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

IE₂ always exceeds IE₁ because removing an electron from an already-positive ion (smaller size, same nuclear charge pulling on fewer electrons) requires more energy than from a neutral atom; ionisation energy overall depends on atomic size, nuclear charge, shielding, and electron configuration stability.

Definitions:

• First ionisation energy (IE₁): the minimum energy required to remove the most loosely held (outermost) electron from an isolated, neutral gaseous atom in its ground state: M(g) → M⁺(g) + e⁻ ; energy required = IE₁.
• Second ionisation energy (IE₂): the minimum energy required to remove an electron from the resulting unipositive gaseous ion: M⁺(g) → M²⁺(g) + e⁻ ; energy required = IE₂.

Why IE₂ > IE₁ for any given atom:

After the first electron is removed, the cation M⁺ has one fewer electron but the SAME nuclear charge (same number of protons) as the neutral atom. This means:

• The effective nuclear charge experienced by each remaining electron increases (fewer electrons now shield/screen each other from the nucleus).
• The ion M⁺ is smaller in radius than the neutral atom M (the remaining electrons are pulled in closer to the nucleus). Both effects mean the remaining electrons are held more strongly, so more energy is needed to remove the next electron. Hence IE₂ is always greater than IE₁ (and IE₃ > IE₂, and so on) for the same element.

Factors affecting ionisation energy (general, IE₁ or any IEₙ):

1. Atomic/ionic size: larger atomic radius means the outer electron is farther from the nucleus and less strongly attracted, so IE is lower; IE generally decreases down a group as size increases.
2. Nuclear charge: a higher effective nuclear charge (more protons pulling on the electron) increases IE; IE generally increases across a period as nuclear charge increases (with size decreasing).
3. Screening/shielding effect: inner-shell electrons shield outer electrons from the full nuclear charge; more shielding lowers the effective nuclear charge felt by the outer electron and thus lowers IE. …