How many electrons are needed in the outer energy levels of most atoms for the atom to be chemically stable?

eight (8), for most atoms.

Explanation:

Atoms gain chemical stability by completing the highest energy level with all the electrons it can have, which is 8 for most atoms.

The elements of the first period (row 1), hydrogen and helium, have outer energy level, n, equal to 1.

So, since n = 1, the atoms of hydrogen and helium (in the ground state) fill the 1s orbital, so they need just two electrons to be chemically stable. That is why helium, having 2 electrons, is a noble gas, and hydrogen, having 1 electron, just needs to add 1 more electron to be chemically stable like helium.

For period 2 through 7 of the periodic table (n = 2, n=3, n =4, n =5, n = 6, and n = 7), the outer energy levels can hold 8 electrons. That is why the noble gases Ne, Ar, Kr, Xe, Rn, and Og, each with 8 electrons in the outer energy level are chemically stable.

Here you have two examples to see how this works:

the awnser is group 12

it can be observed without changing the identity of the object.

it depends on what substance you're talking about as well as many other factors. in quantum mechanics, the observation of particles affects the observers ability to measure certain properties. for example: you can know the velocity of a particle, but you can't know it's position; or you can know it's position, but not it's velocity. it's like trying to find out where a grain of sand is by throwing other grains of sand at it. once you find out where it is, it's been moved. it's also like trying to find an answer to the question of whether or not a tree falling in a forest makes a sound if nobody is there to hear it. paradoxically, the only way to find out is to be there and hear it.