Some p-Block Elements: Comprehensive NEET Chemistry Notes

1. Introduction to p-Block Elements

1.1 Overview

The p-block elements are located in groups 13 to 18 of the periodic table. Their valence shell electronic configuration is $n s^{2} n p^{1 - 6}$ (except Helium, which has $1 s^{2}$ ). The properties of p-block elements are significantly influenced by factors such as atomic size, ionisation enthalpy, electron gain enthalpy, and electronegativity. The absence of d-orbitals in the second period and the presence of d or d and f orbitals in heavier elements (starting from the third period onwards) also play a crucial role in determining their properties.

2. Group 15 Elements

2.1 Occurrence and Properties

Group 15 includes nitrogen, phosphorus, arsenic, antimony, and bismuth. As we move down the group, the elements exhibit a gradual shift from non-metallic to metallic character. Nitrogen and phosphorus are non-metals, arsenic and antimony are metalloids, and bismuth is a metal. Nitrogen makes up about 78% of the Earth's atmosphere by volume, while phosphorus is found in minerals such as fluorapatite.

2.2 Important Atomic and Physical Properties

Property	N	P	As	Sb	Bi
Atomic number	7	15	33	51	83
Atomic mass (g/mol)	14.01	30.97	74.92	121.75	208.98
Electron configuration	$[He] 2 s^{2} 2 p^{3}$	$[N e] 3 s^{2} 3 p^{3}$	$[A r] 3 d^{10} 4 s^{2} 4 p^{3}$	$[Kr] 4 d^{10} 5 s^{2} 5 p^{3}$	$[X e] 4 f^{14} 5 d^{10} 6 s^{2} 6 p^{3}$
Ionisation enthalpy (kJ/mol)	1402	1012	947	834	703
Electronegativity	3.0	2.1	2.0	1.9	1.9
Covalent radius (pm)	70	110	121	141	148
Ionic radius (pm)	171	212	222	76	103
Melting point (K)	63	317	1089	904	544
Boiling point (K)	77.2	554	888	1860	1837
Density (g/cm^3 at 298 K)	0.879	1.823	5.778	6.697	9.808

2.3 Chemical Properties

2.3.1 Oxidation States

The common oxidation states for Group 15 elements are -3, +3, and +5. The tendency to exhibit -3 oxidation state decreases down the group due to increased size and metallic character. Conversely, the stability of the +5 oxidation state decreases, and the +3 state becomes more stable due to the inert pair effect.

2.3.2 Reactivity towards Hydrogen

All Group 15 elements form hydrides of the type $E H_{3}$ (E = N, P, As, Sb, Bi). The stability of these hydrides decreases from $N H_{3}$ to $B i H_{3}$ .

2.3.3 Reactivity towards Oxygen

Group 15 elements form two types of oxides: $E_{2} O_{3}$ and $E_{2} O_{5}$ . The acidic character of these oxides decreases down the group.

2.3.4 Reactivity towards Halogens

These elements react with halogens to form two series of halides: $E X_{3}$ and $E X_{5}$ . Nitrogen does not form pentahalides due to the absence of d-orbitals.

Did You Know? Nitrogen exhibits a maximum covalency of 4 due to the availability of only four orbitals (one s and three p) for bonding, unlike the heavier elements that can expand their covalency by using d orbitals.

2.4 Important Compounds of Nitrogen

2.4.1 Ammonia ( $N H_{3}$ )

Preparation: Ammonia is prepared on a large scale by the Haber process: $N_{2} (g) + 3 H_{2} (g) \leftrightarrow 2 N H_{3} (g); Δ H = - 46.1, k J / m o l$
Properties: Ammonia is a colorless gas with a characteristic pungent odor. It is highly soluble in water and forms ammonium hydroxide: $N H_{3} + H_{2} O \leftrightarrow N H_{4}^{+} + O H^{-}$
Uses: Used in the manufacture of fertilizers, nitric acid, and as a refrigerant.

NEET Tip: Focus on the Haber process conditions (pressure, temperature, and catalysts) as they are frequently asked in NEET.

2.5 Practice Questions

Which of the following does not react with oxygen directly?
- a) Zn
- b) Ti
- c) Pt
- d) Fe
Complete the following reactions:
- a) $C_{2} H_{4} + O_{2} \to$
- b) $4 A l + 3 O_{2} \to$
Explain why $N H_{3}$ acts as a Lewis base.

Answers:

c) Pt
a) $2 C O_{2} + 2 H_{2} O$ b) $2 A l_{2} O_{3}$
$N H_{3}$ acts as a Lewis base because it has a lone pair of electrons on the nitrogen atom that can be donated.

Common Misconception: Students often confuse the stability of the oxidation states in Group 15 elements. Remember that the stability of the +3 oxidation state increases down the group due to the inert pair effect.

Quick Recap

Group 15 elements include nitrogen, phosphorus, arsenic, antimony, and bismuth.
Nitrogen exhibits a maximum covalency of 4, while other elements can expand their covalency.
The common oxidation states are -3, +3, and +5.
Important compounds include ammonia, dinitrogen, and oxides of nitrogen.

Concept Connection: Relate the properties of Group 15 elements with their position in the periodic table and their electron configurations.

3. Group 16 Elements

3.1 Overview and Properties

Group 16 elements include oxygen, sulfur, selenium, tellurium, and polonium. These elements are known as chalcogens. The valence shell electronic configuration is $n s^{2} n p^{4}$ .

3.2 Trends in Properties

3.2.1 Atomic and Ionic Radii

The atomic and ionic radii increase from oxygen to polonium due to the addition of extra electron shells.

3.2.2 Ionisation Enthalpy

Ionisation enthalpy decreases down the group due to the increase in atomic size.

3.2.3 Electronegativity

Electronegativity decreases down the group, with oxygen being the most electronegative element.

3.2.4 Melting and Boiling Points

The melting and boiling points increase with an increase in atomic number. Oxygen and sulfur are non-metals, selenium and tellurium are metalloids, and polonium is a metal.

Did You Know? Oxygen forms 46.6% by mass of the Earth's crust, making it the most abundant element on Earth.

3.3 Chemical Properties

3.3.1 Oxidation States

The elements exhibit oxidation states of -2, +2, +4, and +6. The stability of the +6 oxidation state decreases down the group, while the +4 state becomes more stable.

3.3.2 Reactivity towards Hydrogen

Group 16 elements form hydrides of the type $H_{2} E$ (E = O, S, Se, Te, Po). The acidic character of these hydrides increases from $H_{2} O$ to