Group 15 Elements Test - 1

Concept:

Electronic configuration :

• The arrangement of electrons in the electronic levels of an atom is called its electronic configuration.
• The electrons are arranged according to their energies and space availability.
• There are a few sets of rules which we follow while arranging the electrons in an atom.

They are-

• Aufbau principle-
  • ​It states that the atomic orbitals which have lower energies are filled first.
  • Then the electron goes to occupy the higher atomic energy levels.
  • This ruled is followed to write the electronic configuration of atoms in their ground state.
  • When in the excited state, the electrons might vacant the lower energy state and move on to a higher energy state.
  • The order in which atomic orbitals are filled are: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p.
  • The order follows that the lower ( n + l ) value of atomic orbital will have lower energy, where n = principal quantum number and l = azimuthal quantum number.
• Pauli's exclusion principle:
  • ​An electron is characterized by four quantum numbers n, m, l, and s.
  • Pauli's exclusion principle states that every electron must have its unique set of these four quantum numbers.
  • No two electrons can have the same set of quantum numbers.
  • Simplifying, we may say that an orbital can contain a maximum of two electrons and the two electrons should have opposite spins ( + ½, - ½ ).
• Hund's rule of maximum spin multiplicity:
  • ​While filling up the orbitals of the same energy, or degeneracy, electrons will first singly occupy the subshell.
  • After each subshell has been singly occupied, electron pairing will start.
  • This rule thus implies that maximum spin should be maintained while filling up electrons in a subshell.
  • The maximum spin state involves maximum exchange energy and yields a more stable state.
• Modern periodic law:
  • ​The modern period table involves the arrangement of the elements according to their increasing Atomic number.
  • The law states that the physical and chemical properties of an element are characteristics of its Atomic number.

Explanation:

• The expected electronic configuration of N 1s2, 2s2, 2px2 2py1 but the actual electronic configuration of Nitrogen is 1s2, 2s2, 2px1 2py1 2pz1.
• This is in co-relation with Hund's rule of maximum spin multiplicity.​
• The electronic configuration has 2p subshells (which are the same in energy) singly occupied.
• The total spin of the electrons is

\(S = ½ + ½ + ½ =3/2\)

• Then the maximum spin multiplicity

\(​⇒ 2S + 1 = 3 +1 = 4 \)

• If Hund's rule was being disobeyed, the electronic configuration would have been 1s22s22px22py1.
• In that case, the total spin should have been,

Spin multiplicity \( = 1 + 1 = 2\).

Hence, in order to have maximum spin multiplicity, the electronic configuration of Nitrogen becomes 1s2, 2s2, 2px1 2py1 2pz1.

Important Points

• 1s2, 2s1, 2px2 2py2 this electronic configuration of Nitrogen in ground state is not possible because it does not obey the Aufbau filling of orbitals energy series which is :

\(1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<6p<7s<5f<6d<7p<8s\)

Key Points

• All the elements of the group 15 elements form tri oxides and pentoxides having the general formula M₂O₃ or M₂O₅.
• The oxides of Nitrogen, Phosphorus, and Arsenic are acidic.
• The trioxide of antimony Sb2O3 is amphoteric while that of bismuth Bi2O3 is basic.
• As2O3 is acidic in nature.
• The pentoxides of all these elements are acidic but acidic character decreases down the group.
• Thus, P₂O₅ is the strongest acidic oxide whereas Bi₂O₃  is the weakest acidic oxide.
• The acidic character of the same element increases with an increase in the oxidation state.

Hence, the most acidic oxide among the following is P2O5.

​Important Points

Facts about P₂O₅:

•  It is also written as P₄O₁₀ and is called phosphorus pentoxide.
• It is prepared by burning phosphorus in excess of air or oxygen. It is a white solid which sublimes on heating.
• It is a strong dehydrating agent and dehydrates nitric acid as well as sulphuric acid.
• The structure is:

​

Additional Information

• N₂O₅ is the most acidic of group 15 elements. 

Explanation:

PH₃ has lowest boiling point because NH₃ has intermolecular hydrogen bonding and other hydrides have higher van-der wall forces.

Except NH₃ boiling point smoothly increases down the group due to increase in magnitude of vander Waal's force. Ammonia shows intermolecular H bonding. Its boiling point is higher than AsH₃ , but lower than SbH₃.

Order of Boiling Point:

SbH₃> NH₃> AsH₃> PH₃

Additional Information Most basic is ammonia because of highest electron density on nitrogen.

Highest bond angle is in ammonia as nitrogen has highest electron density , repulsion between lone pairs and bond pairs will be more resulting into more bond angle due to small size of nitrogen.

The correct answer is option 2 i.e.N2O₅

Acid anhydride

• Anhydride means ‘without water’ It can be defined as the chemical compound formed by eliminating water from another compound.
• Anhydrides react with water to produces either base or an acid.
• Acid anhydrides are the molecules that are capable of forming acidic solutions in water.
• Dinitrogen pentoxide (N₂O₅) is the anhydride of nitric acid. 
• It means Hydration of Dinitrogen pentoxide (N2O5) gives Nitric acid.
• N2O₅ + H₂O → 2HNO₃

Explanation:

• PH₃, ASH₃, NH₃, SbH₃ - these are the hydrides of group 15 elements.
• One thing which is important to note is the hydrides of these elements are basic (Lewis Bases) in nature due to the presence of a lone pair of electrons.
• Because electrons diffuse over a larger volume, basicity decreases with the size of the core atom.
• The M-H bond decreases when the size of central atoms increases. (Where M can be any group 15 element - P, N, As, Bi, Sb)
• As a result, as the size of hydrides grows larger as we move down the group, their stability declines and their decreasing character increases.
• In simple words - The thermal stability of hydrides decreases as we move down the group. 
• Hence, the order of their stability is - 
  • NH3 > PH3 > AsH3 > SbH3 
• Hence, the chemical compound ammonia (NH₃ ) is the most thermally stable.

Explanation:

(NH4)2Cr2O7 → 4H2O + N2 + Cr2O3

From the reaction, we can say that on heating we get Chromium Oxide and Nitrogen.

Additional Information

Decomposition reaction:

• Those reactions in which a compound splits up into two or simpler substances are known as decomposition reactions.
• The decomposition reactions are carried out by applying heat, light, or electricity.
• Heat, light, or electricity provide energy that breaks a compound into two or simpler compounds. 
• A decomposition reaction is the opposite of a combination reaction. 

The types of decomposition reaction are:

Thermal decomposition:

• When a decomposition reaction is carried out by heating, it is called thermal decomposition.

Example: CaCO3 → CaO + CO2

• When limestone (CaCO3 ) is heated then it changed to Quick lime (CaO) and carbon dioxide gas.      

Electrolytic decomposition:

• When current passed through an aqueous solution of a substance it breaks down into ions.

Example: 2H2O → 2H2 + O2

Photo decomposition reaction:

• When a compound is broken down by photons or absorption of light, it is called a photodecomposition reaction.

Example: 2AgCl → 2Ag + Cl2

Explanation: 

Nitrogen Family elements form oxides of the types X₂O₃, X₂O_4, and X₂O₅,

The acidic strength of oxides :

N₂O < NO < N₂O₃ 2O₄ < N₂O₅ -

The decreasing order of stability of oxides of group 15 follows.

P₂O5 > As₂O5 > Sb₂O5 > Bi₂O₅

The nature of oxides of group 15 elements is as follows,

N₂O₃, and P₂O₃, (acidic): As₂O₃, and Sb₂O₃, (amphoteric); Bi₂O₃ (basic)

The acidic character of oxides increase with increase in oxidation number of element. However,

(N₂O, NO) → (Neutral)

N₂O₃, NO₂, and N₂O₃ → Acidic character increases

Explanation:

Structures of phosphorous pentoxide and phosphorous trioxide

From the figure it is clear that

P - O - P bridge in both the structures are 6

Additional Information Oxides of phosphorus:

Phosphorus is quite reactive and forms number of compounds in station states of -3, +3 and +5.

Phosphorus forms two common oxides namely, phosphorus trioxide (P₄O₆) and phosphorus penta oxide (P₄O₁₀)

I. Phosphorus trioxide (P4O6): It is formed when P is burnt in a limited supply of air.

P₄ + 3O₂ → P₄O₆

II. Phosphorus penta oxide (P4O10): It is prepared by heating white phosphorous in excess of air 

P4 + 5O2 → P4O10

The correct answer is option 2, i.e., The stability of +5 oxidation state decreases down the group.

• Out of the given options, the correct statement regarding the oxidation state of Group 15 elements (Nitrogen family) is the stability of +5 oxidation state decreases down the group.
• The stability of +5 oxidation state decreases down the group and that of +3 oxidation state increases, with BiF₅ being the only well-characterized Bi (V) compound. Hence, Option 1 is NOT correct while Option 2 is correct.
  • This is because of the 'inert-pair effect'. 
• Inert-pair effect is the tendency of the two s-electrons to not participate in bonding due to the high energy needed for unpairing them.
  • This phenomenon is seen in compounds of post-transition metals (Group 13, 14, 15, and 16).
• The tendency of -3 oxidation state decrease down the group.
• This happens due to the increase in size and metallic character of elements down the group.
• The last member of the Nitrogen family, Bismuth hardly forms any compound with -3 oxidation state.
• Nitrogen is restricted to maximum covalency of 4 because it has 4 orbitals (one s and three p) available for bonding.
• The most common oxidation states exhibited by the Group 15 elements is -3, +3, and +5.
• Nitrogen exhibits +1, +2, and +4 oxidation states also when it reacts with oxygen.
• Phosphorous exhibits +1 and +4 oxidation states in some oxoacids.

Explanation:

Phosphorus is quite reactive and forms a number of compounds in station states of -3, +3, and +5.

Phosphine (PH₃) is prepared by the reaction of calcium phosphide with water or dilute HCl

Properties

→ It is a colorless gas with a rotten fish smell and is highly poisonous.

→ It explodes in contact with traces of oxidizing agents like HNO₃, Cl₂, and Br₂ vapours.

→ It is slightly soluble in water.

→ The solution of PH₃ in water decomposes in presence of light giving red phosphorus and H₂ .

→ Phosphine is weakly basic and like ammonia, gives phosphonium compounds with acids

e.g., PH₃ + HBr →  PH₄Br

Additional Information

PH3, ASH3, NH3, SbH3 - these are the hydrides of group 15 elements.

One thing which is important to note is the hydrides of these elements are basic (Lewis Bases) in nature due to the presence of a lone pair of electrons.

Because electrons diffuse over a larger volume, basicity decreases with the size of the core atom.

The M-H bond decreases when the size of central atoms increases. (Where M can be any group 15 element - P, N, As, Bi, Sb)

As a result, as the size of hydrides grows larger as we move down the group, their stability declines and their decreasing character increases.

In simple words - The thermal stability of hydrides decreases as we move down the group. 

Hence, the order of their stability is - 

NH₃ > PH3 > AsH3 > SbH3