✔ 最佳答案
It is impossible to form only one π bond between two N atoms. In a covalent bond, there is at least one σ bond. A single bond contains one σ bond, a double bond contains one σ bond and one π bond, while a triple bond contains one σ bond and two π bonds.
If there is a single bond (a σ bond) between two N atoms, [N-N]4- anion is formed, which would form Ca2N2 with Ca2+ ion. If there is a double bond (a σ bond as well as a π bond) between two N atoms, [N=N]2- anion is formed, which would form CaN2 with Ca2+ ion. (The number of negative charge on each anion can be determined by octet rule.)
Both [N-N]4- and [N=N]2- anions are very unstable and cannot exist under normal conditions. This is because the two N atoms are both negatively charged, and the repulsion between the negative charges are very large as the two very small N atoms are very close to each other.
In the compound of calcium with nitrogen, calcium exists as Ca2+ cation and nitrogen exists as N3- anion. The formula of the compound is thus Ca3N2.
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2008-09-20 22:18:21 補充：
Ca is a very electropositive and N is very electronegative. Therefore, their compounds must be ionic, and calcium exists as Ca^2+. The only thing you need to consider is what the anion is. In my answer, I have stated that the only stable anion is N^3-, and thus the formula is Ca3N2.
2008-09-20 22:18:49 補充：
I have also stated that both [N-N]^4- and [N=N]^2- are unstable, and the formula would NOT be CaN2 or CaN. If you do not know what they are, forget them because they do not exist.
2008-09-20 22:18:59 補充：
DON'T think about any covalent bond (or even coordination bond) between Ca and N in this case. For example, [N-Ca-N]^4- and Ca=N-Ca-N=Ca NEVER exist.
