Oxidation State of p-Block Elements - Oxidation States, Inert Pair Effect and Chemical Reactivities of Groups 13, 14, 15, 16, 17 and 18 Elements

On special occasions we generally paint the interiors or the exteriors of our house. Also at times, we first strip the wall of the old paints or the rusty surfaces. The term ‘oxidation’ is quite relatable to this. Stripping the paint off a surface is like removing electrons from an atom and thus making it undergo oxidation! On the contrary, painting a surface with new colours is like adding electrons from outside, to an atom. In both cases, the final state (electron count) of the atom decides its oxidation state. 

Every element in the periodic table has a specific range of possible oxidation states. It is important to understand the oxidation states of every element so as to determine the extent and manner of its chemical reactivity! At this point, we shall try to understand the oxidation states of the elements of p-Block. So, without any further ado, let’s get started!

TABLE OF CONTENTS

• What is Oxidation State?
• Introduction to Oxidation State in p-Block Elements
• Oxidation States of Group 13 Elements and Inert pair Effect
• Oxidation States of Group 14 Elements and Inert pair Effect
• Oxidation States of Group 15 Elements and Chemical Reactivity
• Oxidation States of Group 16 Elements and Chemical Reactivity
• Oxidation States of Group 17 Elements and Chemical Reactivity
• Oxidation States of Group 18 Elements and Chemical Reactivity
• Practice Problems
• Frequently Asked Questions - FAQ

What is Oxidation state?

Oxidation state is a fundamental concept in chemistry, and is particularly important in transition metal chemistry, as d-block elements often have a wide range of stable oxidation states. The oxidation state of an atom within a molecule is usually considered to be the formal charge on the atom if hypothetically all of the molecules are composed of ions.

Oxidation numbers are assigned to atoms in a rather arbitrary fashion to designate electron transfer in oxidation-reduction reactions. They represent the charges that atoms would have if the electrons were assigned according to an arbitrary set of rules.

According to IUPAC, the oxidation state of the element is defined as a measure of the degree of oxidation of an atom in a substance.

Introduction to Oxidation State in p-Block Elements

Group 13 to 18 of the periodic table of elements constitute the p-Block. p-Block contains metals, metalloids as well as non–metals. 

• The general valence shell electronic configuration of the p–block elements is ns² np^1-6.

Exception: Helium, 1s²

• The maximum oxidation state shown by a p-block element is equal to the total number of valence electrons (i.e., the sum of the electron count of s and p subshells in the outermost orbit.). 

• The first member of a group has a greater ability to form p𝜋–p𝜋 multiple bonds to itself and to the element of the second row. E.g. C = C, C ≡ C, N ≡ N , C = O, C = N, C ≡ N, N = O

• The highest oxidation state of p–block elements is equal to the [Group number - 10 ].

• Down the group, the oxidation state is two less than the highest-group oxidation state. Which becomes more stable in groups 13 to 16 due to the inert pair effect.

Oxidation States of Group 13 Elements and Inert pair Effect

Boron is a typical non-metal, Aluminium is a metal, but shows many chemical similarities to boron. Gallim, Indium and Thallium are almost exclusively metallic in character.

Valence shell electronic configuration: ns² np¹ 

General oxidation states exhibited: +1 and +3

• Due to the small size of boron, the sum of its first three ionisation enthalpies is very high. This prevents it from forming +3 ions and compels it to form only covalent compounds.

• As we move from B to Al, the sum of the first three ionisation enthalpies of Al considerably decreases. So, Al forms Al³⁺ ions.

Inert Pair Effect

• Down the group, due to poor shielding of d and f-orbitals, the effective nuclear charge increases. This holds the ns electrons tightly and further restricts their participation in bonding.

• Therefore only the electrons in p-orbitals get involved in bonding. In Ga, In and Tl, both +1 and +3 oxidation states are observed. This non-participation of the s-orbital during chemical bonding due to the poor shielding of the intervening electrons is called the inert pair effect.

• Due to this, the relative stability of the +1 oxidation state progressively increases for heavier elements.

• It is shown experimentally that B³⁺ is more stable than B⁺ but, the stability of Tl⁺is more than that of Tl³⁺ due to the inert pair effect.

• In Thallium, +1 oxidation state is predominant and +3 oxidation state is highly oxidising in character. The compounds in +1 oxidation state are more ionic than those in +3 oxidation states.

• In trivalent state, the number of electrons around the central atom in a molecule will be only six (E.g., B in BF₃).

• Such electron deficient molecules tend to accept a pair of electrons to achieve stable electronic configuration and behave as Lewis acids. The tendency to behave as a Lewis acid decreases with the increases in size down the group.

• Due to the absence of d-orbitals, the maximum covalency of B is 4. Maximum covalency can be expected beyond 4 for (M = Al, Ga, In, Tl) due to the availability of d - orbitals. 

Oxidation States of Group 14 Elements and Inert Pair Effect

The general electronic configuration of Group 14 elements is ns² np². These elements have 2 electrons in the outermost p - orbitals.  The elements have four electrons, in their outermost shell.

• The common oxidation states exhibited by these elements are +4 and +2. 

• Carbon also exhibits negative oxidation states. Since the sum of the first four ionisation enthalpies is very high, the compounds in +4 oxidation state are generally covalent in nature.

Inert Pair Effect 

• Down the group, due to poor shielding of d and f - orbitals, the effective nuclear charge increases. This holds the ns electrons tightly and further restricts their participation in bonding.

• So, only the electrons in p-orbital get involved in bonding. For Ge, Sn, and Pb, both +2 and +4 oxidation states are observed. 

• Due to inert pair effect, the stability of +2 oxidation state increases down the group: Ge < Sn < Pb

• Stability of +4 oxidation state decreases down the group: Ge > Sn > Pb

• C and Si mostly show +4 oxidation state. Tin (Sn) forms compounds in both the oxidation states.

• Tin in +2 state is a reducing agent. Lead compounds in +2 state are stable and in +4 state are strong oxidising agents.

Oxidation States of Group 15 Elements and Chemical Reactivity

The valence shell electronic configuration of elements of group 15 is ns² np³. So, the elements here can either lose 5 electrons or gain 3.

• The common oxidation states exhibited by Group 15 elements are  -3, +3, +5. 

• Due to the increase in the atomic radius down the group, the ionisation enthalpy and electrongeativity decreases. This subsequently decreases the tendency to gain three electrons to create a -3 oxidation state down the group. 

• Down the group, the tendency to exhibit -3 oxidation state decreases due to increase in size and metallic character. 

• The stability of +3 oxidation state increases down the group. Bi³⁺ > Sb³⁺ >  As³⁺. This is due to an increase in inert pair effect down the group. 

• The stability of +5 oxidation state decreases down the group. As⁵⁺ > Sb⁵⁺ >Bi⁵⁺ .

• Bismuth hardly forms any compound in -3 oxidation state. In fact, the stability of the +5 state also decreases as we move down the group. BiF₅ is the only well-characterised Bi(V) compound

Special cases: Nitrogen exhibits a large number of oxidation states, from -3 to +5 when it reacts with oxygen. 

• As it does not have d-orbitals to accommodate electrons from other elements to form bonds, nitrogen does not form compounds in +5 oxidation state.

• Phosphorus shows +1 and +4 oxidation states in some oxoacids like H₃PO₂.

• For nitrogen, all states from +1 to +4 tend to be disproportionate in acidic medium. 

3HNO₂ → HNO₃ + H₂O + 2NO

• All intermediate oxidation states disproportionate into +5 and –3, both in acidic and alkaline media.

• For As, Sb and Bi, +3 oxidation states become increasingly stable with respect to disproportionation.

Oxidation States of Group 16 Elements and Chemical Reactivity

The possible oxidation states of this group is -2, +2, +4,+6. Down the group, the tendency to exhibit -2 oxidation state decreases. Polonium hardly shows -2 oxidation state.

• The electronegativity of oxygen is very high, therefore it shows only negative oxidation states such as –2.

• Except in the case of OF₂ where, the oxygen has an oxidation state of +2. This is due to extremely high electronegativity of fluorine.

• Oxygen and sulphur have only s and p electrons, whereas Se, Te, and Po have d-electrons too.

• The filling of the d- shell makes the atom smaller and hence the electrons are tightly packed.

• Due to this reason, Se cannot acquire the highest oxidation state of (+6).

• S, Se, and Te usually show +4 oxidation state in their compounds with oxygen and +6 oxidation in their compounds with fluorine. Bonding in +4 and +6 oxidation states is primarily covalent.

• Light yellow coloured numbers represent oxidations states which are not shown by the respective elements.

• Orange colour represents oxidation state which are shown but less abundant

• Brown colour shows oxidation state which are most abundant for the respective element.

Oxidation States of Group 17 Elements and Chemical Reactivity 

• Halogens in their diatomic elemental forms have the oxidation state 0. Fluorine exhibits the oxidation states of −1 (F^- ion). Astatine is the only radioactive element in the group. 

• They have seven electrons in their outermost shell (ns² np⁵) and are short of one electron from the configuration of the nearest noble gas. 

• The chemical properties and reactivity of an element are determined by the oxidation state exhibited by them.

• All halogens exhibit -1 oxidation states. However chlorine, bromine, iodine exhibit +1,+3, +5,+7 oxidation states.

• Higher oxidation states of halogens are seen when halogens combine with fluorine or oxygen atoms. E.g. In interhalogens (IF₅, ICl₅, IF₇), oxides or oxoacids (Cl₂O₇, Cl₂O₆).

• Fluorine atom has no d-orbitals in its valence shell. Therefore, it cannot expand its octet. Being the most electronegative it exhibits only -1 oxidation state.

• Halogens readily accept electrons and are therefore strong oxidising agents.

• Light yellow coloured numbers represent oxidations states which are not shown by the respective elements.

• Orange colour represents oxidation state which are shown but less abundant

• Brown colour shows oxidation state which are most abundant for the respective element.

Oxidation States of Group 18 Elements and Chemical Reactivity

Group 18 elements have a stable electronic configuration i.e., ns² np⁶ with completely filled orbitals. Due to completely filled orbitals and complete octet configuration, these elements do not have a tendency to lose, gain or share electrons. Hence, they have zero valency and mostly exist as monatomic gases.

Xenon, however, exhibits higher oxidation states, as the paired electrons of the valence shell can be promoted to the higher empty d-orbitals upon excitation by absorption of energy. Fluorine and oxygen being the two strongly electronegative atoms share the unpaired electrons of xenon and form covalent compounds with it. E.g., XeF₂, XeF₄, XeF₆, XeO³ and XeOF₄.

Practice Problems 

Q1. Which gas is obtained during the disproportionation reaction of HNO2?

A) NO₂
B) NO
C) O₂
D) N₂

Answer: The oxidation state of nitrogen in HNO₂ is +3. Nitrogen disproportionates in acidic medium when it shows oxidation states between +1 to +4. 

3HNO₂ → HNO₃ + H₂O + 2NO

So, option B) is the correct answer.

Q2. Which of the following non-metals does not show a high positive oxidation state?

A) Fluorine
B) Iodine
C) Oxygen
D) Chlorine

Answer: Fluorine is the most electronegative element and since it is unable to expand octet due to absence of d-orbitals, it cannot show higher oxidation states. It can exhibit only -1 oxidation state. 

So, option A) is the correct answer.

Q3. Which of the following is a good oxidising agent?

A) PbCl₄
B) SnCl₂
C) PbCl₂
D) None of the above

Answer: For elements like Sn and Pb, d and f-orbitals are filled with electrons. Since the shielding ability of d and f orbitals are very poor, the nuclear charge that seeps through attracts the s-orbital closer to the nucleus. This makes the s orbital reluctant to bond, thereby only the p-electrons involved in bonding.

Therefore for Pb, +2 oxidation state is stabler than the +4 oxidation state. So, Pb⁴⁺ is a very good oxidising agent. 

So, option A) is the correct answer.

Q4. What is the oxidation state of Xe in XeOF₄?

A) +4
B) +6
C) O
D) +8

Answer: Let oxidation state of Xe be x.

The oxidation state of fluorine is -1 and that of oxygen is -2

So, x + ( - 2) + 4( - 1) = 0
⇒ x - 6 = 0
∴ x = + 6

Thus, the oxidation state of Xe in the given compound is +6. 
So, option B) is the correct answer.

Frequently Asked Questions - FAQ

Question 1. Which p-block elements show the inert pair effect?
Answer: The inert-pair effect is only shown by the elements which have inner electrons in d- and f-orbitals influencing their outermost s-orbital electrons by poorly shielding the outer electrons and increasing the effective nuclear charge on them. Inert pair effect is generally exhibited by some heavier nucleus p-block elements [such as, Tl, Sn, Pb, Bi, Po etc. ]. 

For example, the inert pair effect among Group 14 and Group 15 elements. Sn²⁺, Pb²⁺, Sb³⁺ and Bi³⁺ which are the lower oxidation states of the elements are formed because of the inert pair effect. When the outer shell s-electrons remain paired, the oxidation state is lower than the characteristic oxidation state of a particular group.

Question 2. What is the maximum oxidation state of interhalogens?
Answer: The maximum oxidation state for interhalogens is +7. In IF₇, fluorine exists as -1 and iodine exists in +7 oxidation state.

Question 3. What is the effect of increase in oxidation state of a particular halogen atom in an oxoacid of halogen?
Answer: On increasing the oxidation state of a particular halogen atom, the acidic character of corresponding oxoacid increases. This can be explained on the basis of stability of conjugate bases by resonance and charge stabilisation. For example: The acidic strength of oxoacids of chlorine increases in the order:

HClO  <  HClO₂  <  HClO₃  <  HClO4

The charge stabilisation is in the order : ClO¯  <  ClO₂^-  <  ClO₃^-  <  ClO₄^-

Question 4. Give some examples of polyhalide ions.
Answer: Triatomic iodide is an important polyhalide. I₃^- is obtained by reacting diatomic iodine with iodide ion. Some other polyhalide anions are ICl₂^-, ICl₄^- and polyhalonium cations areClF₂⁺, Cl2F⁺ , BrF₂⁺, IF₂⁺ etc.

Related Topics