Which of the following elements has the highest electronegativity ?

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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.

Why the other options are wrong — (a) 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.  (b) 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.  (d) 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.

Concept: 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.

State PCS 2026 Q68 — Science & Technology PYQ with Answer

State PCS 2026 Q68 — Science & Technology PYQ with Answer