Problem 88 - A Complete Resource Book in Chemistry for JEE Main

Question

What is the order of basic nature of hydrides of group V elements? (a) \(\mathrm{AsH}_{3}>\mathrm{SbH}_{3}>\mathrm{PH}_{3}>\mathrm{NH}_{3}\) (b) \(\mathrm{NH}_{3}^{3}>\mathrm{SbH}_{3}>\mathrm{PH}_{3}^{3}>\mathrm{AsH}_{3}^{3}\) (c) \(\mathrm{NH}_{3}>\mathrm{PH}_{3}>\mathrm{AsH}_{3}>\mathrm{SbH}_{3}\) (d) \(\mathrm{PH}_{3}>\mathrm{NH}_{3}>\mathrm{SbH}_{3}>\mathrm{AsH}_{3}\)

Step-by-Step Solution

Answer

The correct order is (c)  >  >  > .

Step 1: Understand Basic Nature

The basic nature of hydrides is determined by their ability to donate a lone pair of electrons. The stronger the ability to donate a lone pair, the stronger the base.

Step 2: Review Electron Pair Donation

In Group V elements, as we move down the group (Nitrogen, Phosphorus, Arsenic, Antimony), the ability to donate a lone pair decreases due to the increasing size of the central atom and decrease in electronegativity.

Step 3: Analyze Electronegativity and Size

Nitrogen, being the smallest and most electronegative, can donate its electron pair most effectively, making ammonia () the most basic. The larger atoms as we go down the group are less effective at this.

Step 4: Determine Order of Basicity

Arranging from most to least basic: 1.  2.  3.  4.  Hence, the order is  >  >  > .

Key Concepts

Understanding the Basic Nature of Hydrides

When we talk about the basic nature of hydrides in Group V elements, we're referring to their ability to donate electrons. This quality is critical in determining how basic these compounds are. The basicity of a hydride emerges from its ability to donate a lone pair of electrons, which are loosely associated with the hydrogen atom due to the hydride's electron configuration.
In general, the stronger the tendency of a hydride to donate a lone pair of electrons, the stronger the base. This is because the donation of electrons facilitates reactions with acids, affirming the hydride's basic character. So, a good basic hydride readily shares its electrons, indicating strong basic properties.

• The basicity is largely determined by the element's propensity to donate its lone electron pair.
• Nitrogen, in ammonia (\( \mathrm{NH}_3 \)), is the most capable of this donation, making it very basic.
• As you go down Group V, this ability diminishes, resulting in weaker bases.

Lone Pair Donation in Group V Hydrides

Lone pair donation is a vital concept when discussing hydrides' basic nature. For Group V elements, the ability of these hydrides to donate electron pairs is dependent on their structure. The presence of a lone pair on the central atom makes these hydrides potentially decent bases.
Moving down Group V, the capacity to donate these electron pairs decreases. This is due to two main factors: the increasing atomic size and decreasing electronegativity.

• Lone pair donation potential is higher in smaller atoms with high electronegativity because such atoms hold electron pairs more closely.
• Nitrogen, being the smallest and most electronegative in the group, has a robust ability to donate its lone pair, making ammonia a strong base.
• Larger atoms like arsenic and antimony have diffused electron clouds, making them less effective in electron pair donation, and hence weaker bases.

Impact of Electronegativity and Atomic Size

Two primary factors influence the basic nature of Group V hydrides: electronegativity and atomic size. Both characteristics directly affect a hydride's ability to donate its lone pairs, hence impacting its basic nature.
Electronegativity refers to how strongly an atom can attract electrons. In Group V, nitrogen is the most electronegative, which makes it more effective at retaining and donating its lone pair. This is a pivotal reason why ammonia is the most basic hydride among its Group V peers.

• As electronegativity decreases down the group, the atoms become less effective at donating electron pairs.
• Atomic size increases as we move down the group, making the electron cloud more diffused and weakening its hold on the lone pair.
• This results in decreased basicity from ammonia to phosphine (\( \mathrm{PH}_3 \)), and further down to arsine (\( \mathrm{AsH}_3 \)) and stibine (\( \mathrm{SbH}_3 \)).