl covalent
5.60. BF 3 and PF 5 are non-octet-rule compounds. Draw an electron dot diagram for each.

Ans. F B F F F
P
F
F F
F
(Fluorine does not
form double bonds.)
5.61. How many electrons remain in the valence shell after the loss of electrons by the neutral atom to form each of the
following ions: (a)Pb 2+ and (b)Pb ?
4+
Ans. (a) The Pb 2+ still has two electrons in its sixth shell. (b) The Pb 4+ has no electrons left in that shell.

5.62. Complete the following table by writing the formula of the compound formed by the cation at the left and the anion
at the top.

− − 2−
ClO 2 ClO 3 C 2 O 4
Fe 2+
Cr 3+