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John Rennie

05:55

@soupless Most elements come in a mixture of isotopes. For example hydrogen is a mixture of ¹H and ²H i.e. a mixture of hydrogen and deuterium. The atomic mass is the mass of one isotope while the atomic weight is an average over all the isotopes present.

So for example ¹H has an atomic mass of 1 and ²H has an atomic mass of 2. Hydrogen has an atomic weight of 1.008 because it's 99.98% ¹H and 0.0156% ²H and when you average the two you get 1.008.

So in general you cannot simply round off the atomic weight to get the atomic mass because they are measuring different things.

John Rennie

06:35

@MoreAnonymous I had a quick scan through the document, and it strikes me that your collision potential cannot behave like a CC because it has the wrong equation of state. If you dilute the gas you also dilute the collision potential, but a CC would have a constant energy density.

soupless

06:55

@JohnRennie Is it better to copy the mass number and just lookup when I need one, or is there a better way?

John Rennie

What are you trying to do? Calculate molar concentrations?

soupless

Making python scripts for me to easily get the data of the elements

I found somewhere on the internet that you get the mass number by rounding the atomic weight. That's why I asked.

John Rennie

No. You need to know the two separately.

soupless

Okay. Is there a name for the 'default' isotope of an element, if that makes sense?

John Rennie

Although it is often true that rounding the atomic weight gives you the atomic mass of the most common isotope.

soupless

07:00

Something like ¹H for hydrogen?

John Rennie

I don't know of any name for the most common isotope.

soupless

Okay, thank you for your help!

John Rennie

You're welcome :-)

2 hours later…

More Anonymous

09:05

@JohnRennie Yes however the time it spends in the potential slope changes according to the pressure (which also decreases in a dilute gas)

@JohnRennie I'd be grateful if you could point out which equation in specific is wrong?

John Rennie

I haven't been through it in detail. I just did a quick scan to get the overall idea of what you're trying to do.

More Anonymous

09:35

@JohnRennie Also there are 2 equations here. S= (number of collisions) * (action per unit volume per unit collision)

2 hours later…

uhoh

11:27

Q: How does JPL define the exact position of Lagrange points in the real solar system where (I thought that) they can't be defined?

My answer to Does the distance to L2 vary? is The correct answer is "No, they are only mathematically defined when the orbit is circular". but @DavidHammen's answer explains that they can be defined in the elliptical restricted three body problem and @PM2Ring's answer shows that JLP's Horizons ...

Has a bounty expiring in 3 says

5 hours later…

Physics Meta

16:01

Q: comments sent to chat and addition of a new comment after that

In this closed question, the comments were moved by @rob to a chat. I was not interested in chatting, I just wanted to make an observation on the question itself, so I wrote a comment, which I found was removed today, as if it were useless to the OP. The comment did not appear in the chat either....