Figure 1 - Lewis's drawings in class of valence electron interactions. |
In 1902, while Lewis was trying to explain valence to his students, he drew atoms as a bunch of cubes with electrons at each corner. This “cubic atom” explained the eight groups in the periodic table, and showed his idea that chemical bonds are formed by atoms trying to complete a set of eight outer electrons (an “octet”). Lewis’s theory of chemical bonding continued to evolve and, in 1916, he published his seminal article suggesting that a chemical bond is a pair of electrons shared by two atoms.
Lewis structures are named after Gilbert Lewis, who drew the picture shown in Figure 1. Source:https://www.sciencehistory.org/historical-profile/gilbert-newton-lewis |
Lewis symbols (also known as Lewis dot diagrams or electron dot diagrams) are diagrams that represent the valence electrons of an atom. Lewis structures are diagrams that represent the valence electrons of atoms within a molecule. These Lewis symbols and Lewis structures help visualize the valence electrons of atoms and molecules, whether they exist as lone pairs or within bonds.
The valence shell is of great interest in chemistry because the electrons it holds are the furthest away from the nucleus, and therefore are the ones that are involved in chemical reactions and bonding. Chemical reactivity depends on the number of electrons in that last, outermost level, called the valence level or valence shell. They allow us to predict how different atoms will react and bond with other atoms -- chemists use them to identify bonds within molecules, which allows us to predict many other properties like boiling point, melting point, viscosity, reactivity, etc. Source: https://courses.lumenlearning.com/boundless-chemistry/chapter/lewis-dot-symbols-and-lewis-structures/ |
Figure 2 - Examples of Lewis structures of covalent compounds. |
Figure 3 - An example of a covalent compound with both atoms having a stable octet through sharing. |
Atoms gain, lose, or share electrons in their valence level in order to achieve greater stability. From this perspective, bonds between atoms form so that the bonded atoms are more stable compared to when they were by themselves. Atoms can achieve this more stable state by having a valence level which contains as many electrons as it can hold. For the first principal energy level, having two electrons in it is the most stable arrangement, while for all other levels outside of the first, eight electrons are necessary to achieve the most stable state. This explains why hydrogen is an exception to the octet rule -- it only needs 2 to be the most stable! Source: https://courses.lumenlearning.com/boundless-chemistry/chapter/lewis-dot-symbols-and-lewis-structures/ |
When we drew Bohr models, you saw that for each atom we drew circles to represent energy levels and dots for electrons. There is a common, shorthand way to represent atoms called Lewis dot diagrams. Dots still represent electrons, but they are drawn around the symbol for the element. Only the electrons in the outermost energy level (valence electrons) are drawn. To draw an electron dot diagram, place one dot around the symbol for every valence electron. Place the first dots alone on each side and then pair up any remaining dots. Source: https://sites.google.com/a/student.isb.ac.th/eve-wongworakul-chemistry-unit/bohr-rutherford-diagrams-lewis-dot-diagrams |
Figure 4 - Comparison of Bohr models and Lewis dot diagrams for potassium (K). |