Q.Define IE1 and IE2. Why is IE2 > IE1 for a given atom? Discuss the factors that effect IE of an element.

First and second ionization enthalpies remove successive electrons from an atom/ion; the second is always larger because removing an electron from an already-positive ion is intrinsically harder — and both are governed by atomic size, nuclear charge, shielding, and electron configuration.

Definitions:

• First ionization enthalpy ($IE_1$): the minimum energy required to remove the most loosely bound (outermost) electron from an isolated, neutral, gaseous atom in its ground state, converting it into a gaseous $+1$ cation: $M(g) \rightarrow M^+(g) + e^-, \quad \Delta H = IE_1$
• Second ionization enthalpy ($IE_2$): the energy required to remove an electron from the resulting gaseous $M^+$ ion, converting it into a $+2$ cation: $M^+(g) \rightarrow M^{2+}(g) + e^-, \quad \Delta H = IE_2$

Why $IE_2 > IE_1$: once the first electron is removed, the ion $M^+$ has one fewer electron but the same nuclear charge as the neutral atom. This means:

1. The effective nuclear charge per remaining electron increases (same number of protons pulling on fewer electrons).
2. Electron-electron repulsion decreases, so remaining electrons are pulled in closer and held more tightly.

As a result, removing a second electron from $M^+$ always requires more energy than removing the first electron from neutral $M$; this holds true for every element ($IE_2 > IE_1$ always, and typically $IE_3 > IE_2$, and so on).

Factors affecting ionization enthalpy:

1. Atomic size: larger atomic radius → outer electron is farther from the nucleus → weaker attraction → lower IE. IE generally decreases down a group as atomic size increases.
2. Nuclear charge: higher nuclear charge (more protons) → stronger attraction for outer electrons → higher IE. IE generally increases across a period as nuclear charge increases (at roughly constant shielding).
3. Shielding/screening effect: inner-shell electrons shield outer electrons from the full nuclear charge; more shielding → lower effective nuclear charge felt by the outer electron → lower IE. …