Electronic configuration 電子之排列

First of all, u need to know that each orbital is composed of sub-shells, these subshells ar namely s sub-shell, p sub-shell, d subshell, f-subshell, and the g-subshell. each s subshell contains 1 orbital, called s-orbital, each p subshell contains 3 orbitals(p-orbital), each d subshell contains 5 orbitals(d-orbitals). each orbital(no matter s, p, d, f, g) can be filled with 2 electrons at most.
in the 1st shell, there is only a s-orbital(called 1s orbital), which can hold 2 electrons at most. in the 2nd shell, there ar a s orbital(2s orbital) and three p-orbitals(2p orbital), since each s orbital can hold 2 electrons, and all the three 2p orbitals can hold all together 6 electrons, the 2nd shell can hold 8 electrons at most. in the 3rd shell, s,p,d orbitals exist, and since there ar 5 d-orbitals, the 3rd shell can hv a maximum of 2+6+10=18 electrons. and the 4th shell so on.
u may doubt that after 3p orbitals ar completely filled(hving the configuration of argon), a new 4s orbital is 'opened'(as in the case in CE level, argon has the configuration of 2,8,8, after that potassium has 2,8,8,1 and Ca, 2,8,8,2), it is because the energy level of 4s orbital is lower than that of the 3d orbital, so electrons go into 4s first, after 4s is completely filled, electrons go into 3d orbital(it starts the first transition metal, scandium)
Electronic configuration refers to the arrangement of electrons, electron structures of elements ar built up according to three rules:
1. The Aufbau principle: electrons enter orbitals in order of ascending energy, that is they would enter the lowest energy orbital first, before the higher energy ones are to be occupied.
2. Pauli exclusion principle:
(a) no orbital can accommodate more than two electrons
(b) if two electrons occupy the same orbital, they must hv opposite spins.
3: Hund's rule: when orbitals of the same energy are to be filled, they will be occupied singly with electrons first, before any pairing of electrons would occur. (this is because electrons repel each other and the atom would hv a higher energy if Hund's rule were not followed.) moreover, all of the unpaired electrons would hv parallel spins.

2006-11-03 21:59:41 補充：
the electronic configuration is still the same, just expressed in a different way, u can count. for Na, 2,8,1, in A-level, it is expressed as 1s2, 2s2, 2p6, 3s1, the no. of electrons still remains the same.