Problem 21 - Chemistry in 30 days

Question

The basic character of the hydrides of \(\mathrm{V}\) group elements decreases in the order (a) \(\mathrm{NH}_{3}>\mathrm{PH}_{3}>\mathrm{AsH}_{3}>\mathrm{SbH}_{3}\) (b) \(\mathrm{SbH}_{3}>\mathrm{AsH}_{3}>\mathrm{PH}_{3}>\mathrm{NH}_{3}\) (c) \(\mathrm{NH}_{3}>\mathrm{SbH}_{3}>\mathrm{PH}_{3}>\mathrm{AsH}_{3}\) (d) \(\mathrm{SbH}_{3}>\mathrm{PH}_{3}>\mathrm{AsH}_{3}>\mathrm{NH}_{3}\)

Step-by-Step Solution

Answer

The correct option is (a) NH_3 > PH_3 > AsH_3 > SbH_3.

Step 1: Identify the Hydrides of Group V

In group V of the periodic table, the elements include nitrogen (N), phosphorus (P), arsenic (As), and antimony (Sb). The hydrides of these elements are ammonia (NH_3), phosphine (PH_3), arsine (AsH_3), and stibine (SbH_3).

Step 2: Recognize the Trend Across the Group

As we move down the group from nitrogen to antimony, the electronegativity decreases, this means the ability to donate a lone pair of electrons for forming a bond (basicity) decreases. As the electronegativity decreases, the hydride's basic character also decreases.

Step 3: Apply the Trend to Determine the Correct Order

Based on the decrease in electronegativity and thus basicity down the group, the correct decreasing order of basic character for the hydrides is: NH_3 > PH_3 > AsH_3 > SbH_3.

Step 4: Match the Determined Sequence to the Options

Compare the determined order of basicity from Step 3 with the given options: (a) NH_3 > PH_3 > AsH_3 > SbH_3 The correct sequence matches option (a).

Key Concepts

Basicity of Hydrides

Hydrides are compounds formed by the combination of hydrogen with another element. When considering the basicity of hydrides, we mainly focus on how readily these hydrides can donate a pair of electrons. The ability to donate electrons directly relates to the compound's basic nature.

In the case of Group 15 elements, which include nitrogen, phosphorus, arsenic, and antimony, each possesses a hydride: ammonia (\(\mathrm{NH_3}\), phosphine (\(\mathrm{PH_3}\), arsine (\(\mathrm{AsH_3}\), and stibine (\(\mathrm{SbH_3}\). When these hydrides dissolve in water, they can release hydrogen ions, therefore, acting as bases. The basicity is strongest in ammonia and weakest in stibine.

The reason lies in their ability to successfully donate lone pairs of electrons. Ammonia is very effective at this due to its higher electronegativity and smaller size, making it a strong base compared to the others. As you move down the group to stibine, the basicity decreases as these elements have larger atomic radii and lower electronegativity, leading to weaker lone pair donation.

Periodic Trends

Periodic trends refer to patterns observed in the properties of elements in the periodic table. These trends allow us to predict and explain the physical and chemical properties of elements. Understanding these trends is crucial for explaining why certain sequences occur in the periodic table.

In Group 15, a key trend to note is that as we move down the group, elements display decreasing electronegativity. This is because the outer electrons are more shielded by inner electrons, making it harder for the nucleus to attract electrons. This reduced attraction results in larger atomic sizes as we move down the group.

• **Electronegativity decreases** as you move down the group, affecting the ability to donate lone pairs.
• **Atomic size** increases, impacting various physical properties, including ionization energy and melting points.

These trends profoundly affect chemical reactivity, including the basic nature of hydrides, as explained in the other sections.

Electronegativity

Electronegativity is the measure of an atom's ability to attract and bind with electrons. This property plays a pivotal role in determining molecular behavior and characteristics. In simpler terms, it's all about how "hungry" an atom is for electrons.

For Group 15 elements, electronegativity decreases as you move down from nitrogen to antimony. Nitrogen sits at the top of the table with the highest electronegativity, making its hydride, ammonia (\(\mathrm{NH_3}\)), the most effective at donating lone pairs and thus, the most basic. On the contrary, antimony, with the lowest electronegativity, forms stibine (\(\mathrm{SbH_3}\)), which has the least tendency to act as a base.

• High electronegativity means stronger pull on electrons, leading to greater basicity.
• Decreasing electronegativity leads to weaker electron pair donation ability.

In essence, understanding electronegativity helps explain why certain elements form stronger or weaker acids or bases, impacting their chemical behavior significantly.