The first ionization enthalpy of boron ($801\ kJ\,mol^{-1}$) is lower than that of beryllium ($899\ kJ\,mol^{-1}$), even though boron has one more proton than beryllium. Explain this anomaly.

The first ionization enthalpy of boron (801kJmol⁻¹) is lower than that of beryllium (899kJmol⁻¹), even though

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801\ kJ\,mol^{-1}

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Step 1. Beryllium's configuration is $1s^2\,2s^2$ — a completely filled $2s$ subshell, which is an unusually stable, symmetric arrangement.

Step 2. Boron's configuration is $1s^2\,2s^2\,2p^1$ — its highest-energy electron starts a brand-new $2p$ subshell rather than continuing to fill $2s$ .

Step 3. A $2p$ electron of the same shell is, on average, farther from the nucleus (less penetrating) and less tightly bound than a $2s$ electron, so it takes less energy to remove even though boron's nuclear charge is one unit higher than beryllium's. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2024Set ANNUAL1 markMCQ
$Q. Assertion: Helium has the highest value of ionisation energy among all the elements known. Reason: Helium has the highest value of electron affinity among all the elements known.$
(a) Assertion is true and the Reason is false.
(b) Both Assertion and Reason are true and Reason is the correct explanation for the Assertion.
(c) Both Assertion and the Reason are false.
(d) Both Assertion and Reason are true but the Reason is not the correct explanation for the Assertion.
›Reveal solutionSolution

Helium's ionisation energy is genuinely the highest of any element, but its electron affinity is NOT the highest — noble gases resist gaining an electron because their shell is already stably filled, so the Reason statement is false even though the Assertion is true.

Assertion — "Helium has the highest value of ionisation energy among all elements": TRUE. Helium's first ionisation energy is about 2372 kJ/mol, the highest of any element, because it has only two electrons held very close to a nucleus with charge +2, with no other electrons to shield them, and it has a completely filled, exceptionally stable 1s2 configuration that strongly resists losing an electron.

…

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CBSE 2020Set ANNUAL1 markMCQ
$Q. Assertion: Helium has the highest value of ionisation energy among all the elements known. Reason: Helium has the highest value of electron affinity among all the elements known.$
(a) Both assertion and reason are false.
(b) Both assertion and reason are true and the reason is correct explanation for the assertion.
(c) Both assertion and reason are true but the reason is not the correct explanation for the assertion.
(d) Assertion is true and reason is false.
›Reveal solutionSolution

The assertion (helium has the highest ionisation energy of all elements) is TRUE, but the reason given (helium has the highest electron affinity) is FALSE, so the two statements are unrelated and option (d) is correct.

Ionisation energy is the energy needed to remove the most loosely bound electron from an isolated gaseous atom. Helium has the smallest atomic radius of any element and a completely filled, very stable 1s2 configuration, so its nucleus holds its two electrons unusually tightly. This makes helium's first ionisation energy (2372 kJ/mol) the highest among all known elements — the assertion is correct.

…

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CBSE 2018Set annual21 markMCQ
$Q. The element having highest ionization enthalpy:$
(a) Sodium
(b) Lithium
(c) Potassium
(d) Caesium
›Reveal solutionSolution
Ionization enthalpy falls down group 1 as atoms get bigger, so the smallest alkali metal, lithium, has the highest ionization enthalpy of the four choices.

Ionization enthalpy is the minimum energy required to remove the outermost (most loosely held) electron from a gaseous atom. Within a group, ionization enthalpy generally decreases as we move down, because:

Atomic size increases (additional electron shells are added).

The outermost electron is farther from the nucleus and is shielded more effectively by the inner electron shells.

As a result, the outer electron is held less tightly by the nucleus and is easier to remove. …
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Q.The first ionization enthalpy of boron ( $801\ kJ\,mol^{-1}$ ) is lower than that of beryllium ( $899\ kJ\,mol^{-1}$ ), even though boron has one more proton than beryllium. Explain this anomaly.

Concept understanding — Ionization Enthalpy

Q.The first ionization enthalpy of boron (801 kJ mol−1801\ kJ\,mol^{-1}801 kJmol−1) is lower than that of beryllium (899 kJ mol−1899\ kJ\,mol^{-1}899 kJmol−1), even though boron has one more proton than beryllium. Explain this anomaly.

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Q.The first ionization enthalpy of boron ( $801\ kJ\,mol^{-1}$ ) is lower than that of beryllium ( $899\ kJ\,mol^{-1}$ ), even though boron has one more proton than beryllium. Explain this anomaly.

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