What is the maximum number of electrons in the last ring?

Question

and how do i know when the atom is stable

Answer 1

"Last ring" is not correct terminology for the atomic-orbital theory. There are orbitals and sub-orbitals. The first orbital has 1 suborbital with 2 electrons max, the second, 2 suborbitals with 8 electrons max, the third, 3 suborbitals with 18 electrons max, and so on. What makes it "messy" is that when you reach the 3 orbital, the electrons don't simply fill up the suborbitals. The sequencing starts with 3s2 followed by 3p 6 (the first two suborbitals in the 3 orbital), then followed by 4s2. After that point, electrons fill up the 3d suborbital in the 3d orbital for 10 elements.

When the smoke clears, you have elements with electrons in the 7s2 orbital, the 5f14 orbital, the 6d10 orbitals and the 6p6 orbitals.

As for stability, you can look it up in the Handbook of Chemistry and Physics (HCP). HCP lists all the isotopes of each element known, and if they are radioactive (what you mean by not stable), how it decays and the half-life. Basically, any element above bismuth (Atomic No. 83) is unstable; bismuth is radioactive, but the half-life is more than the the current age of the universe. There are 2 elements below bismuth which have no stable isotopes, Atomic No. 43, and one of the
"rare earths".

Answer 2

I am pretty sure that the max amount of electrons in the outer most ring is 22. Based on the electron configuration.