Ionisation enthalpy (the energy to pull one electron off a gaseous atom) generally INCREASES from left to right across a transition series. But the rise is small and bumpy, not smooth. This is because the added electrons go into inner (n-1)d orbitals, which shield the outer electrons poorly, so nuclear charge rises but shielding partly cancels it. Memory hook: "d-electrons are weak bodyguards" — they let the pull of the nucleus grow, so IE creeps up slowly.
First ionisation enthalpy rises only slightly and unevenly across the 3d series because inner d electrons shield the outer electrons; Zn is high because of its stable 3d10 4s2 configuration.
Your doubts, answered
Why does ionisation enthalpy increase across a transition series?
As you move left to right (Sc to Zn), the nuclear charge (number of protons) increases one by one. The new electrons enter the inner (n-1)d orbitals. So the outer ns electrons feel a stronger pull from the nucleus. A stronger pull means you need more energy to remove an electron. So the first ionisation enthalpy generally rises. For NEET, remember: more protons = harder to remove an electron = higher IE.
Why is the increase so small and irregular, not steady like in a normal period?
The extra electrons go into inner d orbitals, not the outermost shell. These inner d electrons partly shield (screen) the outer ns electrons from the nucleus. So the effective nuclear charge felt by the outer electron rises only a little. The rise in nuclear charge and the rise in shielding almost balance out. That is why IE increases very slowly and has small ups and downs across the series.
Why is the second ionisation enthalpy of chromium and copper unusually high?
After losing one electron, Cr becomes Cr+ = [Ar]3d5 (stable half-filled d) and Cu becomes Cu+ = [Ar]3d10 (stable fully-filled d). Removing a SECOND electron breaks this extra-stable arrangement, which needs a lot of energy. So the second ionisation enthalpies of Cr and Cu are much higher than expected. Extra-stable d5 and d10 configurations are hard to disturb.
Why is the third ionisation enthalpy of gadolinium low?
Gadolinium is [Xe]4f7 5d1 6s2. When you remove the third electron to form Gd3+, it reaches the very stable half-filled 4f7 arrangement. A half-filled subshell has large exchange energy (extra stability). Because the product ion is so stable, the third electron comes off easily. So Gd has a low third ionisation enthalpy. This exact point was asked in NEET 2022.
Are the ionisation enthalpies of early actinoids lower or higher than early lanthanoids?
They are LOWER. In actinoids the outer electrons enter 5f orbitals. 5f orbitals are more spread out (diffuse) and penetrate less, so the 5f electrons are shielded more and held less tightly than the 4f electrons of lanthanoids. Electrons that are held loosely are removed more easily, so early actinoids have lower ionisation enthalpies. NEET 2023 tested this as an Assertion-Reason.
What is exchange energy and why does it matter for ionisation enthalpy?
Exchange energy is the extra stability an atom gets when it has many parallel-spin electrons in a subshell. It is highest for exactly half-filled (d5, f7) and fully-filled (d10, f14) subshells. If removing an electron CREATES such a stable configuration, IE is low. If removing an electron DESTROYS such a stable configuration, IE is high. This one idea explains the high IE2 of Cr/Cu and the low IE3 of Gd.
⚠️ The NEET trap ✗ Students say ionisation enthalpy increases smoothly and steeply across a transition series, just like across a normal period (e.g. Na to Ar). ✓ Across a transition series it increases only slightly and irregularly, because the added electrons enter inner (n-1)d orbitals that shield the outer electrons, so effective nuclear charge rises slowly. 🧠 Normal period = big jump; transition series = tiny bumpy creep. The d-electrons are the reason.
Real NEET questions
NEET 2023 Phase 2
Assertion (A): Ionisation enthalpy increases along each series of the transition elements from left to right. However, small variations occur. Reason (R): There is a corresponding increase in nuclear charge which accompanies the filling of electrons in the inner d-orbitals. Choose the most appropriate answer.
A · (a) Both (A) and (R) are correct and (R) is the correct explanation of (A) ✓
B · (b) Both (A) and (R) are correct but (R) is not the correct explanation of (A)
C · (c) (A) is correct but (R) is not correct
D · (d) (A) is not correct but (R) is correct
Solution: Across a transition series the first ionisation enthalpy generally increases because nuclear charge increases steadily. The added electrons enter the inner (n-1)d orbitals, which shield the outer electrons imperfectly, so the effective nuclear charge on the valence electrons rises and removing an electron gets harder. The imperfect shielding plus exchange-energy effects cause only small, irregular variations. So both statements are true and (R) correctly explains (A). Answer: (a).
NEET 2022
Gadolinium has a low value of third ionisation enthalpy because of
A · (a) small size
B · (b) high exchange enthalpy ✓
C · (c) high electronegativity
D · (d) high basic character
Solution: Gd is [Xe]4f7 5d1 6s2. On removing the third electron, Gd3+ attains the extra-stable half-filled 4f7 arrangement. A half-filled 4f7 subshell has large exchange energy (high exchange enthalpy), so forming Gd3+ is very favourable. This makes the third ionisation enthalpy of Gd unusually low. Answer: (b).
NEET 2023 Phase 2
Assertion (A): Ionisation enthalpies of early actinoids are lower than for early lanthanoids. Reason (R): Electrons are entering 5f orbitals in actinoids which experience greater shielding from nuclear charge. Choose the correct answer.
A · (a) Both (A) and (R) are true and (R) is the correct explanation of (A) ✓
B · (b) Both (A) and (R) are true but (R) is not the correct explanation of (A)
C · (c) (A) is true but (R) is false
D · (d) (A) is false but (R) is true
Solution: Early actinoids do have lower ionisation enthalpies than early lanthanoids, so (A) is true. In actinoids the differentiating electrons enter 5f orbitals, which are more diffuse and penetrate less. These 5f electrons are shielded more from the nucleus and held less tightly than 4f electrons in lanthanoids, so they are removed more easily, lowering the IE. Hence (R) correctly explains (A). Answer: (a).
Solved D And F Block Elements NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Does ionisation enthalpy increase or decrease across a transition series?
It generally increases from left to right, but only by a small amount and with irregular small variations, because inner (n-1)d electrons shield the outer electrons and partly cancel the rise in nuclear charge.
Why is IE2 of copper and chromium so high?
Cu+ is [Ar]3d10 and Cr+ is [Ar]3d5 — both extra-stable. Removing a second electron breaks a stable fully-filled or half-filled d subshell, which needs a lot of energy, so IE2 is very high.
Which factor lowers the third ionisation enthalpy of gadolinium?
High exchange enthalpy. Removing the third electron gives Gd3+ the stable half-filled 4f7 configuration, so the electron leaves easily and IE3 is low.
Why do early actinoids have lower ionisation enthalpies than lanthanoids?
Their outer electrons enter diffuse 5f orbitals that are shielded more and held less tightly than the 4f electrons of lanthanoids, so they are removed more easily.
What are the two main factors controlling ionisation enthalpy?
The attraction of the electron to the nucleus (effective nuclear charge) and the stability of the resulting ion (exchange energy of half-filled or fully-filled subshells). Higher attraction or breaking a stable subshell raises IE.
