氯(有毒)+鈉(有毒) = 氯化鈉-鹽(無毒)??

氯化鈉並不是由 (氯氣 + 鈉 --> 氯化鈉) 造成的。

現時製造氯化鈉的方法是蒸發海水/鹽水 (evaporation of seawater or brine)。
這些鹽水的來源主要為鹽井，鹽湖，岩鹽礦石。

而氯氣則可由電解氯化鈉中取得。
電解質為氯化鈉，氯氣會在陽極中放出。
2Cl- (氯離子) ----> Cl2 (氯氣) + 2e-
氯化鈉水溶液中的氯離子會進行氧化作用而變成氯氣。
氧化作用是一種化學變化，氯氣比氯離子小一粒電子，所以他們的化學性質是不同的。
即是你所說有毒和無毒(Toxicity 毒性是一種化學性質)。

因為當兩種或更多的物質化合在一起後,其化學性質會改變,就像硫和鐵的化合物硫酸亞鐵,當中的鐵就失去原來的磁性,而氯化鈉的結構稱為巨晶結構

firstly, because in the formation of sodium chloride, a chemical reaction is involved. Therefore the product produced no longer has the same chemical properties with the initial reactants; you cannot really "expect" it to become toxic just because the reactants are toxic. It's a wrong concept that you have in mind. Reaction products DO NOT "inherit" chemical properties.

Secondly, salt can become toxic. Toxicity simply means being harmful to the body.Large amounts of almost ANYTHING can be toxic. even water can be toxic if you drink too much-- have you ever heard of water toxicity?

http://en.wikipedia.org/wiki/Toxicity