Cobalt is a first-row transition element with an atomic number of 27. What is the ground-s… — JAMB UTME

Cobalt is a first-row transition element with an atomic number of 27. What is the ground-state electronic configuration of the cobalt(II) ion, $Co^{2+}$ ?

$[Ar] 3d^7$ Answer
B
$[Ar] 3d^5 4s^2$
C
$[Ar] 3d^6 4s^1$
D
$[Ar] 3d^9$

Answer

The electronic configuration of the cobalt(II) ion,

Co^{2+}

, is

[Ar] 3d^7

Neutral cobalt (

Z = 27

) has the electronic configuration

[Ar] 3d^7 4s^2

. When transition metals ionize, electrons are lost first from the outermost

4s

orbital because

n=4

electrons experience lower electrostatic attraction from the nucleus than

n=3

electrons. Removing two electrons yields

[Ar] 3d^7

Step-by-Step Solution

Determine the ground-state electronic configuration of neutral cobalt (

Co

Z = 27

The neutral cobalt atom has 27 electrons, giving an electronic configuration of

[Ar] 3d^7 4s^2

According to the Aufbau principle, orbitals are filled in increasing order of energy, placing two electrons in the

4s

orbital and seven in the

3d

orbitals.

Apply cation formation rules to remove two electrons for the

Co^{2+}

ion.

Two electrons are removed from the outermost principal energy level (

n = 4

), which is the

4s

orbital.

Although

4s

is filled before

3d

, electrons in the principal shell with the highest principal quantum number (

n = 4

) are held least tightly and are lost first upon ionization.

Write the resulting electronic configuration for

Co^{2+}

[Ar] 3d^7

Removing the two

4s

electrons leaves seven electrons in the

3d

subshell.

Key Concept

Electronic Configuration of Transition Metal Cations

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