Periodic Trends in Atomic Radii

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Questions and Answers

What is the primary reason for the relatively similar atomic radii of the second and third d-series elements?

The lanthanoid contraction, which compensates for the expected increase in atomic size with increasing atomic number, resulting from the filling of the 4f orbitals before the 5d series of elements.

Why do the atomic sizes of the transition elements decrease across a period?

It is due to the addition of electrons to the inner 3d sub-shell without adding a new outer shell, and the increase in effective nuclear charge that acts on the outer electrons and pulls them in more strongly.

What is the trend observed in the atomic radii of the elements from the first to the third d-series?

There is an increase in atomic radii from the first to the second d-series, but the radii of the third d-series are virtually the same as those of the corresponding members of the second series.

What is the effect of the lanthanoid contraction on the physical and chemical properties of the second and third d-series elements?

The lanthanoid contraction results in the second and third d-series elements having very similar physical and chemical properties, much more than expected on the basis of usual family relationships. Signup and view all the answers

What is the relationship between atomic number and atomic size across a period?

As atomic number increases, atomic size decreases. Signup and view all the answers

How do the atomic sizes of the transition elements compare to those of the group 1 and 2 elements in the same period?

The atomic sizes of the transition elements are smaller than those of the group 1 and 2 elements in the same period. Signup and view all the answers

What is the reaction for synthesizing [MnO4]2- salts?

MnO2 (s) + 2KOH (s) + O2 → [MnO4]2- + H2O Signup and view all the answers

What is the oxidation reaction that can be reversed by reaction with chlorine?

4[MnO4]- + 4[OH]- → 4[MnO4]2- + 2H2O + O2 Signup and view all the answers

Why is manganate (VI) not formed in the reduction of acidified [MnO4]-?

Because manganate (VI) is unstable with respect to disproportionation in the presence of weak acids. Signup and view all the answers

What is the structure of the [MnO4]2- ion?

Tetrahedral (Mn-O = 166pm) Signup and view all the answers

How is MnOCl3 prepared?

By reacting KMnO4 with CHCl3 in HSO3Cl Signup and view all the answers

What is the structure of MnOCl3?

Tetrahedral Signup and view all the answers

Why must solutions of [MnO4]3- be strongly alkaline?

To prevent disproportionation, which occurs readily in weakly alkaline or acidic media. Signup and view all the answers

What is the only binary halide of Mn(IV)?

MnF4 Signup and view all the answers

What is the reaction of MnO2 with concentrated HCl?

MnO2 + 4HCl conc → MnCl2 + Cl2 + 2H2O Signup and view all the answers

What is the oxidation state of manganese in MnF3?

Mn(III) Signup and view all the answers

What is the solid-state structure of MnF3?

Jahn-Teller distorted high-spin d4 Signup and view all the answers

What is the reaction of MnO2 with water?

MnO2 + 2H+ + 3e- → MnO2 + 2H2O Signup and view all the answers

What is the thermal stability of MnF3?

Thermally stable Signup and view all the answers

How is Mn2O3 obtained?

By heating MnO2 at 1070 K or by oxidation of Mn(II) in alkaline medium Signup and view all the answers

What is the characteristic color of manganese(II) salts?

Pale pink Signup and view all the answers

What is the structure of MnCl2, MnBr2, and MnI2 in the solid state?

CdI2 layer structure Signup and view all the answers

What is the general trend observed in the highest oxidation state of transition elements across a transition series?

The highest oxidation state increases with the increase of atomic number, reaching a maximum in the middle, and then starts decreasing. Signup and view all the answers

What is the minimum oxidation state shown by a transition metal, and what is the exception to this rule?

The minimum oxidation state is equal to the number of ns-electrons, except for scandium, which has a +3 oxidation state. Signup and view all the answers

Why do chromium and copper have a lowest oxidation state of +1?

It is due to their electronic configurations, Cr(3d54s1) and Cu(3d104s1), which makes the loss of 4s-electrons more favorable. Signup and view all the answers

What is the relationship between the maximum oxidation state and the electronic configuration of the first five elements in the first transition series?

The maximum oxidation state is equal to the sum of the 4s and 3d-electrons. Signup and view all the answers

Why do the elements from Fe to Ni have common oxidation states of +2 or +3?

The maximum oxidation state is not related to their electronic configurations, but is rather a general trend observed in these elements. Signup and view all the answers

What is the role of ns-electrons in the oxidation states of transition elements?

ns-electrons participate in bonding, leading to lower oxidation states. Signup and view all the answers

What is the highest oxidation state shown by any transition metal?

+8, shown by ruthenium and osmium. Signup and view all the answers

Why are d-orbitals more stable than 4s-orbitals after scandium?

The energy associated with the 3d-subshell is less than that of the 4s-subshell. Signup and view all the answers

What is the purpose of adding acids to solutions of transition metal salts, and provide an example of an equilibrium reaction that occurs?

To prevent hydrolysis. For example, FeCl3 + 3H2O ⇌ Fe(OH)3 + 3HCl. Signup and view all the answers

How can sulphides of transition metals be formed, and what are their characteristic properties?

Sulphides of transition metals can be formed by direct combination of metals with Sulphur or by action of H2S or Na2S on the salts of these metals. They are usually black and insoluble in water. Signup and view all the answers

What is the definition of disproportionation, and provide an example of a redox reaction that exhibits this type of reaction?

Disproportionation is a specific type of redox reaction in which a species is simultaneously reduced and oxidized to form two types of products. An example is the reaction: 3MnO42-(aq) + 4H+(aq) → 2MnO4- (aq) + MnO2(s) + 2H2O(l). Signup and view all the answers

What is the effect of pH on the stability of manganate(VI) ion, and provide an example of a reaction that illustrates this?

The stability of manganate(VI) ion is affected by pH, where it is stable in alkaline medium but decomposes rapidly in acidic solution to MnO4- and MnO2. The reaction: 3MnO42-(aq) + 4H+(aq) → 2MnO4- (aq) + MnO2(s) + 2H2O(l) illustrates this. Signup and view all the answers

What is the definition of a stable oxidation state, and how can it be affected by pH?

A stable oxidation state is one that does not undergo disproportionation. The pH of a solution can affect the stability of an oxidation state, making it more or less stable relative to other oxidation states. Signup and view all the answers

Provide an example of a transition metal that undergoes disproportionation in aqueous solution, and write the equation for the reaction.

Copper(I) undergoes disproportionation in aqueous solution, resulting in the formation of copper metal and copper(II) ions. The equation for the reaction is: 2Cu+ → Cu2+ + Cu. Signup and view all the answers

What is the general condition for a particular oxidation state to undergo disproportionation, and provide an example of a reaction that illustrates this?

A particular oxidation state undergoes disproportionation when it becomes less stable relative to other oxidation states, one lower and one higher. The reaction: 3MnO42-(aq) + 4H+(aq) → 2MnO4- (aq) + MnO2(s) + 2H2O(l) illustrates this, where Mn6+ becomes unstable relative to Mn7+ and Mn4+ in acidic medium. Signup and view all the answers

How does the acidity of a solution affect the stability of certain oxidation states, and provide an example of a reaction that illustrates this?

The acidity of a solution can affect the stability of certain oxidation states, making them more or less stable relative to other oxidation states. The reaction: 3MnO42-(aq) + 4H+(aq) → 2MnO4- (aq) + MnO2(s) + 2H2O(l) illustrates this, where the acidity of the solution affects the stability of the Mn6+ oxidation state. Signup and view all the answers

Study Notes

Atomic Radii and Lanthanoid Contraction

The curves in Figure 1 show an increase in metallic and atomic radii from the first (3d) to the second (4d) series of the elements.
The radii of the third (5d) series are virtually the same as those of the corresponding members of the second series due to the intervention of the 4f orbitals.
The filling of 4f orbitals before 5d orbitals results in a regular decrease in atomic radii called lanthanoid contraction, which compensates for the expected increase in atomic size with increasing atomic number.

Trends in Atomic Size

On moving across a period, atomic number increases, effective nuclear charge increases, and atomic size decreases.
This decrease in atomic size across a period is observed in both transition elements and main group elements.
The decrease in atomic size is due to electrons being added to the inner 3d sub-shell without adding a new outer shell, and the increase in effective nuclear charge that pulls the outer electrons in more strongly.

Oxidation States of Transition Elements

The highest oxidation state increases with increasing atomic number, reaching a maximum in the middle of the transition series and then decreasing.
The highest oxidation state shown by any transition metal is +8 (Ru and Os).
The minimum oxidation state shown by a transition metal is equal to the number of ns-electrons.
The highest oxidation state is equal to the sum of the 4s and 3d-electrons for the first five elements of the first transition series (up to Mn).

Sulphides of Transition Metals

Sulphides of transition metals can be formed by direct combination of the metals with sulphur or by action of H2S or Na2S on the salts of these metals.
Sulphides of transition metals are coloured, usually black, and are insoluble in water.

Disproportionation

Intermediate oxidation states have a tendency to disproportionate, which is a specific type of redox reaction where a species is simultaneously reduced and oxidized to form two types of products.
Examples of disproportionation include the manganate(VI) compound K2MnO4 in acidic aqueous solution and the univalent copper in aqueous solution.

Manganese Compounds

Manganese(VII)

Manganese(VII) is unstable with respect to disproportionation in the presence of even weak acids such as H2CO3.
Salts of dark green [MnO4]2- are made by fusing MnO2 with group 1 metal hydroxides in the presence of air or by reaction.

Manganese(V)

Although studies of the MnF3/F2 system indicate the existence of MnF5 in the gas phase, binary halides of Mn(V) have not been isolated.
The only oxohalide is MnOCl3, made by reacting KMnO4 with CHCl3 in HSO3Cl.

Manganese(IV)

The only binary halide of Mn(IV) is MnF4, prepared from the elements, which is an unstable blue solid that decomposes at ambient temperatures.
Manganese(IV) oxide is polymorphic and often non-stoichiometric, and only the high-temperature β-form has the stoichiometry MnO2 and adopts a rutile structure.

Manganese(III)

The only binary halide of Mn(III) is the red-purple MnF3, made by the action of F2 on Mn(II) halides at 520 K, which is thermally stable but immediately hydrolyzed by water.

Manganese(II)

Manganese(II) salts are characteristically very pale pink due to the d-d transitions in the 3d5 of Mn2+ ion.
Manganese(II) salts are obtained from MnO2 by a variety of methods, such as heating MnO2 with concentrated acid or by precipitation from solutions containing Mn2+.

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Description

Understanding the variation in atomic and metallic radii across different series of elements and the phenomenon of lanthanoid contraction.