Can someone explain to me, what does Metastable isotope means?

Question

I already read the definitions on the internet but I really don't understand it.

Thanks in advance.

bobpursley · Answer

I wish I knew your educational level. I will assume undergraduate.

Remember the shell model for electrons in an atom?

Well, the nucleus nucleons occupy energy levels also, they have energy levels, and quantum numbers very similar to the electron shells.

Nucleons all occupy ground levels, but as electrons can be, they can be excited to higher energy states. The occupancy of these different states changes some of the characteristics of the nucleus itself and one of these characteristics is nuclear stability. The so called metastable isotopes are energy variants of specific isotopes which are unstable, but it turns out, the excited state configuration may be 1000 times more stable than the ground state nucleus. These are called metastable isotopes.

http://en.wikipedia.org/wiki/Nuclear_orbital

LG · Answer

How about TC-99m? it is a metastable isotope, but the half life is 6hours. it is said that metastable isotope has lived longer than isotopes in a stable stae I don't get that part.