MPSC 2026 Prelims Paper-I · Question 68 of 100

Correct — option (3), Fluorine. Electronegativity is the tendency of an atom in a chemical bond to draw the shared pair of electrons towards itself. It rises across a period from left to right, because the nuclear charge increases while the electrons are being added to the same shell, so the atom gets smaller and holds the bonding pair more tightly; and it falls down a group, because each successive element has an extra shell, the bonding pair is further from the nucleus and more shielded from it. The highest value in the periodic table therefore lies at the top right, among the elements immediately before the noble gases — and that element is fluorine. On the Pauling scale, which is the one quoted in textbooks, fluorine is assigned 4.0 (more precisely 3.98) and no element exceeds it. Set the four options against that scale and the order is fluorine 3.98, oxygen 3.44, chlorine 3.16, nitrogen 3.04. Note that the ranking is not simply the order in which the four names appear in a period: chlorine, though in period 3, is slightly more electronegative than nitrogen in period 2, because it sits in group 17 while nitrogen sits in group 15, and the group position pulls harder here than the period does. At the other extreme of the scale lie the heavy alkali metals, caesium and francium, which are the least electronegative elements.

• (1)Oxygen — Oxygen is the second most electronegative element and is assigned about 3.44 on the Pauling scale, so it is the strongest competitor in this set — but it is fluorine's immediate neighbour on the left in period 2, and electronegativity rises to the right. Oxygen keeps the distinction of being the most electronegative element after fluorine, and the O-H bond's polarity is what gives water its hydrogen bonding, its high boiling point and its power as a solvent. If a paper ever excludes fluorine from the option list, oxygen becomes the answer; when fluorine is present, it does not.
• (2)Nitrogen — Nitrogen is the lowest of the four on the Pauling scale at about 3.04, below chlorine as well as below oxygen and fluorine. It sits two places to the left of fluorine in period 2, and every step to the left reduces the nuclear pull on a bonding pair. Nitrogen is still electronegative enough to form hydrogen bonds, which is why ammonia is a liquid at modest cooling and why the base pairs of DNA hold together, but it is nowhere near the top of the scale. It appears in this option set as the third of the three period-2 non-metals, to reward anyone who knows the left-to-right trend.
• (4)Chlorine — Chlorine sits directly below fluorine in group 17, one shell further out, so its bonding electrons are further from the nucleus and better shielded and its electronegativity is lower — about 3.16 against fluorine's 3.98. This option carries a genuine trap for a well-prepared candidate, because on a different property chlorine does beat fluorine: chlorine has the highest electron gain enthalpy, or electron affinity, of any element, since fluorine's atom is so small that an incoming electron meets strong repulsion from the electrons already crowded into that tiny shell. Electron affinity and electronegativity are different quantities, and the paper is asking for the second.

Three related properties are constantly confused, and separating them is the whole of this topic. Ionisation enthalpy is the energy needed to remove an electron from an isolated gaseous atom. Electron gain enthalpy, or electron affinity, is the energy change when an isolated gaseous atom accepts an electron. Both are measured on free atoms and both have definite experimental values. Electronegativity is different in kind: it is not a property of an isolated atom at all, but of an atom inside a chemical bond, describing how strongly that atom pulls the shared pair towards its own side. Because it cannot be measured directly, it is expressed on comparative scales, of which Pauling's is the most used. The practical importance of electronegativity is that the difference between two bonded atoms decides the character of their bond: a small difference gives a nearly non-polar covalent bond, a moderate difference a polar covalent bond, and a large difference an essentially ionic bond.

The periodic trends behind the answer are worth holding as a single picture rather than as separate rules. Moving left to right across a period, nuclear charge increases while electrons enter the same shell, so atomic radius falls and ionisation enthalpy, electron gain enthalpy and electronegativity all tend to rise. Moving down a group, a new shell is added at each step, so atomic radius rises and those three properties tend to fall. The result is a diagonal gradient across the table with the most electronegative elements at the top right and the most electropositive at the bottom left. Fluorine's position at the extreme also makes it the most reactive of the halogens, and fluorine gas is the strongest oxidising agent among the elements. The trends have exceptions that examiners like — chlorine's electron gain enthalpy exceeding fluorine's is the most quoted of them — and those exceptions are almost always explained by the unusually small size of the second-period atoms.

• Electronegativity is the tendency of an atom in a chemical bond to attract the shared electron pair towards itself. It is a property of a bonded atom, not of an isolated one, and is expressed on comparative scales, of which Pauling's is standard.
• Fluorine has the highest electronegativity of all elements, about 3.98 (usually rounded to 4.0) on the Pauling scale. Caesium and francium are at the other end, the least electronegative.
• Pauling values for the four elements in this question: fluorine about 3.98, oxygen about 3.44, chlorine about 3.16, nitrogen about 3.04. Chlorine therefore edges out nitrogen despite lying a period lower.
• Trend: electronegativity increases across a period from left to right as nuclear charge rises and atomic size falls, and decreases down a group as an extra shell is added and shielding increases. The maximum is at the top right of the table.
• Electron gain enthalpy (electron affinity) is a different property, and chlorine's is the highest of any element — higher than fluorine's, because the fluorine atom is so small that an added electron meets strong repulsion from the electrons already present.

• Confusing electronegativity with electron affinity. On electron gain enthalpy chlorine beats fluorine, and a question worded around 'affinity for electrons' is testing exactly that switch.
• Assuming the ranking follows the period. Chlorine is more electronegative than nitrogen even though it lies a period below, because group position outweighs period position between group 15 and group 17.
• Marking oxygen because it is the most familiar electronegative element. Oxygen is second; it becomes the answer only when fluorine is absent from the option list.

Periodic properties are asked in three ways. The extreme-value question, as here — which element has the highest or lowest value of some property, with fluorine, oxygen, caesium and helium as the recurring answers. The trend question, often as an assertion-reason pair, where a statement about a property increasing or decreasing along a period or group must be judged along with a reason about atomic radius or nuclear charge. And the comparison question, which asks you to arrange three or four named elements in order. All three are answerable from one mental picture of the table with the gradients drawn on it, plus a short list of the standard exceptions — chlorine's electron affinity above fluorine's being the one that appears most often.

• practice — not a real PYQ

  Which one of the following elements has the highest electron gain enthalpy (electron affinity), higher even than that of fluorine?

  • (a)Oxygen
  • (b)Chlorine
  • (c)Bromine
  • (d)Sulphur

  Answer(b) Chlorine — the standard exception to the periodic trend. The fluorine atom is so small that an incoming electron faces strong repulsion from the electrons already crowded into that shell, so fluorine's electron gain enthalpy falls below chlorine's, even though fluorine remains the most electronegative element.

• practice — not a real PYQ

  As one moves down a group in the periodic table, electronegativity generally :

  • (a)increases, because atomic radius increases
  • (b)decreases, because atomic radius increases and shielding of the nucleus grows
  • (c)remains unchanged, because the number of valence electrons is the same
  • (d)first increases and then decreases

  Answer(b) decreases, because atomic radius increases and shielding of the nucleus grows — each step down a group adds a shell, so the bonding pair lies further from the nucleus and is screened by more inner electrons, and the atom's pull on it weakens. Across a period the reverse happens and electronegativity rises.