JEE Mains Chemistry · Teaching notes

Classification of Elements and Periodicity — JEE Mains Chemistry

Classification of Elements and Periodicity has 117 past-year questions from 2021 to 2026, and only eight of them ask for a number. Almost every question is an order to check or a pair of statements to judge: rank a set of elements or ions by radius, ionization enthalpy, electron gain enthalpy or electronegativity, or say which oxide is acidic. The trends take a line each to state. The marks go on the exceptions, such as B below Be, O below N, Cl above F and Ga level with Al, so learn the values in the tables and not only the arrows.

Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.

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Subtopic notes

Formula & revision sheet

4 formulas · 13 reference tables · 25 gotchas across all subtopics — the exam-eve cheat-sheet

Periodic Law, Blocks and Position in the Table

Formulas (2)

Names for Z above 100 and the block from Z · Temporary IUPAC name $\text{name} = \text{root}(d_1) + \text{root}(d_2) + \text{root}(d_3) + \text{ium}$
Placing an element from its configuration or an ion · Recover Z from an ion $Z = e^{-} + q \qquad A = Z + n$

Reference tables (1)

From atomic weight to atomic number6 rows

Who and when	Sorted by	What they found
Döbereiner, 1829	Atomic weight	Triads such as Li, Na, K: the middle atomic weight is about the mean of the other two
Newlands, 1865	Atomic weight	Law of octaves: every eighth element repeats the first; it worked only up to calcium
Lothar Meyer, 1869	Atomic weight	Plotted atomic volume against atomic weight and saw a repeating curve
Mendeleev, 1869	Atomic weight	Left gaps and predicted eka-aluminium (Ga) and eka-silicon (Ge); reversed some pairs to keep families together
Moseley, 1913	Atomic number	A plot of $\sqrt{\nu}$ against $Z$ is a straight line, so $Z$ is the true basis Only Moseley's work uses atomic number. A statement giving Newlands or Meyer atomic numbers is false.
Modern table	Atomic number	18 groups and 7 periods; blocks s, p, d and f named by the subshell being filled

Everyone before 1913 used atomic weight; the modern law and table use atomic number.

Watch out for (4)

Twice the orbitals, not equal to them
→ From atomic weight to atomic number
Group, not period
→ From atomic weight to atomic number
Count the electrons before naming
→ Names for Z above 100 and the block from Z
Adding the charge the wrong way
→ Placing an element from its configuration or an ion

Atomic and Ionic Radii

Formulas (1)

Counting electrons to find isoelectronic species · Counting particles $e^{-} = Z - q \qquad n = A - Z$

Reference tables (2)

Atomic radius across a period and down a group5 rows

Series	Atomic radius (pm)	Trend
Period 2	Li 152, Be 111, B 88, C 77, N 74, O 66, F 64	Falls steadily across
Period 3	Na 186, Mg 160, Al 143, Si 117, P 110, S 104, Cl 99	Falls steadily across
Group 1	Li 152, Na 186, K 231, Rb 244, Cs 262	Rises down the group
Group 17	F 64, Cl 99, Br 114, I 133, At 140	Rises down the group
Period 4 ends	K 231, Br 114	Largest and smallest in period 4, noble gas excluded

Compare the columns: Be (111) is smaller than Mg (160) and Mg is larger than Al (143).

Ionic radius and isoelectronic series9 rows

Species	Protons	Electrons	Radius (pm)
Na atom	11	11	186
$\mathrm{Na^+}$	11	10	102
Cl atom	17	17	99
$\mathrm{Cl^-}$	17	18	184
$\mathrm{O^{2-}}$	8	10	140 Isoelectronic with $\mathrm{Mg^{2+}}$ (72 pm), yet about twice as large: the same electrons do not mean the same size.
$\mathrm{F^-}$	9	10	133
$\mathrm{Mg^{2+}}$	12	10	72
$\mathrm{Al^{3+}}$	13	10	53.5
$\mathrm{K^+}$	19	18	138

Within the 10-electron rows, every extra proton makes the ion smaller.

Watch out for (5)

Half the bond, not double
→ Atomic radius across a period and down a group
Down a group beats across a period
→ Atomic radius across a period and down a group
Isoelectronic does not mean the same size
→ Ionic radius and isoelectronic series
Isoelectronic ions have different nuclear charges
→ Ionic radius and isoelectronic series
A neutral atom is not its ion
→ Counting electrons to find isoelectronic species

Ionization Enthalpy

Formulas (1)

Successive ionization enthalpies and the energy for a mass · Energy to ionize a mass of gaseous atoms $E = \frac{m}{M}\,\left(\Delta_i H_1 + \Delta_i H_2 + \dots\right)$

Reference tables (2)

First ionization enthalpy across a period8 rows

Group	Period 2 (kJ mol⁻¹)	Period 3 (kJ mol⁻¹)	Why
1	Li 520	Na 496	One s electron outside a noble-gas core: lowest in the period
2	Be 899	Mg 737	Filled $ns^2$ subshell
13	B 801	Al 577	Dip: the lone $np$ electron is less penetrating Group 13 sits BELOW group 2. The option with a smooth rise (Be < B) is the trap.
14	C 1086	Si 786	Rises again
15	N 1402	P 1012	Half-filled $np^3$ : extra stable
16	O 1314	S 1000	Dip: pairing in $np^4$ adds repulsion Group 16 sits BELOW group 15: N > O and P > S.
17	F 1681	Cl 1256	Rises again
18	Ne 2080	Ar 1520	Filled shell: highest in the period

Read down each column for the period order; the two amber rows are the exceptions every question tests.

Down a group, and where it fails6 rows

Group	First ionization enthalpy (kJ mol⁻¹)	Order and exception
1	Li 520, Na 496, K 419, Rb 403, Cs 376	Steady fall
2	Be 899, Mg 737, Ca 590, Sr 549, Ba 503	Steady fall
13	B 801, Al 577, Ga 579, In 558, Tl 589	B > Tl > Ga > Al > In Ga is not below Al, and Tl is above both.
13, second	B 2427, Al 1816, Ga 1979, In 1820, Tl 1971	B > Ga > Tl > In > Al
14	C 1086, Si 786, Ge 761, Sn 708, Pb 715	C > Si > Ge > Pb > Sn Pb is above Sn.
18	He 2372, Ne 2080, Ar 1520, Kr 1351, Xe 1170, Rn 1037	Steady fall; Rn lowest

The exceptions appear only after a filled d or f subshell: from Ga, Tl and Pb onwards.

Watch out for (4)

The smooth order is the wrong option
→ First ionization enthalpy across a period
A simple fall down group 13 or 14 is wrong
→ Down a group, and where it fails
Negative or smaller second values
→ Successive ionization enthalpies and the energy for a mass
Second ionization compares the cations
→ Successive ionization enthalpies and the energy for a mass

Electron Gain Enthalpy

Reference tables (2)

When electron gain releases energy and when it costs energy8 rows

Atom	Electron gain enthalpy (kJ mol⁻¹)	Sign	Why
He	+48	Endothermic	Electron must enter the $2s$ shell
Ne	+116	Endothermic	Most positive noble gas; the electron enters $3s$
Ar	+96	Endothermic	Same value as Kr
Kr	+96	Endothermic	Same value as Ar
Xe	+77	Endothermic	Larger atom, smaller cost
Li	−60	Exothermic	Half-filled $2s$ takes a second s electron
Na	−53	Exothermic	Small but negative
Cl	−349	Exothermic	Most negative of all elements

The largest gap between two elements is Ne and Cl:

116 - (-349) = 465

kJ mol⁻¹.

Groups 16 and 17: why Cl beats F and S beats O4 rows

Group	Electron gain enthalpy (kJ mol⁻¹)	Order by magnitude
17	F −328, Cl −349, Br −325, I −295, At −270	Cl > F > Br > I > At F is second, not first. Cl is the most negative element in the table.
16	O −141, S −200, Se −195, Te −190, Po −174	S > Se > Te > Po > O O is the least negative in group 16, below even Po.
1	Li −60, Na −53, K −48, Rb −47, Cs −46	Li > Na > K > Rb ≈ Cs
Hydrogen	H −73	More negative than any alkali metal

Down each group the value becomes less negative, except that the first member of groups 16 and 17 is out of place.

Watch out for (4)

Neon, not helium, is the most positive
→ When electron gain releases energy and when it costs energy
Electron affinity flips the sign
→ When electron gain releases energy and when it costs energy
F is not the most negative
→ Groups 16 and 17: why Cl beats F and S beats O
Signed or magnitude
→ Groups 16 and 17: why Cl beats F and S beats O

Electronegativity and Metallic Character

Reference tables (3)

Electronegativity on the Pauling scale5 rows

Series	Pauling electronegativity	Trend
Period 2	Li 1.0, Be 1.5, B 2.0, C 2.5, N 3.0, O 3.5, F 4.0	Rises across
Period 3	Na 0.9, Mg 1.2, Al 1.5, Si 1.8, P 2.1, S 2.5, Cl 3.0	Rises across
Group 1	Li 1.0, Na 0.9, K 0.8, Rb 0.8, Cs 0.7	Falls down
Group 17	F 4.0, Cl 3.0, Br 2.8, I 2.5, At 2.2	Falls down
Group 13	B 2.0, Al 1.5, Ga 1.6, In 1.7, Tl 1.8	Falls from B to Al, then rises slightly Poor shielding by d and f electrons again: Ga, In and Tl are above Al.

Values from the Pauling scale; the trend, not the second decimal, is what the questions test.

Metallic character and reactivity6 rows

Compare	More metallic	Reason
Na and Mg	Na	Left of Mg in period 3
Mg and Al	Mg	Left of Al in period 3
Be and Mg	Mg	Below Be in group 2
K and Ca	K	Left of Ca in period 4
Be and Si	Be	Si is a metalloid; Be is a metal
N, P, O, S, Cl, F	P most, F least	P is lowest and furthest left; F is top right

Down and to the left means more metallic.

Metals, non-metals, metalloids and diagonal pairs9 rows

Element	Z	Group	Class
B	5	13	Metalloid
Si	14	14	Metalloid
Ge	32	14	Metalloid
As	33	15	Metalloid
Sb	51	15	Metalloid
Te	52	16	Metalloid
I	53	17	Non-metal
Bi	83	15	Metal
Pb	82	14	Metal

The metalloids run diagonally from B down to Te; Bi and Pb below them are metals.

Watch out for (3)

Electronegativity is not a constant
→ Electronegativity on the Pauling scale
Atomic radius is not always larger than ionic radius
→ Metallic character and reactivity
Bismuth is a metal
→ Metals, non-metals, metalloids and diagonal pairs

Nature of Oxides and Group 14 Trends

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

PYQ weightage by concept

17 concepts · 117 PYQs — where the marks actually sit, so you know what to drill first

Periodic Law, Blocks and Position in the Table14 PYQs · 12%

Concept	PYQs	Share
Placing an element from its configuration or an ion	6	5%
From atomic weight to atomic number	4	3%
Names for Z above 100 and the block from Z	4	3%

Atomic and Ionic Radii21 PYQs · 18%

Concept	PYQs	Share
Ionic radius and isoelectronic series	9	8%
Atomic radius across a period and down a group	7	6%
Counting electrons to find isoelectronic species	5	4%

Ionization Enthalpy27 PYQs · 23%

Concept	PYQs	Share
First ionization enthalpy across a period	15	13%
Successive ionization enthalpies and the energy for a mass	8	7%
Down a group, and where it fails	4	3%

Electron Gain Enthalpy16 PYQs · 14%

Concept	PYQs	Share
Groups 16 and 17: why Cl beats F and S beats O	10	9%
When electron gain releases energy and when it costs energy	6	5%

Electronegativity and Metallic Character19 PYQs · 16%

Concept	PYQs	Share
Metallic character and reactivity	8	7%
Electronegativity on the Pauling scale	6	5%
Metals, non-metals, metalloids and diagonal pairs	5	4%

Nature of Oxides and Group 14 Trends20 PYQs · 17%

Concept	PYQs	Share
Acidic, basic, amphoteric and neutral oxides	8	7%
Group 14: oxides, inert pair effect and bond strength	7	6%
Oxides across a period: basic to acidic	5	4%

Test yourself on Classification of Elements and Periodicity

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Periodic Law, Blocks and Position in the Table

Atomic and Ionic Radii

Ionization Enthalpy

Electron Gain Enthalpy

Electronegativity and Metallic Character

Nature of Oxides and Group 14 Trends

Formula & revision sheet

PYQ weightage by concept

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