Download Atomic Size

40
Atomic Size
(What Size Are Atoms?)
Model 1: Atomic Properties and the Periodic Table.
Ionization energies increase as the core charge
increases across a row (period) of the periodic table.
Ionization energies increase when the core
charge remains the same and the valence
electrons are in a shell closer to the nucleus
(that is, up a column (group) of the periodic table).
We have seen that there are trends in a physical property, IE1, related to the position
of the elements in the periodic table. Many other physical and chemical properties of the
elements in a particular group, such as the alkali metals or the halogens, are relatively
similar. This suggests that many properties of an atom are related to the number of
valence electrons present. The nuclear charge (and core charge) can also be important in
determining atomic characteristics because this determines the strength of attraction
between the nucleus and the valence electrons. This was the basis for our previous
analysis of first ionization energies.
One measure of the size of an atom (or ion) is the covalent radius of the atom. This
can be thought of as the radius of the outermost shell of an atom or ion.
Element
Boron
Carbon
Oxygen
Sulfur
Arsenic
Selenium
Shell
(n)
Valence
Core
Charse
Radius
(pm)
2
+3
89
2
+4
+6
+6
+5
+6
't7
2
J
4
4
66
104
I2I
t17
f:
t
Critical Thinking Questions
'L.
1.
What is the relationship between the valence shell of each atom in Table 1 and its
position in the periodic table?
i:
*
g.i
ChemActivity
6
Atomic
Size
41
2.
Why does the core charge increase as one moves from left to right across a period
in the periodic table-for example, from boron to carbon to oxygen?
3.
What trend in atomic radius is observed as one moves from left to right across a
period? Explain why this trend exists.
4.
What trend in atomic radius is observed as one moves down a group in the periodic
table? Explain why this trend exists.
5.
Estimate the radii of three atoms not listed in Table 1, based on the data presented.
Explain how you are able to estimate these values from the data given.
.
Table
2.
Ion
lonic radii of various isoelectronic ions.
Valence
(n)
sz-
5
cl-
a
J
K+
J
c*+
a
J
Shell
Core
Charse
Radius
(pm)
+6
+7
+9
+10
184
181
133
104
Chemical species that have identical numbers of electrons are isoelectronic.
6.
How many electrons do the ions in Table 2have?
7.
Provide a shell model diagram for K+ showing all electrons explicitly and then
show how the core charse for K+ was calculated.
8.
What is the basis for the trend in ionic radii seen in Table 2?
9.
Predict which is larger: the 02- ion or the F- ion. Explain your reasoning.
ChemActivity
42
Model
Table
6
Atomic Size
2: The Effect of Additional Electrons on Size.
3.
Atom or
Ion
F
F-
o
92-
Atomic radii of various atoms and ions.
Shell
(n)
Valence
Core
2+77
2+78
2+66
2+68
Charge
Number of
Valence Electrons
Radius (pm)
64
133
66
140
Electrons repel electrons. Therefore, as extra electrons are added to the valence shell
of an atom (or ion), the radius of the atom (or ion) increases. Similarly, as electrons are
removed from an atom (or ion), the radius of the atom (or ion) decreases.
Critical Thinking Questions
10.
Provide a shell model diagram for F- showing all electrons explicitly. Then show
how the core charge of F- was calculated (Table 3).
11. F and F- have identical
core charges and the valence shell is
Explain why the radius of F- is larger than the radius of F.
12.
n:2 in both cases.
Predict the radius of N3-
13. What are three characteristics of an atom (or ion) which
determining its relative radius?
must be considered in
ChemActivity
6
Atomic Size
Exercises
l.
Based on the data in Tables 1 and 2, estimate the radius of each of the following
species. Explain your reasoning.
Fd) Ne
a) Ar
b)N
2.
c)
Indicate whether each of the following statements is true or false and explain your
reasoning.
a)
b)
c)
d)
e)
J.
The
The
The
The
The
first ionization energy of Ba is expected to be larger than that of Mg.
Na+ ion is expected to have a larger radius than a Ne atom.
radius of Cl- is expected to be larger than the radius of Ar.
radius of Ar is expected to be larger than that of Ne.
first ionization energy of Ar is.expected to be greater than that of C*+.
Which is the smaller species in each of the following groupings?
a)
b)
c)
d)
e)
N,}\I3K,K*
Cl , ClH, HMg , Mg2*
4.
Which is larger- Fe2+ or Fe3+
5.
Which is the largest species in each of the following groupings?
a)
b)
c)
d)
e)
0
, Plc2+ , Pb4+
Mg,Al , Na
Mg2+ , C** , B**
H, H-,H*
Na, Cl , Br, I,
p3-, 52-, Cl-, F-.
?
Pb
,
Rb
Problems
l.
Which of the following elements should have the largest second ionization energy?
Na; Mg; Al; Si; P. Explain.
2.
Mg atoms are larger than S atoms. Explain why Mg2+ ions are smaller than 52ions.