Transition Metals and the d-Block Elements

The periodic table splits naturally into blocks according to which subshell is being filled by the last electron added to an atom. The s-block and p-block contain the elements you meet first in chemistry, but sitting between them, across periods 4 to 7, is the d-block: a band of ten columns in which electrons are being added to a d subshell (\(3d\), \(4d\), \(5d\), or \(6d\)).

Every element in this band is a "d-block element", but as you will see next, that is not quite the same thing as being a "transition metal".

A transition metal is usually defined as an element that forms at least one stable ion with a partially filled d subshell. This is a slightly stricter definition than simply "being in the d-block", and it excludes two elements at either end of the first row:

• Scandium (Sc): \( [Ar]3d^1 4s^2 \) as an atom, but its only common ion, \( Sc^{3+} \), has the configuration \( [Ar] \) with an empty d subshell.
• Zinc (Zn): \( [Ar]3d^{10}4s^2 \) as an atom, and its only common ion, \( Zn^{2+} \), has the configuration \( [Ar]3d^{10} \), a completely full d subshell.

Because neither ion has a d subshell that is partially filled (somewhere between \(d^1\) and \(d^9\)), scandium and zinc are d-block elements but not transition metals. The elements titanium through copper, however, all satisfy the definition and display the characteristic transition metal behaviour described below.

For the first row of the d-block (period 4), the general filling pattern places electrons into \(4s\) before \(3d\), following the usual order of increasing energy. Most atoms follow this pattern exactly, but two important exceptions arise because a half-filled or fully filled d subshell is unusually stable.

Element	Symbol	Electron configuration
Titanium	Ti	\( [Ar]3d^2 4s^2 \)
Vanadium	V	\( [Ar]3d^3 4s^2 \)
Chromium	Cr	\( [Ar]3d^5 4s^1 \) (anomaly)
Manganese	Mn	\( [Ar]3d^5 4s^2 \)
Iron	Fe	\( [Ar]3d^6 4s^2 \)
Cobalt	Co	\( [Ar]3d^7 4s^2 \)
Nickel	Ni	\( [Ar]3d^8 4s^2 \)
Copper	Cu	\( [Ar]3d^{10}4s^1 \) (anomaly)

Chromium and copper break the expected pattern because a half-filled (\(3d^5\)) or fully filled (\(3d^{10}\)) d subshell, combined with a single \(4s\) electron, is lower in energy than the "expected" \(3d^4 4s^2\) or \(3d^9 4s^2\) arrangement.

Worked example: configuration of iron ions

A common exam-style question asks for the configuration of \( Fe^{2+} \) and \( Fe^{3+} \). It is tempting to remove electrons from the subshell filled last, but ionisation always removes \(4s\) electrons before any \(3d\) electrons, since once the atom becomes an ion the \(3d\) subshell drops below \(4s\) in energy.

1. Start from the atom: \( Fe = [Ar]3d^6 4s^2 \).
2. To form \( Fe^{2+} \), remove both \(4s\) electrons: \( Fe^{2+} = [Ar]3d^6 \).
3. To form \( Fe^{3+} \), remove one further electron from the \(3d\) subshell: \( Fe^{3+} = [Ar]3d^5 \).

Notice that \( Fe^{3+} \) has a half-filled \(3d^5\) subshell, which is part of the reason it is a particularly stable ion.

Because their d electrons are involved in bonding and are close in energy to one another, transition metals share a set of properties not seen in the s-block or p-block:

• Variable oxidation states: elements such as vanadium, manganese and iron can exist in several stable oxidation states, since d electrons can be lost relatively easily and at similar energy costs. This is explored further in reactions of vanadium and other transition metals.
• Coloured compounds and ions: partially filled d subshells allow electrons to absorb visible light as they move between split d orbitals, giving many transition metal ions their characteristic colours.
• Formation of complex ions: transition metal ions readily bond to surrounding ligands through dative bonds, a topic covered in detail in ligands and complex ions.
• Catalytic activity: the ability to change oxidation state and to adsorb reactants onto their surface makes many transition metals effective catalysts, as discussed in transition metal catalysts.

Together, these properties are what make the transition metals such a distinctive and heavily examined section of the A-level chemistry periodic table, quite different from the more predictable trends seen across the s-block and p-block.

Related lessons

• Uk A Level Chemistry