Q.What is ionisation enthalpy ? Why does first ionisation enthalpy increase across the period, whereas decrease down the group ?
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Start your 14-day free trial to unlock the full solution →Ionisation enthalpy is the energy needed to remove an electron from a gaseous atom; it rises across a period (stronger nuclear pull) and falls down a group (larger, more shielded atoms).
Definition: Ionisation enthalpy (or ionisation energy) is the minimum amount of energy required to remove the most loosely bound (outermost) electron from an isolated gaseous atom in its ground state, converting it into a gaseous cation.
M(g) -> M+(g) + e-
Why it increases across a period: Moving across a period (left to right), electrons are added to the same principal shell while protons are also added to the nucleus, so the nuclear charge increases while the shielding by inner electrons stays roughly constant. This raises the effective nuclear charge felt by the outermost electrons, pulling them in closer and holding them more tightly, and also shrinks the atomic radius. Both effects make it progressively harder to remove an electron, so first ionisation enthalpy generally increases across a period.
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