Which of the following is a reactive gas?

1. Cl
2. Na
3. Au
4. Ra
5. Rn

Is it 1. Cl?

I don't know that Cl is a gas. Cl2 is a gas, and it is quite reactive.

I chose Cl since the only other possibility is Rn, which, as a noble gas, isn't very reactive. Would Rn be correct, instead?

No, Radon is quite resistant to reaction.

Cl is the best answer, however, Cl2 is chlorine gas. Discuss this with your teacher.

Yes, you are correct. Chlorine (Cl) is a reactive gas.

To determine the reactivity of a gas, you need to have some knowledge of the periodic table and the properties of the elements.

In this case, chlorine is in Group 17 of the periodic table, also known as the halogens. Halogens are highly reactive elements because they have seven valence electrons, and they are only one electron short of a stable octet (having eight valence electrons). To achieve a stable octet, halogens tend to gain or share one electron with other elements, making them very reactive.

To answer this question, you can refer to the periodic table and identify the elements in the given options. Among them, chlorine (Cl) is the only halogen, and it is known for its reactivity. Thus, option 1, Cl, is the correct answer.