Simulation · Chemistry · Class 11
Pulling an electron off
From the lesson Ionization enthalpy in Classification of Elements and Periodicity in Properties. Change the values and watch what happens.
The idea behind it
NCERT §3.7.1(c)
- Ionization enthalpy ΔᵢH is the energy needed to remove an electron from an isolated gaseous atom in its ground state: X(g) → X⁺(g) + e⁻. It is measured in kJ mol⁻¹ and, unqualified, means the first ionization enthalpy.
- It is always positive. The second, X⁺(g) → X²⁺(g) + e⁻, is larger than the first because the electron leaves a positive ion; the third is larger still.
- Plotted against Z (up to 60), ΔᵢH peaks at the noble gases (closed shells) and dips at the alkali metals, whose low values match their high reactivity. Generally it rises across a period and falls down a group, the reverse of atomic radius.
- Shielding: inner electrons screen the valence electron, so it feels an effective nuclear charge smaller than the full charge. Lithium's 2s electron, screened by 1s², feels much less than +3. Shielding works best when inner shells are complete.
- Across a period, electrons join the same shell and shielding barely grows, so rising nuclear charge wins and ΔᵢH rises. Down a group, the electron is farther out and better shielded, so ΔᵢH falls.
- Exception 1: ΔᵢH(B) < ΔᵢH(Be). Boron loses a 2p electron, which penetrates less and is more shielded than beryllium's 2s electron.
- Exception 2: ΔᵢH(O) < ΔᵢH(N). Nitrogen's three 2p electrons are unpaired in separate orbitals (Hund's rule); in oxygen two 2p electrons share an orbital, and their repulsion makes one easier to remove.
- Actual period 2 order: Li < B < Be < C < O < N < F < Ne. In period 3, with Na 496, Mg 737 and Si 786 kJ mol⁻¹, Al should be near 575 kJ mol⁻¹, below Mg, because its 3p electron is shielded by 3s electrons.
- Per atom to per mole: hydrogen's ground-state electron has E = −2.18 × 10⁻¹⁸ J, so ΔᵢH = 2.18 × 10⁻¹⁸ × 6.022 × 10²³ ≈ 1.31 × 10⁶ J mol⁻¹.
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