Chemistry - A Molecular Science

Chapter 4 The Ionic Bond

ion by also losing its two 6s electrons. Tl

does not form a +2 ion because that would

require the loss of the 6s electrons prior to the loss of the 6p electron.

The charge on cations of Group 1A and Group

2A metals equals their group number.

The elements of Groups 3A and 3B form +3

ions, but In and Tl also form +1 ions by

emptying their valence p sublevel. Most transition metals form +2 ions, but several form +3 ions as well, and a few form +1 ions. Successive ionization energies* get very large because the effective nuclear charge experienced

by the remaining electrons gets high, so

monatomic cations with charges greater than +3 do not form. Similarly, adding electrons to an atom gets progressively more difficult as each electron reduces Z

and the eff

electronegativity of the atom. Consequently,

monatomic anions do not attain charges more

negative than -3. Thus, carbon, a small Group 4A element, does not form isolated ions because a charge of +4 or -4 would be too great. However, the heavier Group 4A elements, Sn and Pb, do form +2 ions by losing only their

p valence electrons.

* A

→

1+ A

+ e

1- - first ionization

A

1+^

→

2+ A

+ e

1- - second ionization

A

2+^

→

3+ A

+ e

1- - third ionization

Example 4.3

What is the charge on a zinc ion? What is

its electron configurat

ion? Is it expected

to form more than one ion? The 3d shell of zinc is full, so the only elec

trons that can be lost are the 4s. Consequently,

zinc forms only the Zn

2+

ion, which has an electron configuration of [Ar] 3d

10.

1A

2A

3A

+1

+2

+3 atom (cage)cation (sphere)

Li

Be

Na

Mg

Al

KC

a

Rb

Sr

Figure 4.2 Relative sizes of metal atoms and their cations The charge on each cation is give

n at the top of the column.

4.3

RELATIVE SIZES OF IONS

Like the size of an atom (Section 3.3), the si

ze of an ion is determined by the n quantum

number of the outermost electron and by the e

ffective nuclear charge experienced by the

outermost electrons. Remember that Dalton’s

billiard ball model is not an ideal way to

view atoms and ions because they are not hard spheres. When we refer to the size of an atom or an ion, we are referring to the size of an electron cloud that does not have a sharp boundary. Thus, the size of an atom or an

ion depends upon its environment and tables

often differ on the sizes. The

relative

sizes of some atoms and their ions are shown in

Figure 4.2. Ionic radii will be discussed in more detail in Section 8.7.

Cations are smaller than their parent atoms

(Figure 4.2). One of the factors

dictating the size of an atom or an ion is the n quantum number of the outermost electrons. In cations of most main group metals, the n

quantum number of the outermost electrons is

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