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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!
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:
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.
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.
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.
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.
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).
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.
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 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.
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.
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.
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.
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.
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.
A strong dibasic acid, used in car batteries, fertilizers, and chemical synthesis. It’s a good dehydrating and oxidising agent.
Contact Process:
H₂SO₄ is a strong dibasic acid, dehydrating and oxidising agent. It carbonises sugar:
C₁₂H₂₂O₁₁ + H₂SO₄ → 12C + 11H₂O
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.
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).
They have high electronegativity and form halide compounds. Fluorine is the most reactive. Their melting and boiling points increase down the group.
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.
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.
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.
They have very low reactivity, boiling points, and are non-polar. Their physical properties change slightly with increasing atomic number.
Used in lighting (Neon signs), helium for balloons and cooling, argon in welding, and Xenon in headlights and lasers.
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.
Xenon also forms XeO₃ and XeO₄, which are powerful oxidising agents and show Xenon in +6 and +8 oxidation states.
They involve sp³d, sp³d², or sp³d³ hybridisation.
XeF₆: Distorted octahedral (sp³d³)
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.
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