Transition Metals (Edexcel A Level Chemistry)

• Transition metals are elements with an incomplete d-subshell that can form at least one stable ion with an incomplete d-subshell
• This definition distinguishes them from d-block elements, because scandium and zinc do not fit the definition
  • Scandium only forms the ion Sc³⁺, configuration [Ar] 3d⁰
  • Zinc only forms the ion Zn²⁺, configuration [Ar] 3d¹⁰

• The elements of the first transition series are therefore titanium to copper

The transition elements and the d-block elements

Electron Configuration

• The full electronic configuration of the first d-series transition metals is shown in the table below
• Following the Aufbau Principle electrons occupy the lowest energy subshells first
• The 4s overlaps with the 3d subshell so the 4s is filled first
• Remember that you can abbreviate the first five subshells, 1s-3p, as [Ar] representing the configuration of argon( known as the argon core)

Table showing the Electronic Configuration of the First d-series Transition Elements

• From AS Chemistry you should recall two exceptions to the Aufbau Principle, chromium and copper
• In both cases an electron is promoted from the 4s to the 3d to achieve a half full and full d-subshell, respectively
• Chromium and copper have the following electron configurations, which are different to what you may expect:
  • Cr is [Ar] 3d⁵ 4s¹ not [Ar] 3d⁴ 4s²
  • Cu is [Ar] 3d¹⁰ 4s¹ not [Ar] 3d⁹ 4s²

• This is because the [Ar] 3d⁵ 4s¹ and [Ar] 3d¹⁰ 4s¹ configurations are energetically more stable

Answer

Step 1: Write out the electron configuration of the atom first:

Mn atomic number = 25

1s²2s²2p⁶3s²3p⁶4s²3d⁵

2 + 2 + 6 + 2 + 6 + 2 + 5 = 25 electrons

Step 2: Subtract the appropriate number of electrons starting from the 4s subshell

Mn(III) = 22 electrons

1s²2s²2p⁶3s²3p⁶3d⁴

General properties

• Although the transition elements are metals, they have some properties unlike those of other metals on the periodic table, such as:
  • Variable oxidation states
  • Form complex ions
  • Form coloured compounds
  • Behave as catalysts

• Like other metals on the periodic table, the transition elements will lose electrons to form positively charged ions
• However, unlike other metals, transition elements can form more than one positive ion
  • They are said to have variable oxidation states
• Because of this, Roman numerals are used to indicate the oxidation state on the metal ion
• • For example, the metal sodium (Na) will only form Na⁺ ions (no Roman numerals are needed, as the ion formed by Na will always have an oxidation state of +1)
  • The transition metal iron (Fe) can form Fe²⁺ (Fe(II)) and Fe³⁺ (Fe(III)) ions

• When transition elements forms ions they lose electrons from the 4s subshell first
• This is because when the orbitals are occupied, the repulsion between electrons pushes the 4s into a higher energy state so that it now becomes slightly higher in energy than the 3d subshell
  • The 4s is now the outer shell and loses electrons first
• The loss of the 4s electrons means that +2 is a common oxidation state in transition metals
• The reason why the transition metals have variable oxidation states all comes down to energy

Table showing the Common Oxidation States of Transition Elements