Writing Equations [ENDORSED]

[Seraphina Joseph 1C]

When we are asked to write chemical equations and they say _ gas, for example Nitrogen gas, do you always make it 2 like N2 and O2 when its a gas?

[Faith Veenker 2K]

Not necessarily. There are 7 diatomic molecules that you will have to memorize: H2, N2, F2, O2, I2, Cl2, and Br2. Other molecules would not need 2 atoms per molecule when you only have one element in a molecule. They are pretty common though, like O2 for combustion reactions.

[Seraphina Joseph 1C]

Thank you so much!

[David Y]

This clarification helped a lot thank you very much!

[AmyHo2K]

This helped a lot. I was under the impression that all gas molecules had the 2 in it.

[Tanya Nguyen 1B]

Not necessarily. There are 7 diatomic molecules that you will have to memorize: H2, N2, F2, O2, I2, Cl2, and Br2. Other molecules would not need 2 atoms per molecule when you only have one element in a molecule. They are pretty common though, like O2 for combustion reactions.

What are diatomic molecules? And why is it only those 7 that have two atoms per molecule?

[Moura Girgis 1F]

So when asking us to write an equation with an element that happens to be diatomic, like oxygen, would we always but a subscript of 2 with it? Because I feel like I have seen instances where there wasn't any subscripts with it.

[Faith Veenker 2K]

Not necessarily. There are 7 diatomic molecules that you will have to memorize: H2, N2, F2, O2, I2, Cl2, and Br2. Other molecules would not need 2 atoms per molecule when you only have one element in a molecule. They are pretty common though, like O2 for combustion reactions.

What are diatomic molecules? And why is it only those 7 that have two atoms per molecule?

Diatomic molecules are molecules that have 2 atoms of the same element bonded together. They cannot be found in nature without two atoms together. For example, you wouldn't find an O atom by itself. This has to do with how the atoms share electrons to be stable enough.

[Faith Veenker 2K]

So when asking us to write an equation with an element that happens to be diatomic, like oxygen, would we always but a subscript of 2 with it? Because I feel like I have seen instances where there wasn't any subscripts with it.

Atoms that make up diatomic molecules cannot be found in nature as solo atoms. So, yes, if you are told to put a molecule of solely this type of atom, I believe you would always put a subscript of 2 on these atoms to make them diatomic.

For example, when told that Iodine reacts with another substance, use I2 in the equation.

However, it is possible to have ions of these elements that aren't diatomic: for example, H+ in the dissociation reaction of HCl.
HCl --> (H+) + (Cl-)

[Ashrita Singh 2F]

You add 2 if they are diatomic molecules (H, N, F, O, I, C, Br).

[Chem_Mod]

Not necessarily. There are 7 diatomic molecules that you will have to memorize: H2, N2, F2, O2, I2, Cl2, and Br2. Other molecules would not need 2 atoms per molecule when you only have one element in a molecule. They are pretty common though, like O2 for combustion reactions.

This is exactly right. There are 7 diatomic gases that are written as X₂ (X being H, N, F, O, I, Cl, and Br). Being a gas does not necessarily mean that it has a subscript of 2. For example, neon gas is written as Ne.

[Desiree Eshraghi 3A]

As others have stated, not all gas molecules are diatomic molecules (such as oxygen). It is a common feature of a select few that we may be more familiar with (such as oxygen and hydrogen) but I would personally try to not make broad generalizations among states of matter and molecular structure.

[Elizabeth Kim 2K]

It depends on the gas. The only gases for which you would include a subscript of 2 are the seven diatomic elements: H2, N2, F2, O2, I2, Cl2, and Br2. These specific gases have a subscript of 2 because these elements are always found as a pair of atoms of the same pure element bonded together and are not found as individual atoms.

[Molly McAndrew 1 1H]

When dealing with diatomic molecules, if asked to find the molar mass, would O₂ have a molar mass of ~16 g.mol^-1 or a molar mass of ~32 g.mol^-1? I'd assume ~32, but I just wanted to confirm.

[Angela Denvir 1H]

32 g/mol would be the molar mass of O2 since each oxygen atom must be accounted for. The same goes for all other diatomic molecules.

[Sasha Gladkikh 2A]

When dealing with diatomic molecules, if asked to find the molar mass, would O₂ have a molar mass of ~16 g.mol^-1 or a molar mass of ~32 g.mol^-1? I'd assume ~32, but I just wanted to confirm.

Hi Molly,

Yes, the molar mass of the oxygen molecule O2 is ≈ 32 grams/mole.

In general, to find the molar mass of a molecule (molecules are two or more atoms chemically joined together):
1. Use the chemical formula to determine the number of each type of atom present in the molecule.
2. Multiply the atomic weight (from the periodic table) of each element by the number of atoms of that element present in the molecule.
3. Add it all together and put units of grams/mole after the number.

Therefore, because there are 2 oxygen atoms, the molar mass of the oxygen molecule O2 is:

molar mass of O2 = 2 * 15.9994 grams/mole of O = 31.9988 grams/mole of O2 ≈ 32 grams/mole of O2

Hope this helps!