Question

Are there unknown elements our periodic table is missing?

Answer 1

If there are, then they are at the end of the periodic table.

How far does your copy of the periodic table go?

Explanation:

The atomic number is the number of protons in the nucleus of an atom of an element. As such, it is a positive integer starting with `1` for Hydrogen then up to `118` for Ununoctium (a.k.a. eka-radon). Only a few atoms of Ununoctium have been synthesized and detected.

Elements `119` (Ununennium or eka-francium) and `120` (Unbinilium or eka-radium) are hypothesised and various attempts are being made to synthesize them. Their predicted lifetimes are very short.

Footnote

In 1959, when the heaviest known element was Nobelium (atomic number `102`) Tom Lehrer wrote a song "The Elements" (to the tune of "A Modern Major General") naming all of them. It ends with a typically Lehrer style rhyme: "These are the only ones of which the news has come to Harvard, and there may be many others but they have not been discovered" (pronounced "dis-car-vard").