The octet rule is one of the fundamental concepts of chemistry. Discover the simple explanation of the rule and what are the three exceptions.
The universe of chemistry is a fascinating world, full of laws, theories and discoveries that have shaped our understanding of matter. Among the chemists who have left an indelible mark on this field is Gilbert Newton Lewis , known for having devised the ” Lewis structure ” and for having introduced the famous ” octet rule “, along with its exceptions .
In this guide, we will explore the octet rule and its exceptions, which fall into three main types.
Octet rule
The octet rule is a rule of thumb developed in 1916 by Gilbert N. Lewis, which serves to approximately describe the formation of chemical bonds between atoms . This rule applies mainly to atoms in the main groups of the periodic table , which are distinguished from the others by Roman numerals.
According to the rule, atoms tend to gain or lose electrons to reach a stable electronic configuration , with eight electrons in their valence shell, that is, in the outermost energy level. Once the valence shell is full, the atom is energetically stable and does not tend to form further bonds.
For example , when sodium combines with fluorine to form sodium fluoride, sodium loses an electron, reaching a 2 + 6 = 8 outer configuration. Fluorine, on the other hand, gains an electron, also completing its outer level with a 2 + 6 = 8 configuration.
Octet Rule: The Three Exceptions
There are three exceptions to the octet rule : atoms with too few electrons, atoms with an excess of electrons, and atoms with 8 electrons.
Electron deficiency
The first concerns atoms with few electrons in their valence shell , such as hydrogen, beryllium, and boron. Hydrogen has only one electron, beryllium has two, and boron has three. No matter how many bonds they form, these atoms will never reach eight electrons in their valence shell.
For example , in the case of borane (BH₃) which is composed of one boron atom and three hydrogen atoms, the hydrogen has one electron and the boron has three. As can be seen, there are not enough electrons to complete the octet of boron, which therefore does not satisfy the octet rule.
Excess electrons
The opposite exception concerns the case where there is an excess of electrons , which leads to the octet being exceeded . This phenomenon occurs for elements that are above the third period of the periodic table.
A typical example is sulfur trioxide (SO₃), which is composed of sulfur and oxygen. Sulfur has six valence electrons, as does oxygen. When bonds are formed, 12 electrons are obtained in the valence shell, which means that the octet has been “expanded”.
Another common example of this behavior is observed with phosphorus , which can also expand its octet.
Behavior of noble gases
The third exception to the octet rule concerns helium , which is the only ” noble gas ” that does not have eight electrons in its valence shell, but has only two. Contrary to the octet rule, helium follows a stable configuration called a “duet” , which occurs when an atom has two electrons in its valence shell.
Some other elements with an atomic number Z close to that of helium can also achieve a stable configuration with two electrons in the valence shell.
In general, each element tends to adopt a stable electronic configuration , which can lead it to become unreactive or poorly reactive , as happens to the noble gases.
Elements in the first groups of the periodic table tend to lose electrons through ionization , assuming the electronic configuration of the noble gas that precedes them. In contrast, elements in groups VI and VII tend to gain electrons by releasing energy (known as electron affinity ), thus achieving the configuration of the noble gas that follows them.
