Chemistry

A more difficult explanation:

A shell may contain subshells. There are 4 types of subshells.
s subshell which can contain at most 2 electrons.
p subshell which can contain at most 6 electrons.
d subshell which can contain at most 10 electrons.
f subshell which can contain at most 14 electrons.

1st shell: 1s subshell.
2nd shell: 2s + 2p subshells
3rd shell: 3s + 3p + 3d subshells.
4th shell: 4s + 4p + 4d + 4e subshells

Energy level for multi-electron atoms:
1s < 2s < 2p < 3s < 3p < 4s < 3d ......

For Ar (atomic number = 18):
The 18 electrons are filled in the lowest energy levels. The electronic arrangement is:
Ar: 1s^2, 2s^2, 2p^6, 3s^2, 3p^6
(^n means n electrons in the subshell)
The electronic arrangement is thus:
Ar: 2, 8, 8

For K (atomic number = 19):
It has one more electron than Ar, and it is filled into the 4s subshell, the next lowest energy level. The electronic arrangement is:
K: 1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 4s^1
The electronic arrangement then becomes:
K: 2, 8, 8, 1

For Ca (atomic number = 20):
It has one more electron than K, and it is also filled into the 4s subshell. The electronic arrangement is:
Ca: 1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 4s^2
The electronic arrangement then becomes:
Ca: 2, 8, 8, 2

= = = = =
An easier explanation:

To filled in electrons to the electronic shell, the number of electrons in the outermost shell should NOT exceed 8.

For Ar (atomic number = 18):
The electronic arrangement is Ar: 2, 8, 8

For K (atomic number = 19):
K has one more electron than Ar, and this electron should NOT be filled into the 3rd shell, because the electronic arrangement should NOT be Ar: 2, 8, 9 as the outermost shell should NOT exceed 8. Therefore, this electron is filled into the 4th shell, and thus the electronic arrangement becomes
K: 2, 8, 8, 1

For Ca (atomic number = 20):
Ca has one more electron than K, and thus electron is filled into the 4th shell to pair up the electrons. The electronic arrangement becomes
Ca: 2, 8, 9, 2

2011-02-11 00:37:00 補充：
見「意見」

2011-02-11 00:37:27 補充：
The last line should be:
Ca: 2, 8, 8, 2

2011-02-12 11:58:35 補充：
2, 8, 16, 3 ? Which element ?

Ca: 2, 8, 8, 2
Sc: 2, 8, 9, 2
Ti: 2, 8, 10, 2
V: 2, 8, 11, 2
*Cr: 2, 8, 13, 1
Mn: 2, 8, 13, 2

2011-02-12 11:58:47 補充：
Fe: 2, 8, 14, 2
Co: 2, 8, 15, 2
Ni: 2, 8, 16, 2
*Cu: 2, 8, 18, 1
Zn: 2, 8, 18, 2
Ga: 2, 8, 18, 3
Ge: 2, 8, 18, 4
.....

*The electronic arrangement of Cr and that of Cu are explained by the extra stability of half-filled and fully-filled 3d subshells. This would be taught in higher form.