What determines the binding energy of an electron?

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Binding energy is the energy required to remove an electron from the atom, which comes from how strongly the electron is attracted to the nucleus. In the simple Coulomb picture, the attraction grows as you get closer to the nucleus, described by the potential that scales with 1/r. The closer the electron is to the nucleus (smaller distance), the more negative the potential energy and the greater the energy needed to escape. So the distance between the nucleus and the electron is the key factor determining binding energy.

The other factors don’t set how tightly an electron is bound in this basic view: the mass of the electron, its spin orientation, or its velocity don’t determine the depth of the attraction to the nucleus. (Velocity relates to the energy distribution but isn’t the cause of binding, and spin or mass don’t change the central Coulomb pull.)