THE PERIODIC TABLE DOE-HDBK-1015/1-93 Fundamentals of Chemistry CH-01 Rev. 0 Page 20 The electron shells represent major energy states of electrons.  Each shell contains one or more subshells called orbitals, each with a slightly different energy.  In order of increasing energy, the orbitals are designated by the small letters s, p, d, f, g, h. No two shells consist of the same number of orbitals.  The first shell contains only one orbital, an s orbital.  The second shell contains s and p orbitals.  In general, each higher shell contains a new type of orbital: the first shell contains an s orbital, the second shell contains s and p orbitals, the third shell contains s, p, and d orbitals, the fourth shell contains s, p, d, and f orbitals, and so on.  Each orbital can hold a definite maximum number of electrons.  There is also a limit to the number of electrons in each shell and the limit increases as one goes to higher shells.  The numbers of electrons that can occupy the different orbitals and shells are shown in Table 4. TABLE 4 Electrons, Orbital, and Shell Relationships in Atomic Structure Shell Number Type of Orbitals Maximum Number of Electrons Maximum Total in Each Orbital Electrons in shell 1 s 2 2 2 8 s 2 p 6 3 18 p 6 s 2 d 10 4 32 s 2 p 6 d 10 f 14 5 50 d 10 s 2 p 6 f 14 g 18