Chemistry · Ch 4 — The d- and f-Block Elements

Electronic Configurations of the d-Block Elements

4.2

Electronic Configurations of the d-Block Elements

The General Pattern

The outer orbitals of d-block elements generally follow the configuration

(n-1)d^{1-10}\,ns^{1-2}

Here $(n-1)$ denotes the inner d sub-shell — the penultimate shell — which can hold anywhere from one to ten electrons, while the outermost $ns$ orbital carries one or two electrons. Palladium is the one clear exception to this general pattern: its ground-state configuration is $4d^{10}5s^{0}$ , with the outer $s$ orbital left empty.

Why the Pattern Has Exceptions

The rule above is a generalisation, and it breaks down in a few places because the energy difference between the $(n-1)d$ and $ns$ orbitals is very small. When that gap is small, an extra factor takes over: half-filled and completely filled sets of orbitals are relatively more stable than partially filled ones. This stability preference is visible in two elements of the 3d series:

Note

Chromium adopts $3d^5 4s^1$ instead of the "expected" $3d^4 4s^2$ , because a half-filled 3d sub-shell (five electrons, one in each orbital) is more stable, and the 3d–4s energy gap is small enough that an electron can be pulled out of 4s to achieve it.

Copper adopts $3d^{10}4s^1$ instead of the "expected" $3d^{9}4s^2$ , for the same reason applied to a completely filled 3d sub-shell.

The complete set of ground-state outer-orbital configurations across all four series is worked out element by element in the accompanying table.

Table 4.1Electronic Configurations of outer orbitals of the Transition Elements (ground state)

1st Series

	Sc	Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn
Z	21	22	23	24	25	26	27	28	29	30
4s	2	2	2	1	2	2	2	2	1	2
3d	1	2	3	5	5	6	7	8	10	10

(first, 3d, series of transition elements — the 4d and 5d series follow the same pattern with 5s/4d and 6s/5d orbitals)

2nd Series

	Y	Zr	Nb	Mo	Tc	Ru	Rh	Pd	Ag	Cd
Z	39	40	41	42	43	44	45	46	47	48
5s	2	2	1	1	1	1	1	0	1	2
4d	1	2	4	5	6	7	8	10	10	10

3rd Series

	La	Hf	Ta	W	Re	Os	Ir	Pt	Au	Hg
Z	57	72	73	74	75	76	77	78	79	80
6s	2	2	2	2	2	2	2	1	1	2
5d	1	2	3	4	5	6	7	9	10	10

4th Series

| | Ac | Rf | Db | Sg | Bh | Hs | Mt | Ds | Rg | Cn | …

Zn, Cd, Hg, and Cn Fall Outside the Definition

Zinc, cadmium, mercury, and copernicium share the general outer configuration $(n-1)d^{10}ns^2$ . In each of these elements the d-orbitals are completely filled, not just in the ground state but also in the oxidation states these elements commonly show. Because a genuinely transition element is defined by incompletely filled d-orbitals (in the atom or in a commonly formed ion), this group of four elements is therefore not counted among the transition elements even though they sit in the d-block.

Why Partly Filled d Orbitals Matter

The d-orbitals of transition elements extend further towards the periphery of the atom than the s and p orbitals do. Because of this, they are more strongly influenced by the atom's surroundings, and in turn they more strongly influence the atoms or molecules around them. Ions that share the same $d^n$ configuration (for $n = 1$ to $9$ ) tend to show broadly similar magnetic and electronic behaviour, regardless of which element they come from.

This is the underlying reason the transition elements display several characteristic properties that set them apart:

A variety of oxidation states
Formation of coloured ions
Formation of complex compounds with a range of ligands

Transition metals and their compounds are also known for catalytic activity and paramagnetic behaviour. Each of these characteristics is examined in detail later in this unit. …

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