Numford
Electronic configuration

Ruthenium (Ru)

[Kr] 4d⁷ 5s¹, an exception. 4 unpaired electrons, period 5, group 8, d block.

Element or ion

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H1He2Li3Be4B5C6N7O8F9Ne10Na11Mg12Al13Si14P15S16Cl17Ar18K19Ca20Sc21Ti22V23Cr24Mn25Fe26Co27Ni28Cu29Zn30Ga31Ge32As33Se34Br35Kr36Rb37Sr38Y39Zr40Nb41Mo42Tc43Ru44Rh45Pd46Ag47Cd48In49Sn50Sb51Te52I53Xe54Cs55Ba56Hf72Ta73W74Re75Os76Ir77Pt78Au79Hg80Tl81Pb82Bi83Po84At85Rn86

RuRuthenium, Z = 44, 101.07 u

[Kr] 4d7 5s1

In full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁷ 5s¹

[Kr]4d5s
  • Blockd block
  • Period5
  • Group8
  • Valence8
  • Unpaired4
  • MagneticPara

Last electron 4d

  • n4shell
  • l2d subshell
  • m−1orbital
  • s−½spin down
Why Ru is an exception

Plain filling would give [Kr] 4d⁶ 5s². For heavier atoms the (n − 1)d, (n − 2)f and ns levels lie very close together, so one electron sits in d. These are the configurations found by experiment and printed in the NCERT tables.

Working

Step by step

  1. Ruthenium has 44 electrons.
  2. Fill in (n + l) order: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d⁶.
  3. That gives [Kr] 4d⁶ 5s², but the accepted configuration is [Kr] 4d⁷ 5s¹. For heavier atoms the (n − 1)d, (n − 2)f and ns levels lie very close together, so one electron sits in d. These are the configurations found by experiment and printed in the NCERT tables.
  4. Write the subshells in order of n: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁷ 5s¹.
  5. Replace the krypton core with [Kr]: [Kr] 4d⁷ 5s¹.
At a glance

Ruthenium (Ru): the numbers

Atomic number44
Atomic mass101.07 u
Period5
Group8
Blockd block
Valence electrons8
Unpaired electrons4
Magneticparamagnetic
Last electron4d: n = 4, l = 2, m = −1, s = −1/2
Questions

Ruthenium (Ru): quick answers

What is the electronic configuration of ruthenium?

Ruthenium has 44 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁷ 5s¹, or [Kr] 4d⁷ 5s¹ in short.

Why is ruthenium [Kr] 4d⁷ 5s¹ and not [Kr] 4d⁶ 5s²?

For heavier atoms the (n − 1)d, (n − 2)f and ns levels lie very close together, so one electron sits in d. These are the configurations found by experiment and printed in the NCERT tables.

What are the quantum numbers of the last electron of ruthenium?

The last electron goes into 4d: n = 4, l = 2, m = −1, s = −1/2. Orbitals fill from m = −l up, spin +½ first.

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