Okay, so here’s something you should know about the development of the basic model of the atom before you go putting too much faith into it…
Long before the word “Electron” was even part of the scientific vocabulary, chemists had noticed that different elements had different chemical characteristics. Some were very active, and some were practically inactive. The active elements were said to have high “valency” and the less active to have “low valency.” When the elements were grouped according to their level of valency, chemists found that they naturally separated themselves into eight groups, each representing a different level of valency.
In the early 20th century, when Niels Bohr was coming-up with one of his successive schemes of atomic structure, he used as his starting-point the Rutherford model, which posited the atom to be like a tiny solar system, with negatively charged Electrons orbiting around a positively charged nucleus. When Bohr was deciding how many electrons and electron-orbits there should be in atoms, he admitted, according to author Theodore Arabatzis, that he determined how many Electrons to place in an atom’s outermost Electron-orbit “arbitrarily”— he figured it sure would be neat if the number of Electrons in the outershell had something to do with an atom’s chemical properties. Without this sort of tie-in, the idea of Electron-orbits really had little connection to the real world and the chemistry of the day. By tying Electron-orbits to chemical properties, Bohr could make these conjectured orbits appear more realistic and, thus, acceptable.
So, Bohr “arbitrarily” (his word, by the way) matched the number of Electrons in each atom’s outermost shell to each atom’s valency-level. In this way, he came up with the scheme that the largest number of Outer Electrons an atom could possess would be eight, and the fewest would be one. For example, an atom in the fourth category of valencies was said to have four Electrons in its outershell.
It is by this pre-determined condition, says Arabatzis, that “the properties of the chemical elements were linked with the number and behavior of the Outer Electrons.” These Outer Electrons, which came to be called Valence Electrons, were thereby EQUATED with the chemical characteristics of an element… no other part of the atom much mattered anymore, chemically speaking.
But the truth is, writes Arabatzis, that Bohr’s electron-configuration scheme “could not be obtained solely from the principles of his theory.” For instance, if chemists had come-up with ten different valency-levels, Bohr would have placed between one and ten Valency Electrons in the outershells of atoms. If five valency categories had been created, Bohr’s atoms would have had between one and five Valency Electrons. There was nothing in his theory that determined the number. It was a matter of convenience and wishful thinking.