[lardygeezer]

  Which of these  Cl2          NaCl         Co          O3  could be considered elements. Do they have to be on the periodic table to be elements?

[mike]

They are all elements: Cl, Na, Co, O.

But NaCl would not be considered an element, it is made of two elements, Na and Cl.

Co is an element

Probably depends on what your teacher has taught you for the others. Most probably the other two are refered to as elements (O3 is an allotrope of oxygen, 02 which is an element)

[Albert]

I would say only Co is an element, since Cl₂ and O₃ can be called compounds.

[xstrae]

I would say only Co is an element, since Cl₂ and O₃ can be called compounds.

So the elemental form of Chlorine is Cl and not Cl₂?

[Donaldson Tan]

since Cl₂ and O₃ can be called compounds.

Compounds refer to chemical substances that are made up of more than 1 element.

So the elemental form of Chlorine is Cl and not Cl₂?

Any allotropes of the same element would qualify to be the element itself.

[Korokian]

what geodome said, O3 and Cl2 are molecules, elements, but not compounds

[Albert]

You are both right. However, what I tried, unsuccessfully, to say was that, while you can think of a Co atom, Cl₂, for instance, is molecule made of two Cl atoms: theoretically speaking, it's not the smallest particle.