The number of possible elements is not known. A very early suggestion made by Elliot Adams in 1911, and based on the arrangement of elements in each horizontal periodic table row, was that elements of atomic weight greater than 256Â± (which would equate to between elements 99 and 100 in modern-day terms) did not exist.
IRS debts can only legally be collected for 10 years, but there are many events that may stop that 10 year clock from running. This is known as tolling the statute of limitations. Events that stop or toll the statute of limitations from running include:
It depends on how accurate your return was, what records you are talking about, etc. It's anywhere from 3 years, to 6 years, to forever. 6 years might be a good idea, especially for collection reasons. In many cases, the IRS will require the last 6 years of returns before working with taxpayers. If the IRS lost your return, you might need to resubmit it--so maybe 6 years generally, and 10 years or longer if you think the return might be incredibly weird.