Understanding Chemical Bonds: Atomic Stability and Lewis Octet Rule
Chemical Bonds
Atomic Stability
3.1 Lewis Octet Rule
3.2 Ionic Bonds: Properties of Ionic Compounds
3.3 Covalent Bonds: Properties of Covalent Compounds. Intermolecular Forces
3.4 Metallic Bonds: Properties of Metallic Compounds
In nature, we observe a variety of simple and compound substances, consisting of combinations of atoms, either of the same or different elements. However, with the exception of noble gases, we rarely find substances consisting of individual atoms. This leads us to ask two questions:
What special feature do the noble gases possess?
Why do the rest of the atoms tend to combine with other atoms?
The answer to both questions lies in a fundamental concept in every physical system: stability. Any system tends to maximize its stability, usually accomplished with minimal energy. A ball rolls down a slope, a stretched spring recovers its form, an electron in an upper layer jumps to a lower layer because the energy that holds it is less than it was at first. In all these situations, if we reverse the process, we supply power.
Similarly, two or more atoms bond together because the whole has less energy than the sum of the separate atoms. In binding, energy has decreased. And therein lies the key: to separate them again, we will supply the amount of energy that was previously released. While that energy is not supplied, they are kept together.
If the noble gases have no tendency to bind to other atoms, it is because they have the highest possible stability. A union with another atom does not release energy.
The common feature of all the noble gases, and the reason they are located in the same group, is their electron configuration. Regardless of the period in which they are, all have 8 electrons in their valence shell (s and p subshells complete) and complete all previous layers. The only exception is Helium (He), but it has only sublayer 1s, and it is complete. It is, therefore, that the configuration (s2p6) in the last layer of the atom provides great stability. Other elements will try to achieve that configuration, taking, giving, or sharing electrons with another atom.
This tendency is called the Lewis octet rule:
- Atoms are most stable when they have 8 electrons in their valence shell, complete with the s and p subshells.
- To achieve the above, in some cases electrons are transferred from one atom to another, forming ions (ionic bond); in others, they share one or more pairs of electrons (covalent bond). This will depend on the electronegativity difference (ΔX).
There are exceptions to this rule. There are elements (Be, B) that can surround themselves with less than 8 electrons, and some (S, P) can be surrounded by 10 up to 12 electrons. Later we will see some cases.
Lewis’s theory has now been largely superseded by theories such as Molecular Orbital Theory (MOT) or Valence Bond Theory (VBT), obtained from the quantum model of the atom. However, it poses a very simple and very useful starting point for studying chemical bonds.
Lewis Diagrams
Lewis diagrams are an easy way to symbolically represent how electrons are distributed on the last layer in an atom. We’ll see with several examples.
As we see, the electrons, represented by points, are paired or unpaired, as found in the respective orbitals. These diagrams are very useful when studying how atoms exchange electrons.