First vs Second Ionization Enthalpy: What Is the Difference?
Chemistry · Periodic Classification Of Properties · NEET
The first ionization enthalpy is the energy to pull one electron off a neutral gas atom (X to X+). The second ionization enthalpy is the energy to pull the next electron off the +1 ion you just made (X+ to X2+). The second is always bigger because you are removing an electron from a positive ion, which holds electrons more tightly. Memory hook: "second costs more" — once an atom is already positive, its remaining electrons feel a stronger pull, so they are harder to take.
First ionization enthalpy removes an electron from the neutral atom X; the second removes one from the resulting X+ ion. Because X+ is already positive and holds electrons more tightly, the second ionization enthalpy is always greater than the first.
Your doubts, answered
What exactly is the first ionization enthalpy?
It is the energy needed to remove ONE electron from an isolated gaseous atom in its ground state. NCERT writes it as the reaction X(g) -> X+(g) + e-. It is measured in kJ/mol. When someone just says 'ionization enthalpy' without a number, they always mean the FIRST one.
What is the second ionization enthalpy?
It is the energy needed to remove the second most loosely bound electron, but now from the +1 ion, not the neutral atom. NCERT writes it as X+(g) -> X2+(g) + e-. So the second ionization enthalpy always acts on a particle that is already positively charged.
Why is the second ionization enthalpy always greater than the first?
Because it is harder to pull an electron away from a positive ion than from a neutral atom. After the first electron leaves, the ion has the same number of protons but fewer electrons. The remaining electrons feel a stronger net pull from the nucleus, so they are held more tightly. This is why every successive value keeps rising: first < second < third, and so on.
What does 'successive ionization enthalpy' mean?
Successive means one after another. If you keep removing electrons one by one from the same atom, each removal has its own ionization enthalpy: 1st, 2nd, 3rd, 4th... These are the successive ionization enthalpies, and they always increase in that order for the same element.
Can the second ionization enthalpy of one element be smaller than the first of another?
Yes. The rule 'second > first' only applies to the SAME element. When you compare different elements, that rule does not carry over. For example, the second ionization enthalpy of sodium is very large (you would be breaking a stable noble-gas core), but that is a between-element comparison, not the same-atom rule.
Is the jump from first to second always small?
No, and this is the exam trap. Sometimes the jump is small, but if the second electron has to come from a stable, fully filled inner shell (a noble-gas core), the jump is HUGE. Example: sodium (2,8,1) gives up its lone outer electron easily (low first IE), but the second electron must come from the stable 2,8 core, so the second IE is enormous. A big sudden jump tells you how many electrons are in the outermost shell.
⚠️ The NEET trap ✗ The second ionization enthalpy is only a little bigger than the first, so the two values are always close. ✓ The second is always larger, but the size of the jump depends on where the second electron comes from. If the second electron must be pulled from a stable noble-gas core (like the second electron of Na), the jump is very large. 🧠 Watch for a SUDDEN big jump in successive values — it marks the point where the valence shell is empty and you start breaking the stable core.
Real NEET questions
NEET 2016 Phase 1
In which of the following options the order of arrangement does NOT agree with the variation of property indicated against it?
B · B < C < N < O (increasing first ionization enthalpy) ✓
C · I < Br < Cl < F (increasing electron gain enthalpy)
D · Li < Na < K < Rb (increasing metallic radius)
Solution: Across a period first ionization enthalpy generally rises, but N has a stable half-filled 2p3 configuration, giving N a HIGHER first ionization enthalpy than O. So the true order is B < C < O < N, and option B (B < C < N < O) is wrong. This tests that you know 'first ionization enthalpy' means removing the first electron and that its trend has half-filled-stability anomalies.
NEET 2019
For the second period elements the correct increasing order of first ionisation enthalpy is:
A · Li < Be < B < C < N < O < F < Ne
B · Li < B < Be < C < O < N < F < Ne ✓
C · Li < B < Be < C < N < O < F < Ne
D · Li < Be < B < C < O < N < F < Ne
Solution: First ionisation enthalpy generally increases across the period with two anomalies: Be > B (B's 2p1 electron leaves more easily than a 2s electron of filled 2s2) and N > O (N's half-filled 2p3 is extra stable). Including both, the order is Li < B < Be < C < O < N < F < Ne. This is the FIRST ionization enthalpy; the second ionization enthalpy of each of these atoms would be a separate, larger value.
Solved Periodic Classification Of Properties NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Which is larger, first or second ionization enthalpy, for the same atom?
The second is always larger. You are removing an electron from a +1 ion, which holds its remaining electrons more tightly than a neutral atom does.
Does the second ionization enthalpy always follow the first by a fixed amount?
No. The gap can be small or very large. A very large jump appears when the second electron must come from a stable inner noble-gas core rather than from the outer valence shell.
What is the equation NCERT gives for second ionization enthalpy?
X+(g) -> X2+(g) + e-. This is reaction 3.2 in NCERT. Notice it starts from the +1 ion, which is why the value is higher than the first ionization enthalpy.
Why does NEET care about successive ionization enthalpies?
Because a sudden big jump in the successive values tells you the number of valence electrons, which reveals the group of the element. It is a common reasoning question.
Is ionization enthalpy positive for both the first and second removals?
Yes. Energy is always required to remove an electron, so every ionization enthalpy, first, second, third and beyond, is positive (endothermic).
