Chapter 4: The d & f Block Elements

(i) Cr³⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d³

Or, [Ar]¹⁸ 3d³

(ii) Pm³⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 4f⁴

Or, [Xe]⁵⁴ 4f⁴

(iii) Cu⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰

Or, [Ar]¹⁸ 3d¹⁰

(iv) Ce⁴⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶

Or, [Xe]⁵⁴

(v) Co²⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁷

Or, [Ar]¹⁸ 3d⁷

(vi) Lu²⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 4f¹⁴ 5d¹

Or, [Xe]⁵⁴ 4f¹⁴ 5d¹

(vii) Mn²⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵

Or, [Ar]¹⁸ 3d⁵

(viii) Th⁴⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶

Or, [Rn]⁸⁶

Electronic configuration:               Fe²⁺ is [Ar]¹⁸ 3d⁶.

Mn²⁺ is [Ar]¹⁸ 3d⁵.

We know that half and completely-filled orbitals are more stable. Hence, Mn with (+2) state has a stable d⁵ configuration, which is why Mn²⁺ shows resistance to oxidation to Mn³⁺. We know that Fe²⁺ has 3d⁶ configuration, and by losing one electron, its configuration changes to a more stable 3d⁵ configuration. Therefore, Fe²⁺ easily gets oxidized to the Fe⁺³ oxidation state.

In the table below, the oxidation states of the first half of the first row of transition metals are displayed.

We can see that, except Sc, all other metals have a +2 oxidation state. As moving in the increasing order of atomic number from 21 to 25, that is, from Sc to Mn, the number of electrons in 3d orbitals increases from 1 to 5.

Sc (+2) = d¹

Ti (+2) = d²

V (+2) = d³

Cr (+2) = d⁴

Mn (+2) = d⁵

+2 oxidation state is attained by the loss of the two 4s electrons by these metals. Since the number of d electrons in (+2) state also increases from Ti(+2) to Mn(+ 2), the stability of +2 state increases (as the d-orbital is becoming more and more half-filled). Mn (+2) has d⁵ electrons (that is, a half-filled d shell, which is highly stable).

The oxidation states with the maximum number (+2 to +7) are exhibited by the elements of Mn oxidation states that are in the first half of the transition series. Ti  With the increase in atomic number, the stability of the +2 oxidation state increases. Once more electrons are filled in the d-orbital, this happens to a great extent.  The +2 oxidation state is not shown by the Sc. Its electronic configuration is 4s² 3d¹. It loses all three electrons to form Sc³⁺, +3 oxidation state of Sc is very stable as by losing all three electrons, it attains stable noble gas configuration, [Ar], Ti (+ 4) and V(+5) are very stable for the same reason. For Mn, the +2 oxidation state is very stable as after losing two electrons, its d-orbital is exactly half-filled, [Ar] 3d⁵

(i) Chromate,

Oxidation state of Cr is + 6.

(ii) Permanganate,

Oxidation state of Mn is + 7.

(iii) Vanadate, 

Oxidation state of V is + 5.

On moving along the lanthanoid series, the atomic number gradually increases by one. With the increase in atomic number, the number of protons and electrons present in the atom also increases by one. The effective nuclear charge increases as electrons are being added to the same shell. Owing to the proton addition being pronounced more above the interelectronic repulsions resulting from electron addition, a nuclear attraction increase happens. Also, with the increase in the atomic number, the number of electrons in the 4f orbital also increases. The 4f electrons have a poor shielding effect. Therefore, the effective nuclear charge experienced by the outer electrons increases. Consequently, the attraction of the nucleus for the outermost electrons increases. This results in a steady decrease in the size of lanthanoids with an increase in the atomic number. This is termed lanthanoid contraction.

Consequences of lanthanoid contraction

(i) It is due to lanthanide contraction that there is variation in the basic strength of lanthanide hydroxides. (Basic strength decreases from La(OH)₃ to Lu(OH)₃.)

(ii) There is a similarity in the properties of the second and third transition series.

(iii) Separation of lanthanoids is possible due to lanthanide contraction.

Transition elements are those elements in which the atoms or ions (in a stable oxidation state) contain partially filled d-orbital. These elements are found in the d-block and show a transition of properties between the s-block and the p-block. Hence, these are called transition elements.

Elements such as Cd, Hg and Zn cannot be called transition elements because they have completely filled the d-sub shell.

Transition metals have a partially filled d−orbital. Hence, the electronic configuration of transition elements is (n − 1)d^1-10 ns^0-2.

The non-transition elements either do not have a d−orbital or have a fully filled d−orbital.

Therefore, the electronic configuration of non-transition elements is ns^1-2 or ns² np^1-6.

In the case of lanthanoids, the +3 oxidation state is the most common.

That is, Ln(III) compounds are predominant.

However, +2 and +4 oxidation states can also be found in the solution or in solid compounds.