Question

what is ionisation enthalpy

Answer 1

Ionization enthalpy is defined as the energy required to remove electron in the ground state of a gaseous isolated atom.

So it should be taken care that the atom is isolated meaning not in a combined state or bound with any other atom or molecule .

It should be in its ground state remember not in excited state.
Ionization enthalpy generally increases across a period and decreases down a group.

Answer 2

Hii friends
Ionization energy is the quantity of energy that an isolated, gaseous atom in the ground electronic state must absorb to discharge an electron, resulting in a cation. This energy is usually expressed in kJ/mol, or the amount of energy it takes for all the atoms in a mole to lose one electron each.