Ionization of Transition and Inner Transition Elements
On the graph of ionizationA process in which an atom, molecule, or negative ion loses an electron; a process in which a covalent molecule reacts with a solvent to form positive and negative ions; for example, a weak acid reacting with water to form its conjugate base (an anion) and a hydrogen (hydronium) ion. energies, it can be seen that ionization energies increase much more slowly across the transition and inner transition elements than for the representative elements. For example, the ionization energyThe quantity of energy required to remove an electron from a neutral atom or molecule or from a positive ion. of the representative-element boron is 800 kJ mol–1. Five elements later we find neon, whose ionization energy is 2080 kJ mol–1, an increase of 160 percent.
* ElectronA negatively charged, sub-atomic particle with charge of 1.602 x 10-19 coulombs and mass of9.109 x 1023 kilograms; electrons have both wave and particle properties; electrons occupy most of the volume of an atom but represent only a tiny fraction of an atom's mass. affinities marked with an asterisk (*) have been obtained from theoretical calculations rather than experimental measurements. The heavy colored line separates metals (ionization energy usually below about 800 kJ mol–1) from nonmetals.
In the fourth period, the transition-element scandium has an ionization energy of 631 kJ mol–1. Five elements later we find iron at 759 kJ mol–1, an increase of only 20 percent. All the lanthanoids have ionization energies from 500 to 600 kJ mol–1, and the actinoids are all between 580 and 680 kJ mol–1. These similarities among the transition and especially the inner transition elements illustrate statements made about electron configurations and the periodic table. The distinguishing electron for a transition element enters a d subshell in the next-to-outermost shell, while for an inner transition element it usually enters an f subshell in the third-from-outermost shell. Thus the distinction between an element and the one preceding it in the periodic table is much smaller than among the representative elements. Furthermore, experimental measurements show that for transition and inner transition elements the electrons lost when ionization occurs are not the last ones which were added to build up the atomic electron configuration. Instead, electrons are usually removed first from the subshell having the largest principal quantum numberOne of a set of numbers that specifies the state of an electron in an atom; the set of quantum numbers summarize results from quantum mechanics..
EXAMPLE Determine the electron configuration of the Fe3+ ion.
SolutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture. Since the charge on the ion is +3, three electrons must have been removed from a neutral iron atom (Fe). The electron configuration of Fe is
Fe...1s22s22p63s23p63d64s2 or [Ar]3d64s2
We now remove electrons successively from subshells having the largest principal quantum number:
Fe+ [Ar]3d64s1 one 4s electron removed
Fe2+ [Ar]3d6 a second 4s electron removed
Fe3+ [Ar]3d5 since no electrons are left in the n = 4 shell, one 3d electron is removed
The behavior described in the previous paragraph and the example may be better understood by comparing the 3d and 4s shells, as in the following figure.
Electrons in the subshell having the largest principal quantum number (4s in the example and the second figure) are, on the average, farther from the nucleusThe collection of protons and neutrons at the center of an atom that contains nearly all of the atoms's mass., and they are first to be removed. The first ionization energy of iron is not much larger than that of scandium because in each case a 4s electron is being removed. The iron atom has five more protons in the nucleus, but it also has five more 3d electrons which spend most of their time between the nucleus and the 4s electrons. The screening effect of such 3d electrons causes the effective nuclear charge to increase very slowly from one transition element to the next. The attraction for 4s electrons, and hence ionization energy, also increases very slowly.
Macroscopic properties such as high thermal and electric conductivity, malleability, and ductility were mentioned in a brief introduction to the elements as characteristics of metals. In addition, most metals have low ionization energies, usually below 800 kJ mol–1. In other words, a metal consists of atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume., each of which has at least one loosely held electron. When such atoms pack close together in a solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. metal, the loosely held electrons are relatively free to move from one atom to another. If excess electrons are forced into one end of a metal wire, it is relatively easy for electrons to flow out of the other end. Thus an electric current may be carried through the wire, and the high conductivity of all metals may be understood.
More detailed microscopic interpretations of metallic properties are given for Metals later, but for the time being we are primarily interested in the location of metallic elements in the periodic table. Ionization energies are smallest near the bottom and on the left of the periodic table, and so this is where metals are found. Moreover, ionization energies increase slowly from one transition element to the next and hardly at all across the inner transition elements. Therefore all transition and inner transition elements are metals. In periodic groups IIIA, IVA, and VA elements near the top of the table have large ionization energies and little metallic character. Ionization energies decrease as one moves downward, however. For example, Al is quite metallic, although the element above it, B, is not. A heavy “stairstep” line is usually drawn on the periodic table to separates the nonmetals (above and to the right) from the metals. Elements such as B, Si, Ge, As, Sb, and Te, which are adjacent to the stairstep, have intermediateIn chemical kinetics, a species that is formed in an early step in a reaction mechanism and then consumed in a later step; evidence of existence of an intermediate may be important for the interpretation of a rate law. properties and are called semimetals. This same class is also referred to as metalloids.