Out of the elements phosphorus (P), sulphur (S), chlorine (Cl) and fluorine (F), the elements having the most negative and least negative electron gain enthalpy, respectively are
Cl and P
F and S
F and Cl
P and S
Answer is Right!
Answer is Wrong!
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UPSC CISF-AC-EXE – 2019
The elements having the most negative and least negative electron gain enthalpy among P, S, Cl, and F, respectively, are Cl and P.
Electron gain enthalpy is the energy change when an electron is added to a neutral gaseous atom to form a negative ion. A negative value means energy is released, indicating the atom has an affinity for electrons. A more negative value signifies a stronger affinity.
We are comparing P (Group 15), S (Group 16), Cl (Group 17), and F (Group 17).
General trends:
– Across a period (left to right), electron gain enthalpy generally becomes more negative (atoms become smaller and nuclear charge increases, attracting the added electron more strongly). Group 18 (noble gases) have positive electron gain enthalpies as they have a stable electron configuration. Group 15 elements have relatively less negative (or slightly positive) values due to the stability of the half-filled p-subshell.
– Down a group, electron gain enthalpy generally becomes less negative (the added electron is further from the nucleus in a larger shell and experiences more shielding).
Let’s consider the given elements:
– F (Period 2, Group 17)
– Cl (Period 3, Group 17)
– S (Period 3, Group 16)
– P (Period 3, Group 15)
Comparing F and Cl (Group 17): Although the general trend is less negative down a group, Cl has a more negative electron gain enthalpy (-349 kJ/mol) than F (-328 kJ/mol). This is an anomaly due to the very small size of F, where the added electron experiences significant repulsion from existing electrons in the compact 2p subshell.
Comparing Cl and S (same period, different groups): Electron gain enthalpy becomes more negative across the period. Cl is in Group 17, S is in Group 16. So, Cl should have a more negative value than S. (Cl: -349 kJ/mol, S: -200 kJ/mol). This is consistent.
Comparing S and P (same period, different groups): S is in Group 16, P is in Group 15. Group 15 elements have unusually low (less negative) electron gain enthalpies due to the stable half-filled configuration. So, P should have a less negative value than S. (S: -200 kJ/mol, P: -74 kJ/mol). This is consistent.
Ranking the elements by electron gain enthalpy from most negative to least negative:
Cl (-349) > F (-328) > S (-200) > P (-74)
The element with the most negative electron gain enthalpy is Cl.
The element with the least negative electron gain enthalpy is P.
The pair is (Cl, P).