ionisation energy

Meaning: Ionisation energy is the minimum energy needed to free a bound electron, leaving it at rest infinitely far from the nucleus (total energy zero). A bound electron has negative energy, so the ionisation energy equals the magnitude of that energy. A photon can ionise the atom only if hf is at least this value, and any excess becomes kinetic energy of the ejected electron. In general chemistry the same quantity gives a periodic trend: it generally rises across a period and falls down a group.

Equation: IE = −En = (13.6 eV)Z²/n², where En is the energy of level n (eV; 1 eV = 1.60 × 10⁻¹⁹ J), Z is the atomic number (unitless) and n is the principal quantum number (unitless). This holds for one-electron species such as H and He⁺. For the ejected electron, KE = hf − IE.

On the MCAT: Given an energy level diagram, you find the photon frequency or wavelength that just ionises the atom (f = IE/h, λ = hc/IE), or the kinetic energy of the ejected electron. A common comparison is He⁺ (Z = 2), whose ground-state IE is four times that of hydrogen.

Common confusion: 13.6 eV applies only to hydrogen's ground state. From n = 2 the value is 3.4 eV. Do not give the negative level energy. Ionisation is the jump from n to n = ∞, so E∞ − En = 0 − En is positive.