This question is about the ionisation energy of elements across a period. a) Define ionisation energy. b) Explain the trend in ionisation energy across a period.

a) Ionisation energy is the minimum amount of energy required to remove an electron from each atom of one mole of gaseous atoms.
b) Across a period, the ionisation energy increases due to the increase in protons within the nucleus, this increases the nuclear charge of the atom and also draws the outer electrons in towards the nucleus, in turn decreasing the atomic radius. Both factors lead to a greater attraction between the nucleus and outer electrons, making it harder to remove an electron from the atom.