0198506961.pdf

50 Helium

− 25

− 20

− 10

− 5

− 15

−13.6

0

5

768

9

4

3

2

1

Helium H

Fig. 3.4The energy levels of the helium atom with those of hydrogen for comparison. The 1s^2 ground configuration is tightly
bound. For the excited configurations of helium the 1s-electron screens the outer electron from the nuclear charge so that the
1snlconfigurations in helium have similar energy to the shell with principal quantum numbernin hydrogen. The hydrogenic
levels are indicated on the right. The interval between the^1 Land^3 L terms (equal to twice the exchange integral) is clear for
the 1s2s, 1s2p, 1s3s, 1s3p and 1s4s configurations but it is smaller for highernandl.