Q.Observe Table no 7.3 and explain the trend in following atomic properties of group 17 elements. i. Atomic size, ii. Ionisation enthalpy, iii. electronegativity, iv. electron gain enthalpy
i. Atomic size increases steadily from F (64 pm) to I (133 pm), since each successive halogen adds a new outermost quantum shell.
ii. Ionisation enthalpy decreases from F (1680 kJ/mol) to I (1008 kJ/mol), because the valence electron sits farther from the nucleus and is more shielded by inner shells as size increases, so it is easier to remove.
iii. Electronegativity decreases from F (4.0) to I (2.7), for the same reason -- a larger atom pulls a shared electron pair toward itself less strongly.
iv. Electron gain enthalpy becomes less negative down the group overall, though not perfectly monotonically -- Cl (-349) is actually MORE negative than F (-333), because fluorine's very small 2p orbitals suffer strong electron-electron repulsion when accepting an extra electron (see Try this|2); from Cl onward (Cl > Br > I) electron gain enthalpy becomes steadily less negative as expected.
Down group 17 (F to I): atomic size increases, ionisation enthalpy decreases, electronegativity decreases, and electron gain enthalpy becomes less negative overall -- all driven by increasing atomic size/shielding, except that Cl's electron gain enthalpy is anomalously more negative than F's due to fluorine's small-size electron-repulsion effect.
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