A chemical bond is the physical phenomenon of chemical substances being held together by attraction of atoms to each other through sharing, as well as exchanging, of electrons -or electrostatic forces.
In general, strong chemical bonds are found in molecules, crystals or in solid metal and they organize the atoms in ordered structures.
Weak chemical bonds are classically explained to be effects of polarity, or the lack of it, of strong bonds.
Important Theories on Chemical Bonding
Lewis diagrams are graphical representations of elements and their valence electrons. Valance electrons are the electrons that form the outermost shell of an atom. In a Lewis diagram of an element, the symbol of the element is written in the center and the valence electrons are drawn around it as dots. The position of the valence electrons drawn is unimportant. However, the general convention is to start from 12o’clock position and go clockwise direction to 3 o’clock, 6 o’clock, 9 o’clock, and back to 12 o’clock positions respectively. Generally the Roman numeral of the group corresponds with the number of valance electrons of the element.
Below is the periodic table representation of the number of valance electrons. The alkali metals of Group IA have one valance electron, the alkaline-earth metals of Group IIA have 2 valance electrons, Group IIIA has 3 valance electrons, and so on. The nonindicated transition metals, lanthanoids, and actinoids are more difficult in terms of distinguishing the number of valance electrons they have; however, this section only introduces bonding, hence they will not be covered in this unit.
To draw the lewis diagrams for molecular compounds or ions, follow these steps below (we will be using H2O as an example to follow):
1) Count the number of valance electrons of the molecular compound or ion. Remember, if there are two or more of the same element, then you have to double or multiply by however many atoms there are of the number of valance electrons. Follow the roman numeral group number to see the corresponding number of valance electrons there are for that element.
Oxygen (O)–Group VIA: therefore, there are 6 valance electrons
Hydrogen (H)–Group IA: therefore, there is 1 valance electron
NOTE: There are TWO hydrogen atoms, so multiply 1 valance electron X 2 atoms
Total: 6 + 2 = 8 valance electrons
2) If the molecule in question is an ion, remember to add or subract the respective number of electrons to the total from step 1.
For ions, if the ion has a negative charge (anion), add the corresponding number of electrons to the total number of electrons (i.e. if NO3– has a negative charge of 1-, then you add 1 extra electron to the total; 5 + 3(6)= 23 +1 = 24 total electrons). A – sign mean the molecule has an overall negative charge, so it must have this extra electron. This is because anions have a higher electron affinity (tendency to gain electrons). Most anions are composed of nonmetals, which have high electronegativity.
If the ion has a positive charge (cation), subtract the corresponding number of electrons to the total number of electrons (i.e. H3O+ has a positive charge of 1+, so you subtract 1 extra electron to the total; 6 + 1(3) = 9 – 1 = 8 total electrons). A + sign means the molecule has an overall postive charge, so it must be missing one electron. Cations are positive and have weaker electron affinity. They are mostly composed of metals; their atomic radii are larger than the nonmetals. This consequently means that shielding is increased, and electrons have less tendency to be attracted to the “shielded” nucleus.
From our example, water is a neutral molecule, therefore no electrons need to be added or subtracted from the total.
3) Write out the symbols of the elements, making sure all atoms are accounted for (i.e. H2O, write out O and 2 H’s on either side of the oxygen). Start by adding single bonds (1 pair of electrons) to all possible atoms while making sure they follow the octet rule (with the exceptions of the duet rule and other elements mentioned above).
4) If there are any leftover electrons, then add them to the central atom of the molecule (i.e. XeF4 has 4 extra electrons after being distributed, so th4 extra electrons are given to Xe: like so. Finally, rearrange the electron pairs into double or triple bonds if possible.
Most elements follow the octet rule in chemical bonding, which means that an element should have contact to eight valence electrons in a bond or exactly fill up its valence shell. Having eight electrons total ensures that the atom is stable. This is the reason why noble gases, a valence electron shell of 8 electrons, are chemically inert; they are already stable and tend to not need the transfer of electrons when bonding with another atom in order to be stable. On the other hand, alkali metals have a valance electron shell of one electron. Since they want to complete the octet rule they often simply lose one electron. This makes them quite reactive because they can easily donate this electron to other elements. This explains the highly reactive properties of the Group IA elements.
Some elements that are exceptions to the octet rule include Aluminum(Al), Phosphorus(P), Sulfur(S), and Xenon(Xe).
Hydrogen(H) and Helium(He) follow the duet rule since their valence shell only allows two electrons. There are no exceptions to the duet rule; hydrogen and helium will always hold a maximum of two electrons.
Types of Chemical Bonds
When substances participate in chemical bonding and yield compounds, the stability of the resulting compound can be gauged by the type of chemical bonds it contains.
The type of chemical bonds formed vary in strength and properties. There are 4 primary types of chemical bonds which are formed by atoms or molecules to yield compounds. These types of chemical bonds include:
- Ionic Bonds
- Covalent Bonds
- Hydrogen Bonds
- Polar Bonds