Transition elements electron shells

Scandium is the first element in the fourth period of the transition elements, which means that the number of protons in their nuclei increases across the period. As with all the transition elements, electrons in scandium are added to an incomplete inner shell rather than to the outer valence shell as with most other elements. This characteristic of using electrons in an inner shell results in the number of valence electrons being similar for these transition elements although the transition elements may have different oxidation states. This is also why all the transition elements exhibit similar chemical activity. 

Such similarities do not hold in low oxidation states, where frequently the halides of the main group elements are monomeric species and those of the transition elements are halide bridged polymers. This divergence in bond type in lower oxidation states is connected with the non-bonding electrons, which, for the main group elements, are largely central-atom valence shell s or , and for the transition elements valence-shell d electrons. 

Naiiow-line uv—vis spectia of free atoms, corresponding to transitions ia the outer electron shells, have long been employed for elemental analysis usiag both atomic absorption (AAS) and emission (AES) spectroscopy (159,160). Atomic spectroscopy is sensitive but destmctive, requiring vaporization and decomposition of the sample iato its constituent elements. Some of these techniques are compared, together with mass spectrometry, ia Table 4 (161,162). 

Metals and alloys, the principal industrial metalhc catalysts, are found in periodic group TII, which are transition elements with almost-completed 3d, 4d, and 5d electronic orbits. According to theory, electrons from adsorbed molecules can fill the vacancies in the incomplete shells and thus make a chemical bond. What happens subsequently depends on the operating conditions. Platinum, palladium, and nickel form both hydrides and oxides they are effective in hydrogenation (vegetable oils) and oxidation (ammonia or sulfur dioxide). Alloys do not always have catalytic properties intermediate between those of the component metals, since the surface condition may be different from the bulk and catalysis is a function of the surface condition. Addition of some rhenium to Pt/AlgO permits the use of lower temperatures and slows the deactivation rate. The mechanism of catalysis by alloys is still controversial in many instances. 

In this discussion of the transition elements we have considered only the orbitals (n— )d ns np. It seems probable that in some metals use is made also of the nd orbitals in bond formation. In gray tin, with the diamond structure, the four orbitals 5s5p3 are used with four outer electrons in the formation of tetrahedral bonds, the 4d shell being filled with ten electrons. The structure of white tin, in which each atom has six nearest neighbors (four at 3.016A and two at 3.17.5A), becomes reasonable if it is assumed that one of the 4d electrons is promoted to the 5d shell, and that six bonds are formed with use of the orbitals 4dSs5p35d. 

As the atomic number increases, so does the positive charge of the nucleus, and the electrons are bound with a higher energy. However, this increase is not linear. For example, the electrons in the d orbital of the third shell have a higher energy than those in the s orbital of the fourth shell, and hence the latter are filled first. The consequence is the unexpected behavior of the first ten transition elements. In the case of the actinides and lanthanides, even more inner orbitals are occupied. Nature is not so simple, but the scheme should help to visualize this complex structure. And if one can assign the electrons of an element, one is a step closer to successfully unraveling the mysteries of the Periodic Table. 

The elements may be divided into types (Fig. 17-10), according to the position of the last electron added to those present in the preceding element. In the first type, the last electron added enters the valence shell. These elements are called the main group elements. In the second type, the last electron enters a d subshell in the next to last shell. These elements are the transition elements. The third type... 

There are several forms in which the elements of the periodic chart may be arranged. The version shown here is one of the forms now in widespread use. Groups I, II, III, etc., and the noble gases are called the Main Group Elements. All of their inner shells are fully occupied with electrons. The other elements are called the Transition Elements. They all have at least one inner shell that is only partially filled with electrons. Referring to the entire table, the numbers written above the symbols of the elements (always whole numbers) are the atomic numbers of the elements, and the numbers written below the symbols of the elements (not necessarily whole numbers) are the atomic weights of the elements. Parentheses indicate insufficient information exists or material is not yet official. 

All the transition elements have large atomic numbers, the smallest (scandium) having 21 protons. All of them have at least four energy shells holding their electrons. Scientists do not know exactly why, but the transition element atoms often put electrons into a new, outer energy shell when some inner shells are not quite full. All the transition elements have one or two electrons in their outer shells, but from element to element each new electron is added to a shell deep inside the electron cloud. 

The number of electrons in the outer shell of each atom determines most of an element s chemical properties. Since the outer shell is not changing, many transition elements are similar to one another. They are all metals. Some are the familiar metals of cars and coins and rocket ships. Others are rare and seldom used. This large group contains over half the total number of elements. 

The Group II elements each have two electrons in their outer energy shells, and the larger ones have an empty shell deep inside them. The transition series added electrons to the inner shell until it was completely filled. So, these last three transition elements are like Group II in construction, with the inner shell filled instead of empty, and two electrons in the outer shell. Those elements with filled shells and those with empty ones are the most stable. 

When an electron is added to a main group element to create the element of next highest atomic number, this electron is added to the outer shell of the atom, far from the nucleus. Thus, it has a major influence on the size of the atom. However, when an electron is added to a transition metal atom to create the atom of next highest atomic number, it is added to the electronic shell inside the outermost. The electron thus has been added to a position close to the nucleus to which it is attracted quite strongly and thus it has small effect on the size of the atom. 

The principal characteristic of the transition elements is an incomplete electronic subshell that confers specific properties on the metal concerned. Ligand systems may participate in coordination not only by electron donation to the 3d levels in the first transition series but also by donation to incomplete outer 4s and 4p shells. Figure 5.1 shows that the differences in orbital energy levels between the 4s, 4p and 3d orbitals are much smaller than, for example, the difference between the inner 2s and 2p levels. Consequently, transitions between the 4s, 4p and 3d levels can easily take place and coordination is readily achieved. The manner in which ligand groups are oriented in surrounding the central metal atom is determined by the number and energy levels of the electrons in the incomplete subshells. 

Bond energy variations over the periodic table will be subject to perturbations which reflect the underlying atomic configurations. Compounds derived from main-group elements of Period 4, for example, will show discontinuities in properties from those of Period 3 because of the extra d-electron shell. Conversely, the insertion of an f-electron shell brings together the properties of the second and third transition series, especially in the earlier groups. 

Some post-transition elements (or the corresponding radicals) containing 3 or more electrons in their valence shell are able to assist the formation of clusters by bonding to several metal atoms. Typical examples of this behaviour are the extraordinarily easy syntheses of large series of compounds such as Co3 (CO)9 (p3-E) (E = Al, CR, CX, GeR, P, As, PS, S, Se, PR, SR) 201 209) and Fe3 (CO)9 (p3 -E)2 (E = S, Se, Te, NR, PR). This type of stabilization is usually found in trinuclear clusters although a few examples in tetranuclear clusters are known, for instance ... 

The orbitals of the d states in clusters of the 3d, 4d, and 5d transition elements (or in the bulk metals) are fairly localized on the atoms as compared with the sp valence states of comparable energy. Consequently, the d states are not much perturbed by the cluster potential, and the d orbitals of one atom do not strongly overlap with the d orbitals of other atoms. Intraatomic d-d correlations tend to give a fixed integral number of d electrons in each atomic d-shell. However, the small interatomic d-d overlap terms and s-d hybridization induce intraatomic charge fluctuations in each d shell. In fact, a d orbital contribution to the conductivity of the metals and to the low temperature electronic specific heat is obtained only by starting with an extended description of the d electrons.7... 

Zinc, cadmium and mercury are at the end of the transition series and have electron configurations ndw(n + l)s2 with filled d shells. They do not form any compound in which the d shell is other than full (unlike the metals Cu, Ag and Au of the preceding group) these metals therefore do not show the variable valence which is one of the characteristics of the transition metals. In this respect these metals are regarded as non-transition elements. They show, however, some resemblance to the d-metals for instance in their ability to form complexes (with NH3, amines, cyanide, halide ions, etc.). 

Transition metah—found in the groups located in the center of the periodic table, plus the lanthanide and actinide series. They are all solids, except mercury, and are the only elements whose shells other than their outer shells give up or share electrons in chemical reactions. Transition metals include the 38 elements from groups 3 through 12. They exhibit several oxidation states (oxidation numbers) and various levels of electronegativity, depending on their size and valence. 

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