Under what circumstances does your atom have a negative charge
No. Not under "all or any circumstances."
A regular helium atom (He) has no ionic charge. The element in its standard form as it appears on the period table and in our everyday environment has two protons and two electrons. The positive charge of the protons cancel out the negative charge of the electrons, and so there is no charge left. Furthermore, helium is a noble gas, a family of elements which are extremely stable and will not react with anything under normal circumstances. If, however, an atom of helium where ionized in a lab, and stripped of its electrons, it would have a charge of plus two.
This is an oddly stated question but I would assume you are talking about elements such as neon, argon, xenon, krypton, i.e. noble gases which are monoatomic elements. They are inert gases that, under ordinary circumstances, do not react with other elements to form compounds.
what changes the charge of the atom
The charge of an unionized atom is negative.
A neutral atom will have a charge of ZERO
An atom with an electrical charge is called an ion.
An atom may have a positive charge, a negative charge, or no charge. If it has a non-zero charge, it is said to be an ion.
Under normal circumstances the Sony VGN-CS260J/R VAIO 14.1 in. 2 GHz Intel Core 2 Duo T6400 Laptop's battery will last about 5 hours on one charge.
No matter where an electron happens to be -- in an atom, outside of an atom, near an atom -- its charge will ALWAYS be negative one atomic charge.
An ion is an atom with a positive or negative charge.
The neutral atom hasn't a charge.