Covalent bond It is a type of interaction between atoms that have high electronegativity, that is, a high tendency to receive electrons. The chemical elements commonly involved in this type of bonding are:
a) Nature of the elements involved
The chemical elements that have high electronegativity and that, consequently, form covalent bonds are:
b) Occurrence of covalent bonding
According to the nature of the chemical elements involved in the covalent bond, it can occur as follows:
Between two hydrogen atoms;
Between a non-metal atom and Hydrogen;
Between atoms of the same chemical element (non-metal);
Between atoms of different chemical elements (both non-metals).
c) Number of electrons that each atom must receive
The number of electrons that each atom of a nonmetal or of Hydrogen receives in a bond is related to the octet rule.
According to the octet rule, an atom is stable when it acquires eight or two electrons (only in the case of Hydrogen) in the valence shell. If an atom has five electrons in the valence shell, for example, it must receive three electrons to achieve stability.
OBS.: Beryllium and Boron are exceptions to the octet rule, as they become stable, respectively, with 4 and 6 electrons in the valence shell.
The number of electrons in the valence shell can be determined easily by analyzing the chemical element’s family. In the table below, we have the number of electrons in the valence shell referring to the family to which the element belongs and the number of electrons it needs to receive to achieve stability:
d) Principle of covalent bonding
As in the covalent bond all the atoms involved present the tendency to receive electrons, obligatorily, there will be a sharing among them of the electrons present in the valence layer (furthest level from the nucleus).
Sharing occurs when an electron in the valence shell of one atom becomes part of the same electron cloud that surrounds another electron in the valence shell of another atom.
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Each hydrogen atom, for example, has an electron in its valence shell. When two electrons become part of the same cloud, each Hydrogen starts to present two valence electrons, that is, it stabilizes.
Electrons from two Hydrogen atoms occupying the same electron cloud
e) Formulas used in covalent bonding
1st) Molecular formula
It is the indication of the number of atoms of each element that form the molecule originated from covalent bonds.
A water molecule has 2 hydrogen atoms and 1 oxygen atom.
2nd) Structural formula
The structural formula is the demonstration of the organization of the molecule, that is, it demonstrates the connections between the atoms. For this, traces are used that represent the bond of each atom:
Simple (?): Indicates that the atom shared only one electron from its valence shell with another atom and vice versa;
Double (?): Indicates that the atom shared two electrons from its valence shell with the other atom and vice versa;
Triple (≡): Indicates that the atom shared three electrons from its valence shell with another atom and vice versa.
structural formula of water
3rd) Electronic Lewis formula
The Lewis electronic formula also represents the organization of the molecule (electronic formula), but aims to demonstrate the sharing of electrons of atoms.
To build it, we just need to respect the organization proposed in a structural formula and replace each trace of the bonds (single, double or triple) by “two little balls”, which represent the electrons.
In the structural formula of water, for example, we have two single bonds between Hydrogen and Oxygen. Thus, between them, we will have only two balls, delimited by an ellipse (which represents the electronic cloud).
electronic Lewis formula of water
By me. Diogo Lopes Dias