Answers and Explanations

REVIEW QUESTIONS

1 . C

Refer   to  the first   paragraph   of  this    chapter.

2 .  Mg2+ < P   <   Mg  <   Sr

The trends  in  atomic  radii   are as  follows:    Going   from    left    to  right   across  a   period, the atomic

radii   decrease    because the atomic  number  increases.  The increasing  number  of  protons in  the

nucleus will    have    a   stronger    attraction  for the outermost   electrons,  causing them    to  be  held

closer  and more    tightly to  the nucleus.    Going   down    a   group,  the atomic  radius  will    increase

because there   are more    filled  principal   energy  levels  separating  the nucleus and the outermost

electrons,  shielding   the attractive  force   between them.   P   has a   small   radius  because it  lies    far

to  the right   and high    in  a   group.  The magnesium   species will    have    smaller radii   than    the

strontium   species because they    are higher  in  Group   II. Finally,    positive    ions    have    smaller

atomic  radii   than    the corresponding   neutral molecules,  because the loss    of  electrons   leads   to

a   decrease    in  electron-electron   repulsion   within  the atom,   which   in  turn    allows  the electrons

to  move    in  closer  to  the nucleus.    Therefore,  Mg2+    will    be  smaller than    Mg. Mg2+    has a   smaller

radius  than    P   because Mg2+ has    no  electrons   in  orbitals    of  the third   principal   energy  level.

3 . A

The least   electronegative elements    are located at  the bottom  left    of  the periodic    table.  Cesium

has the lowest  ionization  energy  and,    likewise,   it  is  the least   electronegative.    Note    that

Francium    (Fr)    would   be  lower   still   but is  not a   stable, naturally   occurring   element.

4 .  Ca3+,  Ca2+,   Ca+,    Ca, Ca–,    Ca2–

Positive    ions    will    have    smaller radii   than    the corresponding   neutral atoms,  and the greater the

positive    charge, the smaller the ionic   radius. Negative    ions    will    have    larger  radii   than    the