NEET ChemistryUpdated 19 Oct 2026

Classification of Elements and Periodicity: NEET PYQs and weightage

Classification of Elements and Periodicity carries 3.1 of 100 Chemistry marks in NEET by past-paper share: 22 of the 707 tagged NEET Chemistry PYQs in the ViJEEta bank (NEET 2013–2026). It ranks 17 of 26 Chemistry chapters.

Take the 25-question Classification of Elements and Periodicity test Key facts ↓All NEET Chemistry chapters

3.1/100 Weight in Chemistry

Exam importance (3/5, by past-paper share)

22 NEET PYQs

448 Practice questions

368 With measured difficulty

How the weight is computed: this chapter's tagged NEET PYQs divided by all tagged NEET Chemistry PYQs in the bank (22 of 707), times 100, with repeated questions counted once. The bank does not store the year of each NEET question, so none is printed. Computed from the bank at build time. Updated 19 Oct 2026.

Classification of Elements and Periodicity: key facts for NEET

16 facts the paper keeps asking, each with how it is asked. Copy it onto one page of your notebook: that page is this chapter's handbook.

Dobereiner's Triads (1829)
In a triad, the middle element's atomic weight is about the average of the other two (Li 7, Na 23, K 39; Cl, Br, I).
Asked as: Which set is a Dobereiner triad, or find the middle element's atomic weight.
Newlands' Law of Octaves (1865)
Every eighth element resembles the first; it held only up to calcium.
Asked as: Up to which element did the Law of Octaves work? (Ca)
Mendeleev's Periodic Law
Properties are a periodic function of atomic weight. He put I after Te despite its lower weight and left gaps for Eka-aluminium (Ga) and Eka-silicon (Ge).
Asked as: Eka-silicon is? (Ge), or which element pair was placed out of weight order (Te–I).
Moseley (1913) and Modern Periodic Law
$ν$ plotted against $Z$ gives a straight line. Modern Periodic Law: properties are periodic functions of atomic number.
Asked as: The basis of the modern periodic table is atomic number, not atomic mass.
Long form of the periodic table
7 periods, 18 groups. Period number $= n$ of the valence shell. Periods 1 to 6 hold 2, 8, 8, 18, 18, 32 elements.
Asked as: Period number indicates which quantum number ( $n$ ), or why period 6 has 32 elements.
IUPAC names for Z > 100
Names for $Z > 100$ use roots nil(0) un(1) bi(2) tri(3) quad(4) pent(5) hex(6) sept(7) oct(8) enn(9) + "ium". $Z = 120$ is Unbinilium (Ubn).
Asked as: IUPAC name or symbol of a given $Z$ , or the official name ( $106$ = Seaborgium, $118$ = Oganesson).
Block configurations
d-block is $(n - 1) d^{1 - 10} n s^{0 - 2}$ (Pd is $4 d^{10} 5 s^{0}$ ). f-block is $(n - 2) f^{1 - 14} (n - 1) d^{0 - 1} n s^{2}$ . Lanthanoids are Ce(58)–Lu(71); actinoids are Th(90)–Lr(103) and are radioactive.
Asked as: Assign group, period and block from a given outer configuration.
Exceptions in placement
He ( $1 s^{2}$ ) is s-block but sits in group 18. H is placed separately because it acts like both group 1 and group 17. Zn, Cd and Hg ( $(n - 1) d^{10} n s^{2}$ ) lack most transition properties.
Asked as: Which d-block elements are not typical transition elements?
Metals, non-metals and metalloids
Metals are more than $78%$ of known elements. Hg is a liquid; Ga ( $303 K$ ) and Cs ( $302 K$ ) melt very low. Metalloids include Si, Ge, As, Sb and Te.
Asked as: Pick the metalloid, or order metallic character ( $P < Si < Be < Mg < Na$ ).
Atomic radius
Atomic radius decreases across a period and increases down a group. Covalent radius of Cl is $99 pm$ (half of $198$ ); metallic radius of Cu is $128 pm$ . Noble gases use the larger van der Waals radius.
Asked as: Arrange atoms by size, or why noble gas radii look anomalously large.
Ionic radius
A cation is smaller than its atom (Na $186 \to N a^{+}$ $95 pm$ ); an anion is larger (F $64 \to F^{-}$ $136 pm$ ). In isoelectronic species, more nuclear charge means a smaller ion.
Asked as: Order $N^{3 -} > O^{2 -} > F^{-} > N a^{+} > M g^{2 +} > A l^{3 +}$ ; largest/smallest among Mg, $M g^{2 +}$ , Al, $A l^{3 +}$ .
Ionization enthalpy
Ionization enthalpy is always positive, and $I E_{2} > I E_{1}$ . It rises across a period (max at noble gases) and falls down a group (min at alkali metals).
Asked as: Na has lower $I E_{1}$ but higher $I E_{2}$ than Mg, or spot the big jump that marks the end of valence electrons.
IE anomalies
B < Be (a $2 p$ electron is more shielded than a $2 s$ ), and O < N (paired $2 p$ electrons repel). Period 2 order: $Li < B < Be < C < O < N < F < Ne$ .
Asked as: Correct IE order for period 2, or IE of Al is closer to $575$ than $760 kJ/mol$ .
Electron gain enthalpy
Electron gain enthalpy is most negative for halogens and positive for noble gases (Ne $+ 116$ ). Cl ( $- 349$ ) is more negative than F ( $- 328$ ), and S more than O, because the small $n = 2$ shell causes repulsion.
Asked as: Most negative $Δ_{e g} H$ among P, S, Cl, F (Cl) and least (P).
Electronegativity
Electronegativity (Pauling, F $= 4.0$ ) cannot be measured and is not constant for an element. It rises across a period and falls down a group, tracking non-metallic character.
Asked as: Difference between electron gain enthalpy and electronegativity; is N always $3.0$ ?
Anomalous second period, diagonal pairs and oxides
Second-period elements are anomalous: small size, maximum covalency 4 ( $B F_{4}^{-}$ but $Al F_{6}^{3 -}$ ), and $p π$ – $p π$ multiple bonds. Diagonal pairs are Li–Mg and Be–Al. Oxides are basic on the left ( $N a_{2} O$ ), acidic on the right ( $C l_{2} O_{7}$ ), and amphoteric ( $A l_{2} O_{3}$ , $A s_{2} O_{3}$ ) or neutral ( $CO$ , $NO$ , $N_{2} O$ ) in the centre.
Asked as: Pick the amphoteric/neutral oxide, or name the diagonal partner; Al in $[AlCl (H_{2} O)_{5}]^{2 +}$ has oxidation state $+ 3$ , covalency $6$ .

Trap

Fluorine is the most electronegative element, but chlorine has the most negative electron gain enthalpy. The extra electron in tiny F suffers more repulsion.

How does NEET ask Classification of Elements and Periodicity?

8 real previous-year questions from this chapter, as printed in the paper, each with its worked solution. Answer the first one here with no sign-in.

Q1ChemistryNEET UG PYQIonization Enthalpy

Try right here · No sign-in

For the second period elements the correct increasing order of first ionisation enthalpy is :

Answer key (A) & worked solution

Correct answer: Option A

'Be' and 'N' have comparatively more stable valence sub-shell than 'B' and 'O'. Correct order of first ionisation enthalpy is : Li < B < Be < C < O < N < F < Ne

Q2ChemistryNEET UG PYQMiscellaneous Examples

Identify the incorrect statement from the following :

AThe IUPAC name of the element with atomic number 107 is Unnilseptium.

BThe largest and the smallest species among

Mg, Mg^{2 +}, Al

and

Al^{3 +}

are Al and

Mg^{2 +}

respectively.

CThe similarity in behaviour of Li with Mg is referred to as 'diagonal relationship'

DThe oxidation state and covalency of Al in

[AlCl (H_{2} O)_{5}]^{2 +}

are 3 and 6, respectively.

Answer key (B) & worked solution

Correct answer: Option B

The largest species is Mg and the smallest one is

Al^{3 +}

among

Mg, Mg^{2 +}, Al

and

Al^{3 +}

. - Unnilseptium element has atomic number 107. -

Li

and Mg are diagonally related and have similar properties. - The oxidation state and covalency of AI in

[AlCl (H_{2} O)_{5}]^{2 +}

are +3 and 6 respectively. ∴ Statement B is incorrect.

Q3ChemistryNEET UG PYQModern Periodic Table

The lUPAC name of an element with atomic number 119 is

Aununennium

Bunnilennium

Cunununnium

Dununoctium

Answer key (A) & worked solution

Correct answer: Option A

IUPAC name of an element with atomic number 119 is ununennium

Q4ChemistryNEET UG PYQAtomic and Ionic Radii

A metal has a fcc lattice. The edge length of the unit cell is 404 pm. The density of the metal is 2.72 g cm

^{- 3}

. The molar mass of the metal is (N

_{A}

Avogadro’s constant = 6.02 × 10

^{23}

mol

^{- 1}

)

A40 g mol

^{- 1}

B30 g mol

^{- 1}

C27 g mol

^{- 1}

D20 g mol

^{- 1}

Answer key (C) & worked solution

Correct answer: Option C

ρ = \frac{Z \times M}{N A \times V}

2.72 = \frac{4 \times M}{6.023 \times 10 ^{23} \times ( 4.04 \times 10 ^{- 8} ) ^{3}}

M = \frac{2.72 \times ( 4.04 ) ^{3} \times 6.023 \times 10 ^{- 1}}{4} = 27 g / mol

Q5ChemistryNEET UG PYQAtomic and Ionic Radii

Which of the following statements are true? A. Unlike Ga that has a very high melting point, Cs has a very low melting point B. On Pauling scale, the electronegativity values of N and Cl are not the same. C. Ar, K

^{+}

, Cl–, Ca

^{2 +}

, and S2– are all isoelectronic species. D. The correct order of the first ionization enthalpies of Na, Mg, Al, and Si is Si > Al > Mg > Na. E. The atomic radius of Cs is greater than that of Li and Rb. Choose the correct answer from the options given below

AC and D only

BA, C, and E only

CA, B, and E only

DC and E only

Answer key (D) & worked solution

Correct answer: Option D

A) Ga melting point is not high. B) N has E.N = 3.04, Cl has E.N = 3.16 C) Ar, K

^{+}

, Cl–, Ca

^{2 +}

, and S2– are all isoelectronic species having 18 electrons D) Si > Mg> Al > Na E) The atomic radius of Cs is greater than that of Li and Rb. Size increases down the group

Q6ChemistryNEET UG PYQOxidizing and Reducing Behaviour

The correct order of increasing metallic character of

Na, Be, P, Mg

and Si is

P < Si < Be < Mg < Na

Be < Si < P < Mg < Na

P < Si < Na < Mg < Be

P < Mg < Be < Si < Na

Answer key (A) & worked solution

Correct answer: Option A

On moving left to right in a period, metallic character decreases and on moving top to bottom in a group, metallic character increases. So, the correct order of increasing metallic character is $$ \mathrm{P}NaMgBeSiPElectronegativity (On Pauling scale)0.91.21.51.82.1

Q7ChemistryNEET UG PYQMiscellaneous Examples

In which of the following options the order arrangement does not agree with the variation of property indicated against it?

ALi < Na < K < Rb (increasing metallic radius)

BAl

^{3 +}

− (increasing ionic size)

CB < C < N < O (increasing first ionization enthalpy)

DI < Br < F < Cl (increasing electron gain enthalpy)

Answer key (C) & worked solution

Correct answer: Option C

Correct order of increasing 1

^{s t}

E.→ B < C < O < N I.E. of N > O as it has half-filled electronic configuration.

Q8ChemistryNEET UG PYQAtomic and Ionic Radii

The element expected to form largest ion to achieve the nearest noble gas configuration is

CNa

Answer key (B) & worked solution

Correct answer: Option B

F^{- 1}, N^{- 3}, N a^{+} & O^{- 2}

all ions are isoelectronic containing

10 e^{-}

Z_{eff} \to N a^{+} > F^{-} > O^{- 2} > N^{- 3}

order of radius

\to N^{- 3} > O^{- 2} > F^{-} > N a^{+}

\to

Nitrogen to achieve Noble gas configuration it gain

3 e^{-}, &

form

N^{- 3}

Which topics in Classification of Elements and Periodicity carry the most questions?

All 12 topics, by tagged PYQs, then practice questions. Counted from the bank. Updated 19 Oct 2026.

Topic	PYQs	Practice questions
Atomic and Ionic Radii	8	81
Ionization Enthalpy	4	111
Modern Periodic Table	3	44
Oxidizing and Reducing Behaviour	3	12
Miscellaneous Examples	2	28
Electron Gain Enthalpy or Electron Affinity	1	49
Electronegativity and Its Applications	1	45
Structure of Modern Periodic Table	0	27
Diagonal Relationship	0	21
Historical Development of Periodic Table	0	17
Valency and Oxidation State	0	8
Screening Effect	0	5

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Unknown is not zero. Until you answer at least 3 graded questions in Classification of Elements and Periodicity, ViJEEta shows – instead of a zero or a 50% default. Book exercises can raise chapter mastery up to 0.60, adaptive practice up to 0.75, and only a timed chapter test or mock can certify 0.80 and above.

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What happens when you test Classification of Elements and Periodicity?

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Chapters either side of Classification of Elements and Periodicity by weight

Some Basic Concepts of Chemistry Weight 3.4/100 Structure of an Atom Weight 3.4/100 Organic Chemistry: Some Basic Principles and Techniques Weight 3.0/100 Chemical Equilibrium Weight 2.8/100

Classification of Elements and Periodicity: NEET PYQs and weightage

Classification of Elements and Periodicity: key facts for NEET

Dobereiner's Triads (1829)

Newlands' Law of Octaves (1865)

Mendeleev's Periodic Law

Moseley (1913) and Modern Periodic Law

Long form of the periodic table

IUPAC names for Z > 100

Block configurations

Exceptions in placement

Metals, non-metals and metalloids

Atomic radius

Ionic radius

Ionization enthalpy

IE anomalies

Electron gain enthalpy

Electronegativity

Anomalous second period, diagonal pairs and oxides

How does NEET ask Classification of Elements and Periodicity?

Which topics in Classification of Elements and Periodicity carry the most questions?

Why does Classification of Elements and Periodicity show "–" until you answer 3 questions?

What happens when you test Classification of Elements and Periodicity?

Chapters either side of Classification of Elements and Periodicity by weight