For example, a helium atom has an atomic mass of 4, but an atomic number (or positive charge) of 2. Since electrons have almost no mass, it seemed that something besides the protons in the nucleus ...

Helium is notoriously hard to contain because, like hydrogen, it is a tiny little atom even by atomic standards. It also expands about 750 times when it turns into a gas, according to the post’s ...

Precision spectroscopy of helium and other atomic systems is a vital area of research in modern physics, particularly in testing fundamental theories like quantum electrodynamics (QED).