Electron configurationAQA A-Level Chemistry: Subtopic test

1. 1
   Transition metal ions give many gemstones their colour. Ruby owes its red colour to Cr³⁺ ions in aluminium oxide, and a pale green mineral contains Fe²⁺ ions. The atomic numbers of chromium and iron are 24 and 26.

   (a)

   Which is the ground-state electron configuration of a chromium atom?

   [1 mark]

   • A1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁴ 4s²
   • B1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹
   • C1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶
   • D1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 4p⁴

   (b)

   Which is the electron configuration of a Cr³⁺ ion?

   [1 mark]

   • A1s² 2s² 2p⁶ 3s² 3p⁶ 3d² 4s¹
   • B1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶
   • C1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁴
   • D1s² 2s² 2p⁶ 3s² 3p⁶ 3d³

   (c)

   Write the full electron configuration of a Fe²⁺ ion, and state which sub-shell the electrons were removed from when the ion formed.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Semiconductor devices are made from p-block elements such as gallium (atomic number 31) and selenium (atomic number 34). A technician needs to know their electron configurations and those of their ions.

   (a)

   Which is the electron configuration of a selenide ion, Se²⁻?

   [1 mark]

   • A1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶
   • B1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p²
   • C1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁵
   • D1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴

   (b)

   How many electrons occupy p orbitals in a ground-state atom of selenium?

   [1 mark]

   • A4
   • B12
   • C16
   • D10

   (c)

   Write the full electron configuration of a gallium atom and explain why gallium is classed as a p-block element.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student writes the ground-state electron configuration of titanium (atomic number 22) as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁴, and states that when titanium forms ions the electrons are lost from the 3d sub-shell first. Vanadium has atomic number 23.

   (a)

   Identify the two errors in the student's statements, and write the correct ground-state electron configuration of titanium.

   [3 marks]

   (b)

   Write the full electron configurations of a Ti²⁺ ion and a V³⁺ ion. Explain, in terms of the electrons lost, why they have the same configuration.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student is studying the first row of the d block, from scandium (atomic number 21) to zinc (atomic number 30). The student knows that the simple filling order 1s, 2s, 2p, 3s, 3p, 4s, 3d would predict the ground-state configurations of chromium and copper to end 3d⁴ 4s² and 3d⁹ 4s² respectively.

   (a)

   State the actual ground-state electron configurations of chromium and copper, and explain why they differ from the predictions. Write the electron configuration of Cu²⁺ and explain how it is formed.

   [6 marks]

   (b)

   The student claims that neither Sc³⁺ nor Zn²⁺ has a partly filled 3d sub-shell. Evaluate this claim by writing the electron configurations of the two atoms and of the two ions, and deduce the number of unpaired electrons in Zn²⁺.

   [6 marks]

   Total for question 4: 12 marks

End of questions