bei48482_FM

134 Chapter Four

The lowest energy level E 1 is called the ground stateof the atom, and the higher

levels E 2 , E 3 , E 4 ,... are called excited states.As the quantum number nincreases,

the corresponding energy Enapproaches closer to 0. In the limit of n, E 0

and the electron is no longer bound to the nucleus to form an atom. A positive

energy for a nucleus-electron combination means that the electron is free and has

no quantum conditions to fulfill; such a combination does not constitute an atom,

of course.

The work needed to remove an electron from an atom in its ground state is called

its ionization energy.The ionization energy is accordingly equal toE 1 , the energy

that must be provided to raise an electron from its ground state to an energy of E0,

when it is free. In the case of hydrogen, the ionization energy is 13.6 eV since the

ground-state energy of the hydrogen atom is13.6 eV. Figure 7.10 shows the ioniza-

tion energies of the elements.

Figure 4.15Energy levels of the hydrogen atom.

n = ∞

n = 5

n = 4

n = 3

n = 2

0

• 0.87 ✕ 10 –^19


• 1.36 ✕ 10 –^19


• 2.42 ✕ 10 –^19


• 5.43 ✕ 10 –^19


• 21.76 ✕ 10 –^19

0

• 0.54


• 0.85


• 1.51


• 3.40


• 13.6 Ground state

Excited states

Free electron

Energy, J Energy, eV

n = 1

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