Evidence that electrons are arranged in shells or energy levels can be obtained by measuring the successive ionisation energies of an element
The first ionisation energy of an element is the energy needed to remove one mole of electrons from one mole of gaseous atoms i.e.
M(g) ® M+(g) + e-
In general, ionisation is easier if the nuclear charge is smaller, the electron is further away from the nucleus and there is more shielding from inner electron shells.
For an element, successive ionisation energies get bigger because the remaining electrons are held more tightly by the unchanged nuclear charge
Jumps occour when going from one shell to another
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