An atom becomes radioactive when its nucleus contains too many or too few ... Up to two electrons may exist in the atom's first shell. Up to eight electrons may exist in the atom's second shell.

Instead, its instability led scientists to conclude that the nuclear shell was not filled and ... research would require observing the atom’s nucleus in a higher-energy state.

The nucleus is very small compared to the atom as a whole: as electrons are even smaller, most of an atom is empty space the diameter of a nucleus can be 100,000 times less than the diameter of an ...

Everyone at that time imagined the atom as a "plum ... in set orbits around the nucleus was the key to the periodic repetition of properties of the elements. The shells in which electrons orbit ...

An atom is so small that a single drop of water contains more than a billion atoms. Although tiny, atoms are mostly empty ...

The word magic is not often used in the context of science. But in the early 1930s, scientists discovered that some atomic nuclei – the center part of atoms, which make up all matter – were more ...

The picture shows a close-up of one carbon atom. A hydrogen atom has one proton as the nucleus and one electron in the region outside the nucleus. The electron and proton are attracted to each other.