radioactivity是歸類於什麼properties？

這是個非常好的問題, 關鍵在於"甚麼是chemical properties", 你從甚麼學科去理解整件事.

化學裡最普通的說法是, chemical properties are the characteristics in which how a substance undergoes chemical reaction.
而 chemical reaction = a process that new substance (i.e. different from reactant) is formed.

如果根據這個說法, 任何會造成 formation of new substance 的過程(包括radioactive decay) 都算是chemical reaction, 而radioactivity (the characteristics of a substance to undergo radioactive decay) 就是chemical properties.


這樣的(廣義) 推論也算是合理; 但是, 合用嗎?
考慮一下日常生活中 chemical reaction 的特點吧:
- there is conservation of mass;
- the number of atoms of different kinds (elements) remains constant before & after reaction.

關於第一點, radioactive decay 明顯的不符合: decay 之所以釋出比chemical rxn. 多得多的能量, 就是(主要) 來自質-能轉化 (E = mc^2) ; decay 後, mass 嚴格上會減少.
[而chem. rxn. 的能量來自 rearrangement of electrons within nuclei, 少很多 ]
第二點, (一般而言) 更明顯地不符合了.
[....還要考慮到有些decay 後那元素仍是那元素, 例如Tc-99m ----> Tc-99 ]

如此一來, 這樣的定義根本無法分辨兩種性質截然不同的reaction, 徒添困擾.
而且 radioactive decay 一般也不是化學家所關心/處理的現象; 他們一般著眼的是用同樣(不變)的原子, 去製造新型的化合物.

因此, 作出這樣的區分比較合理:
- formation of new kind of nuclei = nuclear reaction, related to nuclear properties;
- formation of new substance(s) without altering composition of nuclei = chemical reaction, related to chemical properties;
- without formation of any new substance = physical process, related to physical properties.

又由於nuclear reaction 主要由物理學家研究, 仍歸入physical properties....



整件事之所以如此複雜, 原因在於化學 / 物理並非黑白二分的學科; 嚴格來說, 所有科學學科都有關連, 像是彩虹"七色": 簡單分成七色, 其實界線可以很模糊.

例如天文學家會將helium 以上的element 都歸類成metal, 方便圍內討論; 化學家也不一定是研究合成新化合物 (例如我正在做的研究似physical process 多).
中小學讀書將科學分科, 原因之一是為了方便處理課題, 減少東拉西扯. 我扯得太遠了.


2012-01-21 13:34:23 補充：
Element is a substance that cannot be broken down by chemical reactions.
(Atoms of an) element breaks down in radioactive decay.

Thus, radioactive decay cannot be a chemical reaction.

I'd say it's "nuclear properties", which is classified as "physical" one.

是你聽錯嗎?

這個應該算是化學反應啊, 因為科射物衰變後會產生出和原來的東西完全不同的新東西.

首先要define what is physical property & chemical property of a matter , here is an element . The chemical properties of an element will change the matter but not physical properties . Therefore radioactivity is a chemical property because the decaying of the element will tranform this element to a " new substance " --new element .