atom & molecule & ion

atom係一種element入面最細而且不能分割的部份(但根據quantum mechanics，一粒atom是可以再分割的，但係chemistry中我地會採用以上的definition)，而一粒atom入面會有三種subatomic particles，分別為proton, electron和neutron。proton帶正極電，electron帶負極電，neutron則是中性的，就每種element而言，會有一定比率的atoms中的neutron數目比正常的不同，因而有isotopes的出現(isotopes是指同一種element，有些atoms中的neutron數目比大多數的atoms少了或多了，因此令一些atoms的質量有所不同)。而在atom中，proton和electron的數目是一樣的，所以一粒atom是沒有任何電荷的。

molecule是由最少兩粒atoms組成，中性而且在室溫和氣壓下(25C, 1 atm)的存在形態是stable的一種粒子。例如oxygen在室溫和氣壓下穩定的存在形態是oxygen molecules(即O2，兩粒oxygen atoms，以covalent bonds組合)，而非單獨的oxygen atom(O)。

ion是帶電荷的一種粒子，它的存在是由於atom/molecule得到或者失去electron(也可是proton，但比較少)時而令atom/molecule中的正負電荷失衡，得到淨正或負電荷，而ion的stable狀態只會在水溶液中出現。如金屬K(potassium)，它與水反應後會令每粒K atom失去一粒electron，最後得出K+(potassium ion)。

1 mole of oxygen是指有6.02*10exp23粒oxygen molecules(note: oxygen在室溫和氣壓的stable狀態是O2，而非O)，所以有(6.02*10exp23)*2=1.204*10exp24粒atoms。

2007-06-09 22:20:03 補充：
ion也可在其溶液中存在(即將solid的ionic compound加熱至溶解)

Firstly, lets talk about wt atom is.
It's always describing one kind of pure element. It's the smallest particle still characterizing a chemical element which is uncuttable or indivisible. It's formed by proton(s) (for hydrogen atom only has 1 proton), electron(s) (hydrogen has 1), and neutrons (hydrogen has 0).
An element can has different kinds of atom. In fact, they have same no.s of electrons and protons but different no.s of neutron to form a new chemical characteristic. It's called isotope of a element. A mass of an atom is measured by the no.s of neutron and protons (electrons are always unmeasurable bcuz it's too light and too small.) Chinese:原子

A molecule is called 分子 in Chinese. It's a smallest part of a compound or a substance. The difference of molecule and atom is that molecule and formed by several kinds of element such as CO2, NH4, NaCl, KOH...

An ion(離子) is an atom or group of atoms which have lost or gained one or more electrons, making them negatively or positively charged. An ion consisting of a single atom is called a monatomic ion. A negatively charged ion, which has more electrons in its electron shells than it has protons in its nuclei, is known as an anion. A positively-charged ion, which has fewer electrons than protons, is known as a cation. Ions are denoted in the same way as electrically neutral atoms and molecules except for the presence of a superscript indicating the sign of the net electric charge and the number of electrons lost or gained, if more than one. For example: H+, SO42−. Ions are usually in aqueous and molten state.

Now, may i answer your question??
For 1 mole of subtance and element, it's always containing 6.02 * e23 atoms or molecule. For O2, 1mole contains 1* 6.02 * e23 /2 oxygen atom.

2007-06-10 21:11:39 補充：
NOT /2, should be *2