lowered by one because the outermost electrons
of the atom are lost in forming the ion.
This loss of the outermost shell makes the cation much smaller than the corresponding atom. In addition, the decrease in the number of electrons decreases the shielding experienced by the remaining electrons and increases Z
. The increased effective nuclear eff
charge contracts the outer shell even further.
Anions are larger than their parent atoms
(Figure 4.3)
. Anions have more electrons
than protons; thus, the valence electrons ar
e more effectively shielded from the positive
charge of the nucleus, which decreases Z
. This decrease in effective nuclear charge eff
causes the outer electron shell to expand. As
the negative charge increases, the effective
nuclear charge decreases and the size of
the anion increases. Consequently, N
3- is larger
than O
2-, which is larger than the F
1-.
Example 4.4
Arrange the following in order of increasing size: S
2-, Ar, and Ca
2+.
Ar, S
2- and Ca
2+ each have 18 electrons, so the screening is the same. Therefore,
increasing the number of protons increases Z
and decreases the size. eff
Ca
2+ (20 protons) < Ar (18 protons) < S
2- (16 protons).
Energy
H
®
1+H
Cl
Cl
®
1-
FF®
1-
Cl
Cl
®
1+
(a) HCl
(b) Cl-F
Figure 4.4 Oxidation states in H-Cl and Cl-F
Note that this order is different from that
of atomic size. Calcium atoms are the largest
because they have the highest n quantum num
ber and the smallest effective nuclear
charge. Sulfur is larger than argon because its
effective nuclear charge is less. Thus, the
order of atomic sizes is Ar < S < Ca.
4.4
OXIDATION STATES
The
oxidation state
(or
oxidation number
) of an atom is the charge it would have if its
bonds were ionic. In ionic compounds, the oxidation state of an ion is the charge on the ion. For example, the oxidation states of sodi
um and chlorine in NaCl are +1 and -1,
respectively. However, oxidation states are also used to account for the electrons in compounds that are not ionic (compounds with no metals). In these cases, the bonds are assumed to be ionic by
assigning
all bonding electrons to the more electronegative atom
(atom with lower energy orbitals). Compare
Figures 4.1 and 4.4a. In both, the electron is
transferred from the high-energy orbital into the low-energy orbital to produce ions. The difference is that the orbital energy of hydroge
n is much lower than that of sodium, so the
electron does not actually transfer in HCl;
we simply assume that it does to determine the
oxidation states
. If the transfer took place, the hydrog
en atom would become a +1 ion, so
(a) The valance orbitals of Cl are
lower than the electron in H, so
the bonding electrons would be assigned to Cl in HCl. Loss of an electron by H produces the H
1+ ion, or a +1 oxidation state. Gain of
an electron by Cl produces the Cl
1-, or a -1 oxidation state.
(b) The valence electrons of Cl are
higher than the valence orbitals
on F, so the bonding electrons would be assigned to F in ClF. Loss of an electron from Cl produces the Cl
1+ ion, or a +1 oxidation state.
Gain of an electron to F produces the F
1- ion, or a -1 oxidation state.
5A
6A
7A
-3
-2
-1
anion
(cage)
atom
(sphere)
NO
F
SC
l
Se
Br
Te
I
Figure 4.3 Relative sizes of nonmetal atoms and their anions The char
ge on the anion is
given at the top of each column.
Chapter 4 The Ionic Bond
© by
North
Carolina
State
University