When to use the diatomic atom

[Rebekah Han 2K]

If we're asked about a diatomic element, do we automatically add a subscript of 2? How do we know when to add the subscript of 2 and when to just leave it as one atom of that element?

For example, if we're talking about a certain amount of nitrogen in the atmosphere, would we put x grams N2? Or would we just leave it as x grams N? If it just says 0.110 g of nitrogen was formed, do we automatically put it as 0.110 g N2?

[Jeffrey J Cheng 2L]

Good afternoon Rebeka,
If the question specifies "Nitrogen atom" I will just use N. If the question specifies "Nitrogen molecule", I will use N₂. If the question specifies neither "atom" nor "molecule" and the context doesn't offer any clues as to which form of nitrogen I should use, I default to N₂. But for some questions, it doesn't matter how you interpret it; you will get the correct answer in the end.

For example, let a question ask, "Compound X contains only sodium and nitrogen. It can decompose into its constituent elements, sodium and nitrogen. What is the empirical formula of chemical X if 0.110g of nitrogen and 0.0602g of sodium was produced when Compound X decomposes into its elements?"

The approach here would be to find the moles of each element, sodium and nitrogen, and use their molar ratio to figure out the empirical formula.

Say Student A interprets the question as 0.110g of N₂
When the student tries to find the moles of nitrogen atoms, the student gets:


Say Student B interprets the question as 0.110g mol of N
When the student tries to find the moles of nitrogen atoms, the student gets:


The student's results are identical. Their next steps are also identical, and so they will come out with identical answers, despite interpreting the question differently.

Hope that helped!

[Mrinalini Mishra 2L]

This post above is super helpful! Also remember that if the question says nitrogen gas, it is part of the group of diatomic gases, so you would use N2 vs just N.

[Ellie Fox 2K]

I agree with the posts above just adding to the last one that also just generally, the diatomic gases are Hydrogen (H2), Nitrogen (N2), Oxygen (O2), Fluorine (F2), Chlorine (Cl2), Iodine (I2), and Bromine (Br2). So if a question asks for any of these as gases, you can assume they are diatomic.