electronic arrangements

When electrons are filled into electronic shells, the maximum number of electrons for the 1st shell is 2, for the 2nd shell 8, for the 3nd shell 18, for the 4th shell 32 ...... etc.

However, there is one more criterion that is the maximum number of electrons in the outermost shell is 8 (If the 1st shell is the outermost shell, the maximum number of electrons is 2).

When atomic number is 18 (Ar), the electronic arrangement is 2, 8, 8.
Such electronic arrangement fits the above criteria.

When atomic number is 19 (K), the electronic arrangement SHOULD NOT be 2, 8, 9 because the number of electrons in the outermost SHOULD NOT exceed 8. Therefore, the last electrons to be filled in must be put into an outer shell -- the 4th shell.
The electronic arrangement of K is then 2, 8, 8, 1.

When electrons are filled into shells, it prefers to put electrons in pairs.
When atomic number is 20 (Ca), the last electron should put into the 4th shell to pair up the electrons in the outermost shell.
The electronic arrangement of Ca is then 2, 8, 8, 2.

When atomic number is 21 (Sc), the last electron is put into the 3rd shell. This is because the 3rd shell now is NOT the outermost shell and it can have more than 8 electrons (but not exceed 18 electrons).
The electronic arrangement of Sc is then 2, 8, 9, 2.
...... and so on ......
When atomic number of 30 (Zn), the electronic arrangement is 2, 8, 18, 2.

When atomic number of 31 (Ga), the last electron is put into the 4th shell as all the inner shells are fully filled.
The electronic arrangement of Ga is 2, 8, 18, 3.
.......

In higher forms, you will learn that there are s, p, d and f subshells in electronic shell. The electronic arrangement (electronic configuration) can be explained in a much clearer way.

http://hk.knowledge.yahoo.com/question/question?qid=7006102104275

認為這階段你不應看以上網頁，因為它涉及 s, p, d, f 等亞層，在基礎未穩的情況下，只會引起更多疑問。

http://hk.knowledge.yahoo.com/question/question?qid=7006102104275

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http://www.pf23.com

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