Foreign national with a tourist visa marries an american citizen, does she automatically become a US citizen

No, It's not automatic and there's a lot of paperwork involved and some hearings and fees and a test and a ceremony. And, well, she needs to become a permanent resident with a green card for a few years before she can even try to become a citizen.

When my ex husband and I got married (he was on a tourist visa) we had to show that I would support him rather than allowing him to go on welfare if he couldn't get a job, just so he could get a green card, and then we had to wait a while before we were evan allowed to apply for him to become a citizen.

And they will look at the marriage to make certain that it's not a sham just to avoid immigration. We had an interview where we had to answer stuff like how long we knew each other and what we like about each other and what kind of toothpaste we used and what each other's mother's names were and stuff like that. And of course WHY we got married, which was not allowed to be "we got married so he could become a citizen" or we'd both have gotten arrested and he'd have been deported right there. And after they were sure we were serious and together still, he had to take a test to become a citizen.

And then after passing the interviews and the tests and paperwork and being married for several years, THEN there was a ceremony where he was sworn in. And if he had skipped even the ceremony, he'd STILL not be a citizen!