Outer Shell
In chemistry and atomic physics, an electron shell may be thought of as an orbit followed by electrons around an atom's nucleus. The closest shell to the nucleus is called the "1 shell", followed by the "2 shell", then the "3 shell", and so on farther and farther from the nucleus. The shells correspond to the principal quantum numbers or are labeled alphabetically with the letters used in X-ray notation. A useful guide when understanding electron shells in atoms is to note that each row on the conventional periodic table of elements represents an electron shell. Each shell can contain only a fixed number of electrons: the first shell can hold up to two electrons, the second shell can hold up to eight electrons, the third shell can hold up to 18 and so on. The general formula is that the nth shell can in principle hold up to 2 electrons. For an explanation of why electrons exist in these shells, see electron configuration.
