JEE Mains Chemistry · Classification of Elements and Periodicity

Nature of Oxides and Group 14 Trends

Oxides turn from basic to amphoteric to acidic across a period, a few oxides of nitrogen and carbon are neutral, and in group 14 the oxides grow more basic and the lower oxidation state more stable going down.

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Why this matters

Twenty PYQs, seventeen of them multiple choice, and five from 2026. Five follow the change in oxide nature across a period or what an oxide gives with water; eight classify a list of oxides as acidic, basic, amphoteric or neutral, often as a count; seven test group 14, from its oxides to the inert pair effect and bond enthalpies.

Concept 1 of 3: Oxides across a period: basic to acidic

A metal oxide gives a hydroxide with water, so it is basic. A non-metal oxide gives an oxoacid, so it is acidic. Across a period the elements turn from metals into non-metals, and their oxides turn from basic, through amphoteric in the middle, to acidic.

Definition

Extreme left (group 1): strongly basic oxides. Extreme right (group 17): strongly acidic oxides.
Middle: amphoteric (reacts with both acids and bases) or neutral (reacts with neither).
Basic strength falls across and rises down: $\mathrm{K_2O > Na_2O > MgO > Al_2O_3}$ .
A higher oxidation state gives a more acidic oxide.

Oxide	Nature	With water
$\mathrm{Na_2O}$	Strongly basic	$\mathrm{Na_2O + H_2O \to 2NaOH}$
$\mathrm{MgO}$	Basic	Forms $\mathrm{Mg(OH)_2}$ , sparingly soluble
$\mathrm{Al_2O_3}$	Amphoteric	Insoluble; dissolves in both acids and alkalis
$\mathrm{SiO_2}$	Acidic	Insoluble; reacts with hot NaOH to give a silicate
$\mathrm{P_4O_{10}}$	Acidic	$\mathrm{P_4O_{10} + 6H_2O \to 4H_3PO_4}$
$\mathrm{SO_3}$	Acidic	$\mathrm{SO_3 + H_2O \to H_2SO_4}$
$\mathrm{Cl_2O_7}$	Strongly acidic	$\mathrm{Cl_2O_7 + H_2O \to 2HClO_4}$

Period 3 oxides in their highest oxidation states, from sodium to chlorine.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2025 · 22 Jan 2025 · Q114Moderate

Example 1 · Classification of Elements and Periodicity · Nature of Oxides and Group 14 Trends

Given below are two statements : Statement (I) : An element in the extreme left of the periodic table forms acidic oxides. Statement (II) : Acid is formed during the reaction between water and oxide of a reactive element present in the extreme right of the periodic table. In the light of the above statements, choose the correct answer from the options given below :

CO is not acidic

\mathrm{CO_2}

is acidic, but CO is neutral: it forms no acid with water and does not react with alkalis. Do not carry the nature of one oxide of an element over to another.

Drill 4 more on oxides across a period: basic to acidic

Concept 2 of 3: Acidic, basic, amphoteric and neutral oxides

Most counting questions are settled by a short list. Learn the three neutral oxides and the common amphoteric ones; almost everything else is acidic if it comes from a non-metal or a metal in a high oxidation state, and basic if it comes from a metal in a low one.

Definition

Neutral: CO, NO, $\mathrm{N_2O}$ . Only these three.
Amphoteric: $\mathrm{Al_2O_3}$ , $\mathrm{As_2O_3}$ , $\mathrm{Cr_2O_3}$ , BeO, ZnO, SnO, $\mathrm{SnO_2}$ , PbO, $\mathrm{PbO_2}$ , $\mathrm{V_2O_5}$ .
Acidic: $\mathrm{CO_2}$ , $\mathrm{SiO_2}$ , $\mathrm{B_2O_3}$ , $\mathrm{N_2O_3}$ , $\mathrm{NO_2}$ , $\mathrm{N_2O_5}$ , $\mathrm{P_4O_{10}}$ , $\mathrm{SO_2}$ , $\mathrm{SO_3}$ , $\mathrm{Cl_2O_7}$ , $\mathrm{Mn_2O_7}$ , $\mathrm{CrO_3}$ .
Basic: $\mathrm{Na_2O}$ , $\mathrm{K_2O}$ , MgO, CaO, BaO, $\mathrm{V_2O_3}$ , CrO.
Hydroxides follow their oxides: $\mathrm{Be(OH)_2}$ and $\mathrm{Al(OH)_3}$ are amphoteric, $\mathrm{B(OH)_3}$ is acidic, NaOH and $\mathrm{Ca(OH)_2}$ are basic.

Element	Low oxidation state oxide	High oxidation state oxide
Nitrogen	$\mathrm{N_2O}$ , NO: neutral	$\mathrm{N_2O_3}$ , $\mathrm{NO_2}$ , $\mathrm{N_2O_5}$ : acidic
Carbon	CO: neutral	$\mathrm{CO_2}$ : acidic
Vanadium	$\mathrm{V_2O_3}$ : basic	$\mathrm{V_2O_5}$ : amphoteric
Chromium	CrO: basic; $\mathrm{Cr_2O_3}$ : amphoteric	$\mathrm{CrO_3}$ : acidic
Manganese	MnO: basic	$\mathrm{Mn_2O_7}$ : acidic
Sulphur	$\mathrm{SO_2}$ : acidic	$\mathrm{SO_3}$ : more strongly acidic

For one element, the higher the oxidation state, the more acidic the oxide.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2022 · 25 July 2022 · Q130Moderate

Example 2 · Classification of Elements and Periodicity · Nature of Oxides and Group 14 Trends

The total number of acidic oxides from the following list is:

NO,N_{2}O,B_{2}O_{3},{\text{ }N}_{2}O_{5},CO

SO_{3},P_{4}O_{10}

Not every nitrogen oxide is acidic

\mathrm{N_2O}

and NO are neutral, while

\mathrm{N_2O_3}

\mathrm{NO_2}

and

\mathrm{N_2O_5}

are acidic. In a counting question, sort the nitrogen oxides one by one.

NO is neutral, not amphoteric

\mathrm{Al_2O_3}

is amphoteric, but NO reacts with neither acids nor bases. A statement that calls both amphoteric is false.

Drill 7 more on acidic, basic, amphoteric and neutral oxides

Concept 3 of 3: Group 14: oxides, inert pair effect and bond strength

Group 14 runs from a non-metal (C) through two metalloids (Si, Ge) to two metals (Sn, Pb), so its oxides run from acidic to amphoteric. Going down, the two s electrons are held back more and more (the inert pair effect), so +2 becomes more stable than +4 at lead.

Definition

Dioxides: $\mathrm{CO_2}$ , $\mathrm{SiO_2}$ , $\mathrm{GeO_2}$ acidic; $\mathrm{SnO_2}$ , $\mathrm{PbO_2}$ amphoteric. $\mathrm{CO_2}$ is the most acidic.
Monoxides: CO neutral, GeO acidic, SnO and PbO amphoteric.
Inert pair effect: $\mathrm{Pb^{2+}}$ is more stable than $\mathrm{Pb^{4+}}$ , so $\mathrm{PbO_2}$ is an oxidant; $\mathrm{Sn^{4+}}$ is more stable than $\mathrm{Sn^{2+}}$ , so $\mathrm{SnCl_2}$ is a reductant.
Bond enthalpy falls down the group: C–C 348, Si–Si 297, Ge–Ge 260, Sn–Sn 240 kJ mol⁻¹.
Carbon: $^{12}\mathrm{C}$ and $^{13}\mathrm{C}$ are stable, $^{14}\mathrm{C}$ is radioactive; carbon cannot exceed a covalency of four, having no d orbitals; it shows −4 to +4.

Element	Class	Oxides	Electronegativity	Melting point (K)
C	Non-metal	CO neutral, $\mathrm{CO_2}$ acidic	2.5	4373
Si	Metalloid	$\mathrm{SiO_2}$ acidic	1.8	1693
Ge	Metalloid	GeO and $\mathrm{GeO_2}$ acidic	1.8	1218
Sn	Metal	SnO and $\mathrm{SnO_2}$ amphoteric	1.8	505 Tin has the lowest melting point in the group, below lead.
Pb	Metal	PbO and $\mathrm{PbO_2}$ amphoteric	1.9	600

Electronegativity does not fall steadily from Si to Pb: it stays at 1.8 and then rises to 1.9.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2024 · 31 January 2024 · Q49Moderate

Example 3 · Classification of Elements and Periodicity · Nature of Oxides and Group 14 Trends

Consider the oxides of group 14 elements

SiO_{2},GeO_{2},SnO_{2},PbO_{2},CO

and

GeO

. The amphoteric oxides are

GeO is not amphoteric

SnO and PbO are amphoteric, and it is tempting to extend that to GeO. Germanium's monoxide is distinctly acidic, like its dioxide.

Lowest melting point is tin, not lead

Lead is lower in the group, but tin melts at 505 K and lead at 600 K. The melting points fall from C to Sn and rise again at Pb.

Drill 6 more on group 14: oxides, inert pair effect and bond strength

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (3)

Oxides across a period: basic to acidic7 rows

Oxide	Nature	With water
$\mathrm{Na_2O}$	Strongly basic	$\mathrm{Na_2O + H_2O \to 2NaOH}$
$\mathrm{MgO}$	Basic	Forms $\mathrm{Mg(OH)_2}$ , sparingly soluble
$\mathrm{Al_2O_3}$	Amphoteric	Insoluble; dissolves in both acids and alkalis
$\mathrm{SiO_2}$	Acidic	Insoluble; reacts with hot NaOH to give a silicate
$\mathrm{P_4O_{10}}$	Acidic	$\mathrm{P_4O_{10} + 6H_2O \to 4H_3PO_4}$
$\mathrm{SO_3}$	Acidic	$\mathrm{SO_3 + H_2O \to H_2SO_4}$
$\mathrm{Cl_2O_7}$	Strongly acidic	$\mathrm{Cl_2O_7 + H_2O \to 2HClO_4}$

Period 3 oxides in their highest oxidation states, from sodium to chlorine.

Acidic, basic, amphoteric and neutral oxides6 rows

Element	Low oxidation state oxide	High oxidation state oxide
Nitrogen	$\mathrm{N_2O}$ , NO: neutral	$\mathrm{N_2O_3}$ , $\mathrm{NO_2}$ , $\mathrm{N_2O_5}$ : acidic
Carbon	CO: neutral	$\mathrm{CO_2}$ : acidic
Vanadium	$\mathrm{V_2O_3}$ : basic	$\mathrm{V_2O_5}$ : amphoteric
Chromium	CrO: basic; $\mathrm{Cr_2O_3}$ : amphoteric	$\mathrm{CrO_3}$ : acidic
Manganese	MnO: basic	$\mathrm{Mn_2O_7}$ : acidic
Sulphur	$\mathrm{SO_2}$ : acidic	$\mathrm{SO_3}$ : more strongly acidic

For one element, the higher the oxidation state, the more acidic the oxide.

Group 14: oxides, inert pair effect and bond strength5 rows

Element	Class	Oxides	Electronegativity	Melting point (K)
C	Non-metal	CO neutral, $\mathrm{CO_2}$ acidic	2.5	4373
Si	Metalloid	$\mathrm{SiO_2}$ acidic	1.8	1693
Ge	Metalloid	GeO and $\mathrm{GeO_2}$ acidic	1.8	1218
Sn	Metal	SnO and $\mathrm{SnO_2}$ amphoteric	1.8	505 Tin has the lowest melting point in the group, below lead.
Pb	Metal	PbO and $\mathrm{PbO_2}$ amphoteric	1.9	600

Electronegativity does not fall steadily from Si to Pb: it stays at 1.8 and then rises to 1.9.

Watch out for (5)

CO is not acidic
→ Oxides across a period: basic to acidic
Not every nitrogen oxide is acidic
→ Acidic, basic, amphoteric and neutral oxides
NO is neutral, not amphoteric
→ Acidic, basic, amphoteric and neutral oxides
GeO is not amphoteric
→ Group 14: oxides, inert pair effect and bond strength
Lowest melting point is tin, not lead
→ Group 14: oxides, inert pair effect and bond strength

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