By Robert M. Hazen, Ph.D., George Mason University
Chemical bonding occurs between almost all elements. The periodic table of elements systematizes all known elements. It was the great discovery of Dmitri Mendeleev who, along with his contemporaries, knew about 63 different chemical elements. Mendeleev and his contemporaries knew that they were far too many to be fundamental particles. They assumed there had to be some underlying order.
Mendeleev’s Great Strategy to Compose the Periodic Table
Dmitri Mendeleev saw the patterns in the chemical elements, as did many of his contemporaries because groups of elements showed such similar behavior. He made a hypothesis and a prediction about the 63 elements, which ended up being the periodic table of the elements. That was his strategy.
For example, the alkaline metals—lithium, sodium, potassium—are all silvery metals, all soft, all highly reactive. In fact, they all would react violently to water. We had the halogens—fluorine, chlorine, bromine, iodine—these elements are all nonmetals. In fact, they’re gases and liquids, very corrosive, very reactive.
The halogens all react in 1:1 ratios with the alkaline metals. And there were other groups, the alkaline earth metals, like magnesium and calcium and barium—once again, similar chemical properties amongst themselves, and very different from the other elements that were known.
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Great Triumph of the Periodic Table
Mendeleev seized on these patterns, and he also seized on the fact that elements have different weights. You can order the elements from left to right by increasing weight, hydrogen being the lightest element, and increasing and so forth. So he arranged this table with vertical columns of elements that displayed similar properties left to right, according to their increasing weight.
This led to the great periodic table of the elements. And the periodic table was great, not just because it systematized the known elements, but because it made predictions, as all good scientific hypotheses must. It predicted elements that had not been seen, and lo and behold those elements, within a few years, were discovered. This was a great triumph of the periodic table.
Magic Numbers and Chemical Bonding
The fact is, though, elements don’t occur in isolation in nature. You don’t find single isolated atoms. What you find is atoms in combination. For example, water is H2O: two atoms of hydrogen combining with one atom of oxygen.
You have window glass that contains primarily silicon and oxygen combined in a 1:2 ratio, SiO2. And even the air you breathe is composed of atoms in combination: oxygen, O2: two oxygen atoms bound tightly together. And 80 percent of the atmosphere is made of nitrogen, N2: two atoms of nitrogen bound together.
So why should this be? Well, one reason lies in the fact that they’re just such stable arrangements of electrons, the magic number of electrons that you’ve seen: two electrons or 10 electrons or 18 or 36, and so forth. These are the magic numbers of electrons, and they correspond to filled shells of electrons in Mendeleev’s periodic table.
The first row has two elements, so a filled shell has two electrons. The next row has 8 electrons, so 2 plus 8 gives you 10, the magic number when you have two filled shells. The third row has 8 more electrons leading you up to 18. So, 2, 10, and 18 are these magic numbers.
Learn more about what makes some types of atoms particularly unstable and reactive.
Why Does Chemical Bonding Happen?
The key to understanding why atoms link together is energy. All natural systems tend to adopt a state of lowest energy. Imagine a situation of a steep, walled valley strewn with boulders, and this will be an analogy to electrons themselves. Imagine that you’re driving down a road on the very floor of the valley, and to either side of the road are giant boulders that are perched just on the side of the roadway.
They’re on the valley floor though; they’re all the way down at the lowest possible energy state. They’re also boulders that are perched in depressions and ledges on the side of the valley and some boulders all the way up at the very tip-top, at the rim of the valley.
Now, those boulders have a lot more energy. If there were an earthquake or someone came along and pushed them off, they’d come crashing down, and they’d release their gravitational potential energy, turning that into heat, as they did destruction when they landed on the valley floor below. Well, electrons are very much the same. The arrangement of an atom’s electrons is, in fact, exactly analogous to the positions of boulders on a valley.
Learn more about carbon’s unparalleled ability to form covalent bonds.
When Atoms Try to Reach the Magic Number
Now, if we go to the periodic table and look at the extreme right-hand column, you’ll see the inert gases. These are atoms that have completely filled outer shells: helium with 2, neon with 10, argon with 18, and so forth. These elements are extremely stable because they have the lowest possible energy, analogous to boulders on the valley floor. They wouldn’t want to gain electrons. They wouldn’t want to lose electrons.
Well, now think about the atoms that are slightly off that magic number. Halogens, like fluorine and chlorine, and bromine have one too few electrons, 9 instead of 10, 17 instead of 18. And so the halogens are desperate to gain an electron. As a result, when they do, they release huge amounts of energy, sometimes in a flash, sometimes in an explosion.
By the same token, one could look all the way over at the alkali earth and the alkali metals. For example, the first column with lithium, sodium, and potassium—these are elements that desperately want to get rid of an electron. Sodium has 11 electrons; it wants to have 10. Potassium has 19; it wants to have 18. And so, these elements try to get rid of an electron.
Common Questions about Mendeleev’s Great Work and the Underlying Causes of Chemical Bonding
The magic numbers are related to the filled shells of electrons in the periodic table. The chemical bonding that occurs between atoms depends entirely on magic numbers.
Inert gases have all the magic numbers (2, 10, 18, etc.) in the electron shells. The outer shell of this atom is completely filled, hence it doesn’t tend to do any chemical bonding with other atoms.
The outer shells of halogens aren’t completely filled with electrons, leading to some chemical bonding with other atoms. Halogens desperately try to gain those missing electrons, and consequently, they are very reactive.