Chemistry · Periodic Classification Of Properties · NEET
The sign tells you whether energy is released or needed. When an atom gains an electron and energy comes OUT, the electron gain enthalpy is NEGATIVE (this is the common case, like halogens). When you must PUT IN energy to force an electron on the atom, it is POSITIVE (like noble gases). A more negative number means the atom accepts the electron more happily. For NEET, remember: negative = easy to add electron = stable anion forms.
This is the most tested trap. Going down group 17 the value should become less negative, so you expect F to be the most negative. But fluorine's 2p subshell is very small and packed tightly. The new electron entering this compact n=2 shell feels strong repulsion from the electrons already there. This repulsion cancels part of the energy released. Chlorine's 3p shell is bigger, so less repulsion, more energy released. Order of magnitude: Cl > F > Br > I.
They describe the same event (adding an electron) but with opposite sign convention. Electron affinity is the energy RELEASED, so it is usually written as a positive number when energy is given out. Electron gain enthalpy (Δ_eg H) is the enthalpy CHANGE, so energy released makes it NEGATIVE. NCERT uses electron gain enthalpy. So 'high electron affinity' = 'very negative electron gain enthalpy'. Do not mix the signs in the exam.
Across a period the effective nuclear charge increases and atomic size decreases. A smaller atom pulls the incoming electron closer to the positive nucleus, so more energy is released. That makes the value more negative from left to right. This is why the most negative values sit toward the upper right, just before the noble gases.
Noble gases already have a stable, completely filled outer shell. To add one more electron, that electron must enter the NEXT higher energy level (a new shell). This creates a very unstable arrangement, so you have to push energy IN. Because energy is absorbed, the electron gain enthalpy is large and positive. Noble gases simply do not want extra electrons.
The first electron makes the atom a negative ion (like O to O⁻). Now you try to add a SECOND electron to an already negative ion. The negative ion repels the incoming negative electron. To overcome that repulsion you must supply energy, so the second electron gain enthalpy is always positive. Example: for oxygen, first is negative but second is strongly positive.
Among the following, the correct trend in electron gain enthalpy (most negative first) is:
In which of the following options the order of arrangement does NOT agree with the variation of property indicated against it?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the energy change when one electron is added to a neutral gaseous atom to form a negative ion. If energy is released the value is negative; if energy must be supplied it is positive.
Chlorine has the most negative electron gain enthalpy of all elements, more negative than fluorine, because chlorine's larger 3p shell has less electron-electron repulsion than fluorine's small 2p shell.
It generally becomes more negative across a period (left to right) as size decreases, and less negative down a group as size increases. Noble gases are the exception with positive values.
Oxygen is small, so its compact 2p shell gives strong repulsion to the added electron. Sulfur's larger 3p shell has less repulsion, so sulfur's value is more negative than oxygen's, just like Cl beats F.
Yes. A more negative electron gain enthalpy means more energy is released when the electron is added, so the atom forms a stable anion more easily. Halogens are the best anion formers.
