Class 12 Chemistry Chapter 7 The P Block Elements

We know P-Block Elements sounds scary at first, so many elements, groups, and reactions! But trust us, once you get the hang of it, it’s actually not that bad.

This chapter is full of interesting elements from Group 13 to Group 18. You’ll learn cool stuff like why nitrogen behaves differently, why chlorine forms so many compounds, and what makes noble gases so special.

No need to cram everything at once. Go step by step, and keep these p-block elements Class 12 notes with you during revision. You’ve got this, and yes, P-Block is totally scoring if you prepare smartly!

NCERT Chapter 7 The p-block Elements Class 12 Notes

The p-block includes elements from Group 13 to 18 of the periodic table. In this chapter, we focus on Group 15 to 18, which means:

• Group 15 (Nitrogen family)
• Group 16 (Oxygen family)
• Group 17 (Halogens)
• Group 18 (Noble gases)

These elements have their last electron in the p-orbital, and they show a lot of interesting behavior like variable oxidation states, covalent bonding, acidic and basic oxides, and more.

The p-block Elements Class 12 Notes: Group 15 - The Nitrogen Family

Let’s get familiar with the Group 15 elements, also known as the nitrogen family. These elements show a mix of non-metallic, metalloid, and metallic properties.

• General Properties of Group 15 Elements

This group includes Nitrogen (N), Phosphorus (P), Arsenic (As), Antimony (Sb), and Bismuth (Bi). These elements have 5 valence electrons (ns² np³) and show a mix of non-metallic, metalloid, and metallic characters as you go down the group. They form trivalent (−3) and pentavalent (+5) compounds.

• Physical and Chemical Properties

The atomic and ionic sizes increase down the group. Electronegativity and ionisation enthalpy decrease. They form covalent compounds, and their basic character increases from nitrogen to bismuth.

• Oxidation States and Trends

They mostly show −3, +3, and +5 oxidation states. The stability of the +5 state decreases, while +3 becomes more stable down the group due to the inert pair effect (reluctance of s-electrons to participate in bonding).

• Anomalous Behaviour of Nitrogen

Nitrogen is very different from others because it is small in size, has high electronegativity, and forms multiple bonds (like N≡N). It exists as diatomic gas (N₂) and has no d-orbitals, so it cannot expand its octet.

Now let us study some important compounds:

1. Ammonia (NH₃)

A colourless gas with a pungent smell, ammonia is basic in nature and forms hydrogen bonds. It is used in fertilizers and is prepared using the Haber process. 

Important Reaction:  Preparation of Ammonia (Haber's Process)

N₂ + 3H₂ ⇌ 2NH₃ ; ΔH = −92.4 kJ/mol

This is a reversible, exothermic reaction carried out at 450°C, 200 atm, with iron catalyst + molybdenum as a promoter.

Ammonia is basic and reacts with acids to form ammonium salts:

NH₃ + HCl → NH₄Cl

2. Nitric Acid (HNO₃)

A strong acid and powerful oxidising agent. It reacts with metals and non-metals, releasing nitrogen oxides. It’s used in explosives and fertilizers.

Nitric Acid (HNO₃) Preparation: Ostwald’s Process

1. 4NH₃ + 5O₂ → 4NO + 6H₂O
2. 2NO + O₂ → 2NO₂
3. 3NO₂ + H₂O → 2HNO₃ + NO

Nitric acid is a strong acid and a powerful oxidising agent. It reacts with copper as:

Cu + 4HNO₃ (conc.) → Cu(NO₃)₂ + 2NO₂ + 2H₂O

3. Oxides of Nitrogen

Nitrogen forms many oxides such as N₂O, NO, NO₂, N₂O₅, etc. They show different oxidation states ranging from +1 to +5 and are important in atmospheric chemistry and pollution.

4. Phosphorus - Allotropes and Important Compounds

Phosphorus exists in forms like white, red, and black phosphorus. White phosphorus is reactive and toxic, while red phosphorus is more stable and used in matchsticks.

5. Phosphine (PH₃)

A toxic and colourless gas with a rotten fish smell. It is less basic than ammonia and is formed by hydrolysis of metal phosphides.

It is prepared by hydrolysis of calcium phosphide:

Ca₃P₂ + 6H₂O → 2PH₃ + 3Ca(OH)₂

6. Phosphorus Halides

Phosphorus forms PX₃ and PX₅ types of halides (like PCl₃, PCl₅). They are reactive and used in making organic compounds.

Phosphorus reacts with halogens to form compounds like PCl₃ and PCl₅:
P₄ + 6Cl₂ → 4PCl₃
PCl₃ + Cl₂ → PCl₅

7. Oxyacids of Phosphorus

These include H₃PO₂ (hypophosphorous acid), H₃PO₃ (phosphorous acid), and H₃PO₄ (orthophosphoric acid). Their acidity depends on the number of OH groups attached to phosphorus.

Class 12 The p- block elements Notes: Group 16 - The Oxygen Family

Now, let’s move on to Group 16, known as the oxygen family or chalcogens. This group has elements with properties changing from non-metallic to metallic down the group.

• General Properties of Group 16 Elements

This group includes Oxygen (O), Sulphur (S), Selenium (Se), Tellurium (Te), and Polonium (Po). All have 6 valence electrons (ns² np⁴). Oxygen and sulphur are non-metals, selenium and tellurium are metalloids, and polonium is a metal.

• Physical and Chemical Properties

They show a gradual change in metallic character down the group. The melting and boiling points increase, and electronegativity decreases down the group. Oxygen is unique as it forms double bonds (O=O) and exists as diatomic O₂.

• Anomalous Behaviour of Oxygen

Oxygen is different because it is small, highly electronegative, and has a strong tendency to form hydrogen bonds. It does not form a +6 oxidation state like sulphur does.

• Oxidation States and Trends

Common oxidation states are −2, +2, +4, and +6. The +6 oxidation state is stable in sulphur (e.g., H₂SO₄), but not in oxygen.

Let us go through some important compounds.

1. Dioxygen (O₂)

This is the normal form of oxygen we breathe. It supports combustion and acts as an oxidising agent. It supports combustion and acts as an oxidant.

It is formed in labs by heating potassium chlorate:

2KClO₃ → 2KCl + 3O₂ (in presence of MnO₂)

2. Ozone (O₃)

A triatomic molecule of oxygen. It’s a strong oxidising agent and forms a protective layer in the stratosphere against UV rays.

It is formed when oxygen is passed through electric discharge:

3O₂ → 2O₃ (ΔH = +142 kJ/mol)

Ozone is a strong oxidising agent. It oxidises lead sulphide to lead sulphate:

PbS + 4O₃ → PbSO₄ + 4O₂

3. Sulphur - Allotropes

Two major forms – rhombic sulphur (stable at room temp) and monoclinic sulphur (stable at 369 K).These are different crystal forms with different structures and stabilities. 

4. Sulphur Dioxide (SO₂)

A pungent-smelling gas used as a bleaching agent and preservative. It is acidic and forms sulphurous acid in water. 

It is formed by burning sulphur:

S + O₂ → SO₂

SO₂ is a colourless gas with a choking smell. It is acidic and reacts with water:

SO₂ + H₂O → H₂SO₃

It is a reducing agent and bleaches coloured materials by reduction.

5. Oxyacids of Sulphur

Sulphur forms various oxyacids like H₂SO₃ (sulphurous acid) and H₂SO₄ (sulphuric acid). These acids are strong and have industrial importance.

• Sulphuric Acid (H₂SO₄)

A strong dibasic acid, used in car batteries, fertilizers, and chemical synthesis. It’s a good dehydrating and oxidising agent. 

Contact Process:

1. S + O₂ → SO₂
2. 2SO₂ + O₂ ⇌ 2SO₃ (V₂O₅ catalyst)
3. SO₃ + H₂O → H₂SO₄

H₂SO₄ is a strong dibasic acid, dehydrating and oxidising agent. It carbonises sugar:

C₁₂H₂₂O₁₁ + H₂SO₄ → 12C + 11H₂O

NCERT Class 12 The p-block Elements Notes: Group 17 - The Halogen Family

Next up is Group 17, known as the halogen family. The elements in this group are famous for forming salts and showing powerful oxidising behaviour.

• General Properties of Group 17 Elements

This group has Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At). These are highly reactive nonmetals, with 7 valence electrons (ns² np⁵). They exist as diatomic molecules (X₂).

• Physical and Chemical Properties

They have high electronegativity and form halide compounds. Fluorine is the most reactive. Their melting and boiling points increase down the group.

• Oxidation States and Trends

Common oxidation state is −1, but they also show +1, +3, +5, +7 in compounds. Fluorine only shows −1 oxidation state.

Let us learn more about Group 17 Elements in specific.

1. Anomalous Behaviour of Fluorine

Fluorine is unique due to its small size, high electronegativity, and no d-orbitals. It forms strong bonds and shows different behaviour from other halogens.

2. Hydrogen Halides

Formed by reacting halogens with hydrogen (like HF, HCl, HBr, HI). These are acidic in water and their strength increases from HF to HI.

Prepared by direct reaction with hydrogen:

H₂ + Cl₂ → 2HCl

They dissolve in water to give hydrohalic acids. Their acid strength increases down the group (HF < HCl < HBr < HI).

3. Interhalogen Compounds

Formed when two different halogens react, like ClF₃, BrF₅. These are more reactive than normal halogens and have polar covalent bonds. These are more reactive than pure halogens due to weaker X–X’ bonds.

4. Oxoacids of Halogens

These are acids where halogens are bonded to oxygen and hydrogen, like HClO, HClO₂, HClO₃, HClO₄. They are strong oxidising agents and used in bleaching and disinfection. Their acidity and oxidising strength increase with oxygen content.

CBSE Class 12 The p-block Elements Notes Group 18 - The Noble Gases

Finally, we have Group 18, known as the noble gases. These gases are chemically inert, colourless, and monoatomic due to their completely filled outer shells. 

• General Properties of Group 18 Elements

Includes Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn). These are inert gases, colourless, and monoatomic. They have completely filled orbitals (ns² np⁶), making them stable.

• Physical and Chemical Properties

They have very low reactivity, boiling points, and are non-polar. Their physical properties change slightly with increasing atomic number.

• Uses of Noble Gases

Used in lighting (Neon signs), helium for balloons and cooling, argon in welding, and Xenon in headlights and lasers.

• Formation of Compounds - Xenon Compounds

Even though noble gases are inert, Xenon forms compounds like XeF₂, XeF₄, and XeF₆ under special conditions. These are fluorides and show Xenon in +2, +4, +6 oxidation states.
Xe + F₂ → XeF₂
Xe + 2F₂ → XeF₄
Xe + 3F₂ → XeF₆

These compounds are hydrolysed by water:

XeF₄ + 2H₂O → XeO₃ + 4HF

Xenon also forms oxides like XeO₃, XeO₄, which are powerful oxidising agents.

• Oxides of Xenon

Xenon also forms XeO₃ and XeO₄, which are powerful oxidising agents and show Xenon in +6 and +8 oxidation states.

• Structure and Bonding in Xenon Compounds

They involve sp³d, sp³d², or sp³d³ hybridisation.

• XeF₂: Linear (sp³d)
• XeF₄: Square planar (sp³d²)

XeF₆: Distorted octahedral (sp³d³)

Conclusion

And that brings us to the end of The p-Block Elements. From nitrogen in the air to xenon in headlights, we’ve explored elements that are all around us and often show up in exams too!

If going through these notes made things even a little clearer, that’s a win worth celebrating. One more chapter checked off your list, and with that, you’re one step closer to being fully exam-ready! Keep the momentum going.

Q1. What are p-block elements in Class 12?
Ans. p-block elements are those where the last electron enters the p-orbital. They include groups 13 to 18 of the periodic table, like B, C, N, O, halogens, and noble gases.

Q2. Why are p-block elements important?
Ans. They’re super important because most of the everyday life elements (like oxygen, nitrogen, carbon, silicon, chlorine) belong here. They form the backbone of life, environment, and industry.

Q3. What are the main topics in p-block elements Class 12 notes?
Ans. Key topics include group-wise trends, important compounds like borax, ammonia, nitric acid, phosphorus, sulfur, halogens, noble gases, and their uses.

Q4. Which group is most important in p-block for exams?
Ans. Groups 15 (N family) and 16 (O family) are most asked in exams because of compounds like ammonia, nitric acid, phosphorus, sulfuric acid, etc.

Q5. What are the common properties of p-block elements?
Ans. They show variable oxidation states, form covalent bonds, and have both metals and non-metals. Reactivity trends also vary across groups.