Electron No. 3 will accordingly not be bound in the same group with 1 and 2. It must be satisfied with a 2-quantum orbit, 2₁, which consists of oblong loops, and, when nearest the nucleus, comes into the territory of the 1-quantum orbits. It is but loosely bound compared to the first two electrons, and the lithium atom, which has only three electrons, can therefore easily let No. 3 loose so that the atom becomes a positive ion. Lithium is therefore a strongly electropositive monovalent metal. The element beryllium (No. 4) will probably have two electrons in the orbits 2₁; it will therefore be divalent. But during the short visit of these electrons to the nucleus they are subject to a greater nuclear charge than in lithium. The 2₁ electrons are therefore, in beryllium, more firmly bound than in lithium, and the electropositive character of beryllium is therefore less marked.
We have something essentially new in the boron atom (atomic No. 5) where the two electrons No. 3 and No. 4 are taken into 2₁ orbits, but where No. 5 will very probably be bound in a circular 2₂ orbit. How the conditions will be in the normal state of the following atoms preceding neon is not known with certainty. We only know that the electrons coming after the first two will be captured in 2-quantum orbits, the dimensions of which get smaller, according as the atomic number increases.
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.
| Atomic Number | Quantum Numbers. | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1₁ | 2₁ | 2₂ | 3₁ | 3₂ | 3₃ | 4₁ | 4₂ | 4₃ | 4₄ | ||
| Helium | 2 | 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 |
| Atomic Number | Quantum Numbers. | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 5₁ | 5₂ | 5₃ | 5₄ | 5₅ | 6₁ | 6₂ | 6₃ | 6₄ | 6₅ | 6₆ | 7₁ | 7₂ | 7₃ | ||
| Helium | 2 | ||||||||||||||
| Neon | 10 | ||||||||||||||
| Argon | 18 | ||||||||||||||
| Krypton | 36 | ||||||||||||||
| Xenon | 54 | 4 | 4 | - | - | - | |||||||||
| Niton | 86 | 6 | 6 | 6 | - | - | 4 | 4 | - | - | - | - | |||
| ? | 118 | 8 | 8 | 8 | 8 | - | 6 | 6 | 6 | - | - | - | 4 | 4 | - |
The elements fluorine, oxygen and nitrogen can attain the ideal neon-architecture by binding respectively one, two and three additional electrons. Naturally they do not become neon atoms, but merely negative atomic ions with single, double or triple charge; and their tendency in this direction appears in their character of monovalent, divalent and trivalent electronegative elements respectively. If we return to carbon it can probably not become a tetravalent negative ion by binding four free electrons; but in the typical carbon compound, methane (CH₄), the neon ideal is realized in another manner. In fact, it is reasonable to assume that the four electrons of the hydrogen atoms together with the six of the carbon atom give approximately a neon-architecture. The four hydrogen nuclei naturally cannot be combined with the carbon nucleus; the mutual repulsions keep them at a distance. They will probably assume very symmetrical positions within the electron system which holds them together. The nitrogen atom may in a similar way find completion in a neutral molecule with neon-architecture, if it unites with three hydrogen atoms to form ammonia NH₃; but the three hydrogen nuclei, although having symmetrical positions, will not lie in the same plane as the nitrogen nucleus. The electric centre of gravity for the positive nuclei will therefore not coincide with the centre of gravity for the negative electron system. The molecule obtains thus what might be called a positive and a negative pole, and this dipolar character appears in the electrical action of ammonia (its dielectric constant). Something similar holds true for the water molecule, where, in a neon-architecture of electrons, in addition to the oxygen nucleus in the centre there are two hydrogen nuclei which are not co-linear with the oxygen nucleus.
If we go on from neon in the periodic system we come to sodium (11). When the sodium nucleus captures electron No. 11, this cannot find room in the neon-architecture formed by the first ten electrons. Since the eleventh electron thus cannot find a place in either a 2₁ or a 2₂ orbit, it is bound in a 3₁ orbit ([cf. Fig. 29] and diagram at the end). The atom then has a character like that of the lithium atom, and we can therefore understand the chemical relationship between the two elements, which are both monovalent electropositive metals.
We shall not dwell longer upon the individual elements of the atomic series. If we pass from neon through sodium (11), magnesium (12), aluminium (13), etc., to argon (18), we get what is essentially a repetition of the situation in the series from lithium to neon. We first get two orbits of the 3₁ type in magnesium, a 3₂ orbit is for the first time added in aluminium, and for the atomic number 18, eight 3-quantum orbits, together with the eight orbits of the inner 2-quantum group and the two of the innermost 1-quantum group, give the symmetric architecture of argon ([cf. table on p. 196], and [diagram at the end]).