Periodic Classification and Periodicity Questions and Answers

Q: The correct decreasing order of the first ionization enthalpies among the elements C, N, O, F is

The order is F > N > O > C.Across a period, ionization enthalpy generally rises with nuclear charge, so F is highest and C lowest. The exception is N and O: nitrogen's half-filled 2p3 configuration is extra stable, while in oxygen (2p4) the fourth p electron is paired and repelled by its partner, so it is easier to remove. Hence N > O.The values (in kJ mol-1, approximately) are F 1681, N 1402, O 1314 and C 1086. Every other option either places F below N or O, or puts C above them.

Q: List-I contains some group-I elements and List-II contains their electron gain enthalpy values (in kJ mol-1)List-IList-II(i) H(a) -48(ii) Li(b) -53(iii) Na(c) -73(iv) K(d) -60Choose the correct match.

The correct match is (i)-(c), (ii)-(d), (iii)-(b), (iv)-(a).The electron gain enthalpies (kJ mol-1) of these elements are: H -73, Li -60, Na -53 and K -48. Going down group 1 the atoms get larger, the added electron is held farther from the nucleus, and the enthalpy becomes less negative.Hydrogen, small and with a half-filled 1s shell, has the most negative value of the four. Option D, which pairs the order H, Li, Na, K with -48, -53, -73, -60, gets the trend backwards.

Q: Which one of the following is not isoelectronic with other isoelectronic species?

The odd one out is \(\mathrm{Ca^{2+}}\).Count the electrons in each species:\(\mathrm{O^{2-}}\): 8 + 2 = 10\(\mathrm{F^{-}}\): 9 + 1 = 10\(\mathrm{Na^{+}}\): 11 − 1 = 10\(\mathrm{Mg^{2+}}\): 12 − 2 = 10\(\mathrm{Ca^{2+}}\): 20 − 2 = 18The first four all have 10 electrons, the neon configuration, so they are isoelectronic with one another. \(\mathrm{Ca^{2+}}\) has 18 electrons, the argon configuration.

Q: Among the noble gas elements the one with the highest electron gain enthalpy is

Among the noble gases, neon has the highest electron gain enthalpy.Noble gases have completely filled valence shells, so an added electron must go into the next, higher shell. Energy has to be supplied, which makes their electron gain enthalpies large and positive. The textbook values are about +48 kJ mol-1 for helium, +116 for neon, +96 for argon and krypton, +77 for xenon and +68 for radon, so neon's is the highest.Helium is a common wrong choice because it is the smallest atom, but its value (+48) is well below neon's, and krypton, xenon and radon are all lower than neon too.

Q: Which of the following statements are true about electronegativity?(i) Electronegativity generally increases across a period and decreases down a group.(ii) The electronegatitivity of a given element is constant.(iii) The electronegativity values decreases with the increase in atomic radii.(iv) Elecronegativity is directly related to the metallic property of the elements.(v) Electronegativity is inversely related to the non-metallic property of the elements.

The true statements are (i) and (iii), so the answer is (i) and (iii).Electronegativity generally increases across a period and decreases down a group (i), because it is linked to atomic size: as atomic radius increases, the nucleus holds the bonding pair less tightly and electronegativity falls (iii).Statement (ii) is false: an element's electronegativity is not strictly constant but varies with its oxidation state and hybridisation. Statements (iv) and (v) reverse the relationships: electronegativity is directly related to non-metallic character and inversely to metallic character.

Q: The correct increasing order of the ionic radii of the isoelectronic species O2-, N3-, F-, Mg2+, Na+ and Al3+ is

The correct order is \(\mathrm{Al^{3+} \lt Mg^{2+} \lt Na^{+} \lt F^{-} \lt O^{2-} \lt N^{3-}}\).All six species have 10 electrons. With the same number of electrons, the radius depends on the nuclear charge: the more protons, the more strongly the electrons are pulled in and the smaller the ion. The nuclear charges are N 7, O 8, F 9, Na 11, Mg 12 and Al 13.So \(\mathrm{Al^{3+}}\), with 13 protons, is smallest and \(\mathrm{N^{3-}}\), with 7, is largest. Every other option breaks this order somewhere, for example by placing an anion smaller than a cation.

Q: The IUPAC symbol of the element with atomic number 110 is

The symbol is Uun.In the IUPAC system for temporary names, each digit of the atomic number gets a root: 0 = nil, 1 = un, 2 = bi, 3 = tri, 4 = quad, 5 = pent, 6 = hex, 7 = sept, 8 = oct, 9 = enn. For 110 the digits 1, 1, 0 give un-un-nil-ium, Ununnilium, symbol Uun.Unh (Unnilhexium) is element 106, Unq (Unnilquadium) is 104, Uue (Ununennium) is 119 and Uus (Ununseptium) is 117. Element 110 is now named darmstadtium.

Q: Four elements and their electronegativity values (Pauling Scale) are given below. Match the element with its electronegativity value.AtomElectronegativity valueRb(a) 1.0Na(b) 0.8I(c) 3.0Cl(d) 2.5(e) 0.9

The correct matching is (i)-(b), (ii)-(e), (iii)-(d), (iv)-(c), taking the atoms in the order listed: Rb, Na, I, Cl.On the Pauling scale (textbook values), rubidium is 0.8 (b) and sodium 0.9 (e): electronegativity falls down group 1. Iodine is 2.5 (d) and chlorine 3.0 (c): it falls down group 17 as well.Value (a), 1.0, belongs to lithium and is the extra distractor. The other options give Rb or Na a halogen's value, or swap iodine and chlorine, which would make iodine the more electronegative halogen.

Q: The pair of elements that has similar atomic radii is

The pair with similar atomic radii is Mo and W.Normally atomic radius increases down a group. But between the 4d and 5d series come the fourteen lanthanoids, whose poorly shielding 4f electrons let the nuclear charge pull the 5d atoms in. This lanthanoid contraction almost exactly cancels the expected increase, so 5d elements have nearly the same radii as the 4d elements above them: Mo (140 pm) and W (141 pm), Zr and Hf, Nb and Ta.The other pairs are not in the same group (Ti and La, Mn and Os) or are in different groups and periods with clearly different radii (Ag and Ni, V and W).

Q: Which of the following ions are isoelectronic?

The isoelectronic set is K+, S2-, Ca2+, Cl-.Count the electrons: K+ has 19 - 1 = 18, S2- has 16 + 2 = 18, Ca2+ has 20 - 2 = 18 and Cl- has 17 + 1 = 18. All four have the argon configuration.Each other set mixes 10-electron ions (O2-, Na+, Mg2+, Al3+, all with the neon configuration) with 18-electron ions, or includes Sc2+, which has 19 electrons.

21.

The correct decreasing order of the first ionization enthalpies among the elements C, N, O, F is

A.N > O > F > C
B.O > F > N > C
C.C > N > O > F
D.F > N > O > C
E.C > O > N > F

Exam Year(s)

KEAM - 2026,

Answer: D

The order is F > N > O > C.

Across a period, ionization enthalpy generally rises with nuclear charge, so F is highest and C lowest. The exception is N and O: nitrogen's half-filled 2p³ configuration is extra stable, while in oxygen (2p⁴) the fourth p electron is paired and repelled by its partner, so it is easier to remove. Hence N > O.

The values (in kJ mol^-1, approximately) are F 1681, N 1402, O 1314 and C 1086. Every other option either places F below N or O, or puts C above them.

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22.

List-I contains some group-I elements and List-II contains their electron gain enthalpy values (in kJ mol^-1)

List-I	List-II
(i) H	(a) -48
(ii) Li	(b) -53
(iii) Na	(c) -73
(iv) K	(d) -60

Choose the correct match.

A.(i)-(a), (ii)-(c), (iii)-(d), (iv)-(b)
B.(i)-(c), (ii)-(d), (iii)-(b), (iv)-(a)
C.(i)-(d), (ii)-(b), (iii)-(c), (iv)-(a)
D.(i)-(a), (ii)-(b), (iii)-(c), (iv)-(d)
E.(i)-(b), (ii)-(a), (iii)-(d), (iv)-(c)

Exam Year(s)

KEAM - 2026,

Answer: B

The correct match is (i)-(c), (ii)-(d), (iii)-(b), (iv)-(a).

The electron gain enthalpies (kJ mol^-1) of these elements are: H -73, Li -60, Na -53 and K -48. Going down group 1 the atoms get larger, the added electron is held farther from the nucleus, and the enthalpy becomes less negative.

Hydrogen, small and with a half-filled 1s shell, has the most negative value of the four. Option D, which pairs the order H, Li, Na, K with -48, -53, -73, -60, gets the trend backwards.

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23.

Which one of the following is not isoelectronic with other isoelectronic species?

A.\(\mathrm{Ca^{2+}}\)
B.\(\mathrm{O^{2-}}\)
C.\(\mathrm{Mg^{2+}}\)
D.\(\mathrm{Na^{+}}\)
E.\(\mathrm{F^{-}}\)

Exam Year(s)

KEAM - 2026,

Answer: A

The odd one out is \(\mathrm{Ca^{2+}}\).

Count the electrons in each species:

\(\mathrm{O^{2-}}\): 8 + 2 = 10
\(\mathrm{F^{-}}\): 9 + 1 = 10
\(\mathrm{Na^{+}}\): 11 − 1 = 10
\(\mathrm{Mg^{2+}}\): 12 − 2 = 10
\(\mathrm{Ca^{2+}}\): 20 − 2 = 18

The first four all have 10 electrons, the neon configuration, so they are isoelectronic with one another. \(\mathrm{Ca^{2+}}\) has 18 electrons, the argon configuration.

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24.

Among the noble gas elements the one with the highest electron gain enthalpy is

A.helium
B.radon
C.krypton
D.xenon
E.neon

Exam Year(s)

KEAM - 2026,

Answer: E

Among the noble gases, neon has the highest electron gain enthalpy.

Noble gases have completely filled valence shells, so an added electron must go into the next, higher shell. Energy has to be supplied, which makes their electron gain enthalpies large and positive. The textbook values are about +48 kJ mol^-1 for helium, +116 for neon, +96 for argon and krypton, +77 for xenon and +68 for radon, so neon's is the highest.

Helium is a common wrong choice because it is the smallest atom, but its value (+48) is well below neon's, and krypton, xenon and radon are all lower than neon too.

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25.

Which of the following statements are true about electronegativity?

(i) Electronegativity generally increases across a period and decreases down a group.
(ii) The electronegatitivity of a given element is constant.
(iii) The electronegativity values decreases with the increase in atomic radii.
(iv) Elecronegativity is directly related to the metallic property of the elements.
(v) Electronegativity is inversely related to the non-metallic property of the elements.

A.(i) and (iv)
B.(ii) and (iii)
C.(i) and (iii)
D.(iv) and (v)
E.(ii) and (v)

Exam Year(s)

KEAM - 2026,

Answer: C

The true statements are (i) and (iii), so the answer is (i) and (iii).

Electronegativity generally increases across a period and decreases down a group (i), because it is linked to atomic size: as atomic radius increases, the nucleus holds the bonding pair less tightly and electronegativity falls (iii).

Statement (ii) is false: an element's electronegativity is not strictly constant but varies with its oxidation state and hybridisation. Statements (iv) and (v) reverse the relationships: electronegativity is directly related to non-metallic character and inversely to metallic character.

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26.

The correct increasing order of the ionic radii of the isoelectronic species O^2-, N^3-, F^-, Mg²⁺, Na⁺ and Al³⁺ is

A.\(\mathrm{O^{2-} \lt N^{3-} \lt F^{-} \lt Mg^{2+} \lt Na^{+} \lt Al^{3+}}\)
B.\(\mathrm{Al^{3+} \lt N^{3-} \lt F^{-} \lt Mg^{2+} \lt Na^{+} \lt O^{2-}}\)
C.\(\mathrm{Al^{3+} \lt Mg^{2+} \lt Na^{+} \lt F^{-} \lt O^{2-} \lt N^{3-}}\)
D.\(\mathrm{F^{-} \lt O^{2-} \lt N^{3-} \lt Al^{3+} \lt Mg^{2+} \lt Na^{+}}\)
E.\(\mathrm{O^{2-} \lt N^{3-} \lt F^{-} \lt Mg^{2+} \lt Al^{3+} \lt Na^{+}}\)

Exam Year(s)

KEAM - 2026,

Answer: C

The correct order is \(\mathrm{Al^{3+} \lt Mg^{2+} \lt Na^{+} \lt F^{-} \lt O^{2-} \lt N^{3-}}\).

All six species have 10 electrons. With the same number of electrons, the radius depends on the nuclear charge: the more protons, the more strongly the electrons are pulled in and the smaller the ion. The nuclear charges are N 7, O 8, F 9, Na 11, Mg 12 and Al 13.

So \(\mathrm{Al^{3+}}\), with 13 protons, is smallest and \(\mathrm{N^{3-}}\), with 7, is largest. Every other option breaks this order somewhere, for example by placing an anion smaller than a cation.

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27.

The IUPAC symbol of the element with atomic number 110 is

A.Unh
B.Uun
C.Uue
D.Unq
E.Uus

Exam Year(s)

KEAM - 2026,

Answer: B

The symbol is Uun.

In the IUPAC system for temporary names, each digit of the atomic number gets a root: 0 = nil, 1 = un, 2 = bi, 3 = tri, 4 = quad, 5 = pent, 6 = hex, 7 = sept, 8 = oct, 9 = enn. For 110 the digits 1, 1, 0 give un-un-nil-ium, Ununnilium, symbol Uun.

Unh (Unnilhexium) is element 106, Unq (Unnilquadium) is 104, Uue (Ununennium) is 119 and Uus (Ununseptium) is 117. Element 110 is now named darmstadtium.

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28.

Four elements and their electronegativity values (Pauling Scale) are given below. Match the element with its electronegativity value.

Atom	Electronegativity value
Rb	(a) 1.0
Na	(b) 0.8
I	(c) 3.0
Cl	(d) 2.5
	(e) 0.9

A.(i)-(b), (ii)-(e), (iii)-(d), (iv)-(c)
B.(i)-(e), (ii)-(d), (iii)-(b), (iv)-(c)
C.(i)-(b), (ii)-(c), (iii)-(d), (iv)-(e)
D.(i)-(c), (ii)-(d), (iii)-(e), (iv)-(b)
E.(i)-(c), (ii)-(d), (iii)-(a), (iv)-(b)

Exam Year(s)

KEAM - 2026,

Answer: A

The correct matching is (i)-(b), (ii)-(e), (iii)-(d), (iv)-(c), taking the atoms in the order listed: Rb, Na, I, Cl.

On the Pauling scale (textbook values), rubidium is 0.8 (b) and sodium 0.9 (e): electronegativity falls down group 1. Iodine is 2.5 (d) and chlorine 3.0 (c): it falls down group 17 as well.

Value (a), 1.0, belongs to lithium and is the extra distractor. The other options give Rb or Na a halogen's value, or swap iodine and chlorine, which would make iodine the more electronegative halogen.

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29.

The pair of elements that has similar atomic radii is

A.Mo and W
B.Ti and La
C.Ag and Ni
D.Mn and Os
E.V and W

Exam Year(s)

KEAM - 2026,

Answer: A

The pair with similar atomic radii is Mo and W.

Normally atomic radius increases down a group. But between the 4d and 5d series come the fourteen lanthanoids, whose poorly shielding 4f electrons let the nuclear charge pull the 5d atoms in. This lanthanoid contraction almost exactly cancels the expected increase, so 5d elements have nearly the same radii as the 4d elements above them: Mo (140 pm) and W (141 pm), Zr and Hf, Nb and Ta.

The other pairs are not in the same group (Ti and La, Mn and Os) or are in different groups and periods with clearly different radii (Ag and Ni, V and W).

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30.

Which of the following ions are isoelectronic?

A.\(\mathrm{K^{+}, Al^{3+}, Ca^{2+}, Sc^{2+}}\).
B.\(\mathrm{O2^{-}, Al^{3+}, Cl^{-}, Sc^{2+}}\).
C.\(\mathrm{K^{+}, S^{2-}, Ca^{2+}, Cl^{-}}\).
D.\(\mathrm{Na^{+}, Mg^{2+}, Ca^{2+}, Sc^{2+}}\).
E.\(\mathrm{Mg^{2+}, Al^{3+}, Ca^{2+}, S^{2-}}\).

Exam Year(s)

KEAM - 2026,

Answer: C

The isoelectronic set is K⁺, S^2-, Ca²⁺, Cl^-.

Count the electrons: K⁺ has 19 - 1 = 18, S^2- has 16 + 2 = 18, Ca²⁺ has 20 - 2 = 18 and Cl^- has 17 + 1 = 18. All four have the argon configuration.

Each other set mixes 10-electron ions (O^2-, Na⁺, Mg²⁺, Al³⁺, all with the neon configuration) with 18-electron ions, or includes Sc²⁺, which has 19 electrons.

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