1. As it has equal number of protons and electrons.
2. Fullfilled valence orbital.
3. Because of protons and neutrons in it. Due to protons nucleus is positively charged. Both proton and neutrons have mass which makes nucleus a dense area.
4. This is not true always.
In full, it has to do with their orbitals. Take oxygen for example. It has 8 electrons when not an ion, but this leaves its orbitals imbalanced. Oxygen, in terms of electron configurations has 2 electrons in the 1s energy level, which is balanced. It then has 2 more in the 2s energy level, also balanced. when it reaches the 2p energy level, it becomes imbalanced. A p energy level contains three orbitals. Oxygen has 1 electron in the first two orbitals and 2 in the last orbital. Orbitals are imbalanced when they don't contain their full 2 electrons, therefore making the parent oxygen less stable than an oxygen ion.
We hope that clarifies your query.