By Ron B. Davis Jr., Georgetown University
Noble gases almost never combine with other elements to form compounds. They are invisible gases that don’t react with other elements in their environment. But, why are they so unreactive? The answer to this question reveals one of the most powerful driving forces behind the behavior of elements—the octet rule.
Discovery of the Noble Gases
The final decade of the 19th century was exciting time for element discoveries. Rather suddenly, starting in 1894, a whole new group of a half-dozen elements were discovered in rapid succession. First argon, then helium, then neon, krypton, and xenon, and finally radon.
These gases had proven so elusive because one of their defining characteristics is their lack of reactivity. Mendeleev’s table had been organized based on valence—the ratio in which an element reacts with oxygen or hydrogen. However, these noble gases almost never combine with other elements to form compounds. They do not react with other elements in their environment, and basically remain invisible.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Relation between the Nucleus and Electrons
It was 1916, just two years after Henry Moseley had already blown the lid off of the century-old practice of ordering elements by atomic weight. Chemists now knew that the atomic number, given by the number of protons in the nucleus, was the property that tracked perfectly with the periodic properties of elements.
But what was surprising was that, somehow, the composition of a nucleus concentrated at the center of an atom was influencing the valence—the combining power—which should take place near that atom’s outer edges. This seemed a bit strange.
After all, the outer portion of an atom is where we find electrons, not protons. So how are the nucleus and the electrons related?
The Octet Rule
For an atom to be neutral, it must contain the same number of electrons and protons. So, in a neutral atom, the number of protons in the nucleus is equal to the number of electrons surrounding it. This was the starting-point for the work of Gilbert Lewis, an American chemist at the University of California, Berkeley. Lewis thought he had found a trend in the electron cloud that he could use to explain valence.
By now it had been determined that electrons arranged themselves in layers around the nucleus, a model sometimes called the ‘Bohr model’ of the atom. So naturally, Lewis focused his attention on those electrons in the outermost shell, which is also called the ‘valence shell’.
Lewis noted that four of the noble gases—that is, neon, argon, krypton, and xenon—were the only elements in existence that contained exactly eight electrons in their valence shell. No other element in existence could boast exactly eight—not even helium, despite being a noble gas. It seemed clear that there was something special about having exactly eight valence electrons that made atoms of those noble gases unusually stable.
This configuration is so stable that other types of atoms—those that do not have eight valence electrons—are locked in a constant and never-ending struggle to somehow obtain a total of eight. This remarkably strong tendency for elements to seek out a configuration of eight valence electrons would come to be called ‘the octet rule’.
The Lewis Structure
Lewis took to sketching atoms out in the shape of cubes in his Berkeley classroom. This isn’t to say that he thought atoms were shaped like a cube. Rather, cubes contain eight corners, and at each corner, he placed a dot to indicate the presence of a valence electron. A cube gave him a simple means of keeping track of the number of valence electrons in a given atom.
Later versions have electrons arranged in pairs at four points around the element’s symbol to depict valence electrons. Sodium would have just one dot. Magnesium would have two, and so on.
This depiction is still widely used today and is referred to as the ‘Lewis Structure’ of an atom.
Elements React to Reach a Full Octet
Looking at the periodic table, we see that group I elements like lithium, sodium, and potassium have just one valence electron each. According to the octet rule, these elements will react with others in an attempt to shed that outermost electron, giving them a full octet in their new valence shell.
On the far side of the table, we have elements like fluorine, chlorine, and bromine, all of which have seven valence electrons. These atoms would love nothing more than to steal or borrow just one more electron from another atom to reach a noble gas electron configuration with an octet.
But the noble gases, by contrast, already have 8 valence electrons. So, they have no incentive to lose or gain electrons.
Common Questions about the Octet Rule and the Lewis Structure
For an atom to be neutral, it must contain the same number of electrons and protons. So, in a neutral atom, the number of protons in the nucleus is equal to the number of electrons surrounding it.
Gilbert Lewis was an American chemist at the University of California, Berkeley. He thought he had found a trend in the electron cloud that he could use to explain valence. He drew electrons arranged in pairs at four points around the element’s symbol to depict valence electrons. This depiction is still widely used today and is referred to as the ‘Lewis Structure’ of an atom.
The tendency of an atom to reach the configuration of eight valence electrons is called the octet rule. According to this rule, which was explained by Gilbert Lewis, atoms with more or less than eight valence electrons tend to gain or give up electrons to reach a full octet in their valence shell. This tendency causes the elements to interact with one another.