Electronic configurations of the d-block elements

What are the electronic configurations of the d-block elements?

In plain words: across a d series, the ground-state configurations progressively fill the d subshell one shell in, with occasional redistribution between d and s occupations. Two things bend this neat pattern: the d and s levels are so close that an electron can move between them, and a d subshell that is exactly half full or full is extra stable.

General outer configuration (n − 1)d^¹⁻¹⁰ ns^¹⁻²

For the 3d series NCERT’s Table 4.1 gives, beyond the argon core (NCERT page 90):

• The heavier series follow the same pattern with more exceptions. In the 4d series niobium is 4d⁴ 5s¹, molybdenum 4d⁵ 5s¹, ruthenium 4d⁷ 5s¹, rhodium 4d⁸ 5s¹, palladium 4d¹⁰ and silver 4d¹⁰ 5s¹. In the 5d series platinum is 5d⁹ 6s¹ and gold 5d¹⁰ 6s¹, but tungsten is 5d⁴ 6s², unlike chromium and molybdenum above it (NCERT Table 4.1, page 91). The 5d elements after lanthanum, hafnium to mercury, also hold a full 4f¹⁴ inside (Wikipedia, “Electron configurations of the elements”).
• The 6d series beyond actinium is made of short-lived synthetic elements. From meitnerium on, their configurations are marked as predictions (Wikipedia, “Electron configurations of the elements”). NCERT’s entries agree with that list from rutherfordium to darmstadtium and for copernicium. For roentgenium NCERT gives 6d¹⁰ 7s¹, like gold; the predicted configuration is 6d⁹ 7s². In an exam, use NCERT’s.
• Ions lose the ns electrons first. When d-block elements form ions, ns electrons are lost before (n − 1)d electrons, so the usual ions of the first transition series with charge +2 or more have dⁿ configurations with no 4s electrons and an intact argon core (NCERT page 95). Iron, [Ar] 3d⁶ 4s², gives Fe²⁺, [Ar] 3d⁶, and Fe³⁺, [Ar] 3d⁵ (reasoned). Removing an electron also changes the relative energies of the 4s and 3d orbitals (NCERT page 95).
• Why the d electrons matter. NCERT: the d orbitals of the transition elements “protrude to the periphery of an atom more than the other orbitals (i.e., s and p)”, so they are more influenced by their surroundings and affect neighbouring atoms or molecules more, and in some respects ions of a given dⁿ configuration (n = 1–9) have similar magnetic and electronic properties (NCERT page 91); give that in an exam. Strictly, the same d count does not guarantee the same properties, since ligands, geometry and spin state matter (reasoned).
• NCERT slip (Table 4.1, page 91): the table lists technetium (Z = 43) as 4d⁶ 5s¹. Its ground state is [Kr] 4d⁵ 5s², confirmed by the NIST atomic level tables; the listed 4d^⁶5s^¹ configuration corresponds to excited levels. NCERT’s own Class 11 Chemistry, Table 2.6 (page 66), gives exactly this, and it matches manganese (3d⁵ 4s²) and rhenium (5d⁵ 6s²) in Table 4.1 itself (Wikipedia, “Electron configurations of the elements”). In an exam, write 4d⁵ 5s².

Configurations of chromium and copper

By the filling order alone, chromium would be 3d⁴ 4s² and copper 3d⁹ 4s². In fact chromium is 3d⁵ 4s¹ and copper is 3d¹⁰ 4s¹. The 3d and 4s energies are close enough for an electron to move into 3d, and half-filled and completely filled sets of orbitals are relatively more stable (NCERT page 90).

• NCERT slip (page 90): the sentence on chromium says the 3d–4s gap “is small enough to prevent electron entering the 3d orbitals”. The configuration it explains, 3d⁵ 4s¹, shows the opposite: the gap is small enough for one electron to go into 3d instead of pairing in 4s. NCERT Class 11 Chemistry (page 64) puts it the right way round: an electron shifts from 4s to 3d. Answer that way in an exam.

NCERT Class 11 gives the reasons for the extra stability (Class 11 configurations page):

• Symmetry. Half-filled and completely filled subshells have a symmetrical distribution of electrons, which leads to stability (NCERT Class 11 Chemistry, page 65).
• Exchange energy. NCERT: electrons with the same spin in the degenerate orbitals of a subshell “tend to exchange their positions and the energy released due to this exchange is called exchange energy”, and the number of exchanges is maximum when the subshell is half filled or completely filled (NCERT Class 11 Chemistry, page 65); give that account in an exam. NCERT leaves the details of exchange energy for “higher classes” (Class 11, page 65). Beyond the NCERT text: the exchange interaction is a quantum constraint on indistinguishable particles, whose wave function changes sign when two electrons are exchanged (Wikipedia, “Exchange interaction”), so it is not a process of electrons literally swapping places; the total energy also includes attraction, repulsion and pairing costs (reasoned). Five parallel d electrons give 4 + 3 + 2 + 1 = 10 same-spin pairs (NCERT Class 11 Chemistry, Figure 2.18, page 65); four allow only 3 + 2 + 1 = 6 (reasoned).

The tendency repeats down the groups but is not a law. Molybdenum, 4d⁵ 5s¹, follows chromium, and silver and gold, (n − 1)d¹⁰ ns¹, follow copper; tungsten, 5d⁴ 6s², does not follow chromium (NCERT Table 4.1, page 91).

Why zinc, cadmium and mercury are not transition elements

• Full d subshells throughout. Zinc, cadmium, mercury and copernicium have the outer configuration (n − 1)d¹⁰ ns². Their d orbitals are completely filled in the ground state and in their common oxidation states, so they are not regarded as transition elements (NCERT page 91). Zinc, for example, is 3d¹⁰ 4s², and Zn²⁺, which loses the 4s pair, is 3d¹⁰ (reasoned).
• Still studied with them. As the end members of the 3d, 4d and 5d series, their chemistry is studied along with that of the transition metals (NCERT page 89).
• Scandium, by contrast, has an incompletely filled 3d subshell, 3d¹, in its ground state, so it is a transition element (NCERT Example 4.1, page 91).
• The ions count too. The definition asks for an incomplete d subshell in the atom or in its ions (NCERT page 89). Silver’s atom has a full 4d¹⁰, but silver can show the +2 state, with an incomplete 4d subshell, so it is a transition element (NCERT Intext Question 4.1, answer on page 117). Copper in Cu²⁺, 3d⁹, is the same case (reasoned).

Solved examples

Common mistakes to avoid

Quick revision summary

• General outer configuration (n − 1)d^1–10 ns^1–2; palladium is 4d¹⁰ 5s⁰.
• Cr is 3d⁵ 4s¹ and Cu is 3d¹⁰ 4s¹: half-filled and full sets are extra stable (symmetry and exchange energy).
• Ions lose ns electrons first; ions of charge +2 or more have no 4s electrons.
• Zn, Cd, Hg and Cn are (n − 1)d¹⁰ ns², full in the atom and the common ions, so they are not transition elements; Ag is, through Ag(II).
• NCERT Table 4.1 lists Tc as 4d⁶ 5s¹; the ground state is 4d⁵ 5s².

Questions students ask

Why do transition metals lose their 4s electrons before their 3d electrons?

For the usual first-row transition-metal cations, the observed configurations lose 4s occupation before further 3d occupation. Orbital energies depend on the atom, charge and occupation, and the remaining electrons relax during ionisation; a single fixed 4s/3d energy ladder is not valid for every species. NCERT notes that removing an electron alters the relative energies of 4s and 3d (NCERT page 95). So 4s filling first, in potassium and calcium, does not clash with 4s emptying first: the filling order compares one element with the next, ionisation the electrons of a single atom (why 4s fills first).

Should 4s be written before or after 3d in an electronic configuration?

4s may be written before or after 3d, because an electronic configuration records only how many electrons each subshell holds: titanium can be written [Ar] 4s² 3d² or [Ar] 3d² 4s² (beyond the NCERT text; Wikipedia, “Electron configuration”). Writing 4s first follows the order in which the subshells fill from potassium on; NCERT writes the 3d first, shell by shell, as in its general formula (n − 1)d^1–10 ns^1–2 (NCERT page 90). The shell-by-shell order has a bonus: the 4s electrons, written last, are the first to go when the atom forms a positive ion (NCERT page 95).

How do you write the electronic configuration of a transition element from its atomic number?

To write a transition element’s configuration, take the noble-gas core of the period before, add two ns electrons and, after lanthanum and actinium, a full (n − 2)f¹⁴, then put the rest in (n − 1)d (reasoned from NCERT Table 4.1, pages 90–91). Osmium, Z = 76, has 76 − 54 − 2 − 14 = 6 left for 5d, so it is [Xe] 4f¹⁴ 5d⁶ 6s², as in the table. Last, check the table for exceptions with one s electron or none, such as platinum, 5d⁹ 6s¹, but read its technetium, 4d⁶ 5s¹, as 4d⁵ 5s² (NCERT Class 11 Chemistry, Table 2.6, page 66).

How do you find a transition element from the electronic configuration of its ion?

For an ion of the first transition series with a charge of +2 or more, add the argon core and the charge back to the 3d electrons: Z = 18 + (3d electrons) + charge, because such ions keep no 4s electrons (NCERT page 95). An M⁴⁺ ion that is [Ar] 3d¹, for example, has Z = 18 + 1 + 4 = 23, so it is V⁴⁺, from vanadium, [Ar] 3d³ 4s² (NCERT Table 4.1, page 90). Do not use the rule for M⁺ ions, which can keep a 4s electron: Mn⁺ is 3d⁵ 4s¹ (NCERT page 96).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Why is copper 3d¹⁰ 4s¹ and not 3d⁹ 4s²?

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   The 3d and 4s energies are very close, and a completely filled 3d set is relatively more stable, so one 4s electron moves into 3d (NCERT page 90). Completely filled subshells are extra stable (NCERT Class 11 Chemistry, page 65); strictly, that is one stabilising factor in the total-energy balance, not an automatic rule overriding every d–s energy difference (reasoned).

2. Q2. In what way is the electronic configuration of the transition elements different from that of the non-transition elements? (NCERT Exercise 4.9.)

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   In the s- and p-block elements the last electron enters the outermost shell, ns or np. In the transition elements it enters the (n − 1)d subshell, one shell in, which is partly filled, while outer ns occupation can be zero, one or two. A transition element may have a full d subshell in its neutral atom if it forms a cation with an incomplete d subshell (NCERT pages 89–90). These partly filled d orbitals give the transition elements their variable oxidation states, coloured ions and complexes (NCERT page 91).

3. Q3. Which of the d-block elements may not be regarded as transition elements, and why? (Part of NCERT Exercise 4.8.)

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   Zinc, cadmium and mercury, and copernicium below them. Their configuration is (n − 1)d¹⁰ ns², and NCERT classifies their ordinary chemistry using filled d subshells (page 91). Copernicium’s placement also involves predictions and analogy, since its experimental chemistry is very limited.