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  Atomic Number of the Element with Lowest First Ionisation Enthalpy | JEE Chemistry 📌 Question The atomic number of the element from the following with the lowest first ionisation enthalpy is: 📚 Concept Used – First Ionisation Enthalpy Definition: First ionisation enthalpy is the energy required to remove the most loosely bound electron from an isolated gaseous atom in its ground state. Periodic Trend: Across a period → IE increases (nuclear charge ↑, atomic size ↓) Down a group → IE decreases (atomic size ↑, shielding effect ↑) Lowest IE Element: The element at the bottom-left of the periodic table has the lowest IE, because it has large size and weak nuclear attraction for the outermost electron. 🖋 Solution The element with the lowest first ionisation enthalpy is Francium (Fr) , but in practical chemistry, Cesium (Cs) is often considered because Francium is radioactive and rare . Francium’s atomic number = 87 Therefore, the answer ...