The work of assigning electrons to shells was continued from 1913 to 1925 by many chemists and a few physicists. They are used in the spectroscopic Siegbahn notation. These letters were later found to correspond to the n values 1, 2, 3, etc. However, later experiments indicated that the K absorption lines are produced by the innermost electrons. It was not known what these lines meant at the time, but in 1911 Barkla decided there might be scattering lines previous to 'A', so he began at 'K'. The other second diffraction beam he called 'fluorescent' because it depended on the irradiated material. Barkla described these two types of X-ray diffraction: the first was unconnected with the type of material used in the experiment, and could be polarized. Barkla, who worked independently from Moseley as an X-ray spectrometry experimentalist, first noticed two distinct types of scattering from shooting X-rays at elements in 1909 and named them 'A' and 'B'. The origin of this terminology was alphabetic. This led to the conclusion that the electrons were in Kossel's shells with a definite limit per shell, labeling the shells with the letters K, L, M, N, O, P, and Q.

Moseley measured the frequencies of X-rays emitted by every element between calcium and zinc, and found that the frequencies became greater as the elements got heavier, leading to the theory that electrons were emitting X-rays when they were shifted to lower shells. Moseley was part of Rutherford's group, as was Niels Bohr. However, because in a neutral atom, the number of electrons equals the number of protons, this work was extremely important to Niels Bohr who mentioned Moseley's work several times in his interview of 1962. Moseley's work did not directly concern the study of electron shells, because he was trying to prove that the periodic table was not arranged by weight, but by the charge of the protons in the nucleus. The existence of electron shells was first observed experimentally in Charles Barkla's and Henry Moseley's X-ray absorption studies. The multiple electrons with the same principal quantum number ( n) had close orbits that formed a 'shell' of positive thickness instead of the circular orbit of Bohr's model which orbits called 'rings' were described by a plane. Sommerfeld retained Bohr's planetary model, but added mildly elliptical orbits (characterized by additional quantum numbers ℓ and m) to explain the fine spectroscopic structure of some elements. During this period Bohr was working with Walther Kossel, whose papers in 1914 and in 1916 called the orbits 'shells'. The shell terminology comes from Arnold Sommerfeld's modification of the Bohr model. Periodic table of Bohr in 1913 showing electron configurations in his second paper where he went to the 24th element. 'From the above we are led to the following possible scheme for the arrangement of the electrons in light atoms: Bohr built his 1913 model of electrons in elements thus: The 1913 Bohr model of the atom attempted an arrangement of electrons in their sequential orbits, however, at that time Bohr continued to increase the inner orbit of the atom to eight electrons as the atoms got larger. 5 List of elements with electrons per shell.Įach shell consists of one or more subshells, and each subshell consists of one or more atomic orbitals. For an explanation of why electrons exist in these shells, see electron configuration. The general formula is that the nth shell can in principle hold up to 2( n 2) electrons. The shells correspond to the principal quantum numbers ( n = 1, 2, 3, 4 .) or are labeled alphabetically with the letters used in X-ray notation (K, L, M, .).Įach 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 (2 + 6) electrons, the third shell can hold up to 18 (2 + 6 + 10) and so on.

The closest shell to the nucleus is called the ' 1 shell' (also called the 'K shell'), followed by the ' 2 shell' (or 'L shell'), then the ' 3 shell' (or 'M shell'), and so on farther and farther from the nucleus. In chemistry and atomic physics, an electron shell may be thought of as an orbit followed by electrons around an atom's nucleus.

Krypton number of valence electrons