The _neon_ atom (compare the diagram) has a particularly stable structure, both “closed” and symmetric, which besides two 1₁-orbits contains four electrons in 2₁ orbits and four electrons in 2₂ orbits. As regards the four electrons in 2₁ orbits, they do not have symmetrical positions at every moment or move simultaneously either towards or away from the nucleus; on the contrary, it must be assumed that the electrons come closest to the nucleus at different moments at equal intervals of time. The name of inert or inactive gases is given to the entire series of helium (2), neon (10), argon (18), krypton (36), xenon (54) and niton (86), the O-column in the periodic system given in the table on p. 23. All these elements are monatomic and quite unwilling to enter into chemical compounds with other elements (although there is about 1 per cent. of argon in the air about us this element has, on this account, escaped the observation of chemists until about 1895, when it was discovered by the English chemist, Ramsay). This complete chemical inactivity is explained by the fact that the atoms of all these elements have a nicely finished “architecture” with all the electrons firmly bound in symmetrical configurations. They may be said to form the mile posts of the periodic system, and to be the ideals towards which the other atoms aspire. The table shows how the electrons in the atoms of these gases are divided among the types of orbits corresponding to the different quantum numbers. TABLE SHOWING THE DISTRIBUTION OF THE ELECTRONS OF DIFFERENT ORBITAL TYPES IN THE NEUTRAL ATOMS OF THE INACTIVE GASES. -------+------+---------------------------------------+ | | Quantum Numbers. | |Atomic+---+---+---+---+---+---+---+---+---+---+ |Number| 1₁| 2₁| 2₂ | 3₁| 3₂| 3₃| 4₁| 4₂ | 4₃| 4₄| -------+------+---+---+---+---+---+---+---+---+---+---+ Helium | 2 | 2 | | | | | | | | | | Neon | 10 | 2 | 4 | 4 | | | | | | | | Argon | 18 | 2 | 4 | 4 | 4 | 4 | - | | | | | Krypton| 36 | 2 | 4 | 4 | 6 | 6 | 6 | 4 | 4 | - | - | Xenon | 54 | 2 | 4 | 4 | 6 | 6 | 6 | 6 | 6 | 6 | - | Niton | 86 | 2 | 4 | 4 | 6 | 6 | 6 | 8 | 8 | 8 | 8 | ? | 118 | 2 | 4 | 4 | 6 | 6 | 6 | 8 | 8 | 8 | 8 | -------+------+---+---+---+---+---+---+---+---+---+---+