Bohr could have easily stopped there, satisfied with inventing a (semi) quantum model for the one-electron hydrogen. But he didn’t. His was an inquiring mind, and it wanted to know what insights could be gleaned by applying his postulate to atoms with 2, 3, or even 92 electrons.
He approached this problem by extending his quantization condition beyond a single electron. For a helium atom, which contains two electrons, he demanded that the total angular momentum of the electron pair be quantized. He carried out the math and discovered that the two electrons would occupy orbits of near-identical size about the nucleus. However, when he considered a three-electron lithium atom, he discovered that the first two electrons would occupy nearby orbits, similar to those of helium, but the third electron would sit in a much larger orbit that was attracted much less to the nucleus.
He coined the term electron shell to differentiate between the two inner electrons (in the “inner” shell) and the outermost electron (in the “outer” shell). He also concluded that a hydrogen atom, which had a single electron in its outermost shell, would have chemical properties similar to lithium, which also had a single electron in its outermost shell. You may remember this from high school chemistry as the concept of valence, the number of electrons in an atom’s outermost shell, which is the strongest determinant for an element’s chemical properties.
DEFINITION
Electron shells are distinct orbits or layers about an atomic nucleus, each containing a specific number of electrons.
Valence refers to the number of electrons in an atom’s outermost shell; it is a very strong indicator of an element’s chemical properties.
Bohr famously extended this concept in 1922 during a series of lectures (known as the “Bohrfest”) in Goettingen, Germany. Here, he went so far as to speculate that sodium, which also has chemical properties similar to hydrogen and lithium, must also have one electron in its outermost shell (which must therefore be the third shell). This implied that the second shell must be able to hold a total of eight electrons compared to the first shell’s capacity of two.
As you can see, Bohr’s postulates suggested that the structure of atoms with more and more electrons was determined by the steady filling of their atomic shells. But this, as every chemist since Mendeleev already knew, was precisely the basis upon which elements were packed into the period table!
Mendeleev invented the periodic table by grouping elements into rows and columns empirically, based upon the observation and comparison of their chemical properties. For example, hydrogen, lithium, and sodium all sat in the first column (or period) on account of their similar chemical properties. Until Bohr’s theory, however, chemists could give no fundamental reason for all the observed periodicities.
This is a schematic rendition of the first three rows of the periodic table, as explained using Bohr’s shell model. As you move across any given row, one particular shell is steadily filled by electrons. All the atoms in any given column have the same number of electrons in the outermost shell.
More successes for Bohr’s model were to come, largely with respect to predictions of various spectral lines. But his crowning achievement was successfully predicting the chemical properties of hafnium, which has 72 electrons orbiting its nucleus, a year before it had even been discovered. Fittingly, hafnium was discovered by physicists working in Bohr’s hometown, and its name was derived from the Latin term for “Copenhagen.”
Bohr’s shell model also provided insight into the formation of molecules. Chemists had long known that the noble gases (helium, neon, argon, etc.) were the least chemically active, and therefore most stable, of all the elements. Bohr’s shell model implied that the reason for this is that the outermost shell of noble gas atoms was filled to capacity. Filled shells therefore equate to stability. Bohr applied this logic to suggest that atoms seek to form molecules when it allows them to “share” electrons and effectively fill their outermost shells.
Take, for example, the two-atom molecule formed by sodium and chlorine. Chemists refer to this as sodium chloride, while you call it table salt. Sodium has one lone electron in its outer shell and chlorine is one electron shy of filling its outer shell. If the sodium atom “lends” its electron to the chlorine atom, by forming a molecule, then they have both effectively filled their outermost shells. As a result, both atoms are nice and stable and our food tastes all the better.
Table salt is formed because sodium (Na) would rather have one less electron and chlorine (Cl) one more. By forming a molecule and “sharing” the outermost electron of sodium, they each effectively fill their outermost shells and become more stable.
For all its strengths, though, Bohr’s explanation of the periodic table was lacking in one chief respect. It failed to explain why each shell had a limit to how many electrons it could hold. Why, for example, could the innermost shell only hold two electrons? And why did the second shell max out at eight, such that the eleventh electron of sodium was banished to an even more distant shell? The world would have to wait a few years before this mystery was unraveled.
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