Atoms and Isotopes Questions

Question:

Examples of Isotopes: 

Carbon 11 has 6 protons and 5 neutrons. [unstable]

Carbon 12 has 6 protons and 6 neutrons. [stable]

Carbon 13 has 6 protons and 7 neutrons. [stable]

Carbon 14 has 6 protons and 8 neutrons. [unstable]

-What makes Carbon 12 and 13 stable? What makes Carbon 11 and 14 unstable?

. From my recent research, I found that if the number of neutrons compared to the number of protons in an isotope is too much or too little, the isotope is unstable, or radioactive. 

  - But if in Carbon 13, there are 6 protons and 7 neutrons and is stable then why isn't Carbon 11 stable if there are 6 protons and 5 neutrons in that isotope?

Question:

Technetium and Promethium have no stable isotopes at all.

Tin has 10 stable isotopes.

-Because Technetium and Promethium have no stable isotopes at all, does that mean they are both very dangerous and radioactive? Why were they eventually put into the Periodic Table of Elements?

-Because Tin has 10 stable isotopes, does that mean that it is the most safest isotope?

Question:

The Electron Balance:

The 1st shell of an atom can contain 2 electrons.

The 2nd & 3rd shells of an atom can contain 8 electrons.

That makes 18 electrons in all at the maximum.

-For example, Aluminum has an atomic number of 13. 2 electrons are in the 1st shell, 8 electrons are in the 2nd shell, and 3 electrons are in the outer shell. [Which makes 3 the electron balance]

   If Calcium has an atomic number of 20, there would be 2 electrons in the 1st shell, 8 electrons in the 2nd shell, and 8 electrons in the 3rd shell. That leaves 2 electrons left. Where are they? What is the electron balance?