Hydrochloric acid is a colorless and transparent solution of hydrogen chloride in water. It is a strong acid with a pungent smell. The melting and boiling points of the hydrogen chloride solution are not fixed as they depend upon the concentration of its aqueous solution. Similarly, the values of pH and density of the hydrogen chloride solution are not fixed. If we open a bottle of concentrated HCl solution, we can observe the fumes in the air with a pungent smell. Hydrochloric acid is present on our stomach and helps in the digestion of food. The high concentration of this acid in the stomach leads to acidity problems. HCl is used in the synthesis of various organic and inorganic compounds and it is also used for the regeneration of ion exchanges, which are used for the purification of water. So, is HCl Ionic or Covalent? HCl is a covalent compound because the electronegativity difference between hydrogen and chloride is less than 2.0. However, it is not a true covalent compound as chlorine is more electronegative than hydrogen and hence, it will attract a shared pair of electrons towards itself. Hence, the HCl molecule is a covalent compound with a 17 % ionic character. The H-Cl bond will act as a dipole with a partial positive charge and a partial negative charge on the hydrogen atom and chlorine atom, respectively, in the HCl molecule. The value of partial charge in HCl is ±0. 027 x 10−18 C. Let us discuss the covalent nature of hydrogen chloride in detail. For that, we need to start with the basics of chemical bonding, i.e., what is a chemical bond and how it is formed.
Chemical Bond
First of all, let us have a look at a modern periodic table.
As we all are aware that atoms of the modern periodic table do not exist freely in nature except for group 18 members. Hence, they combine either with the same atom or with a different atom, leading to the formation of elements and compounds, respectively. This force of attraction between two same atoms or different atoms, which helps in bringing them together is known as a chemical bond.
How chemical bonds are formed?
An atom consists of subatomic particles i.e., protons, electrons, and neutrons. Out of these three particles, only electrons participate in the formation of a bond. We know that an atomic number of an atom corresponds to the number of electrons in an atom. Now, a question arises that whether all electrons present in an atom participate in the bond formation? No, all electrons do not participate in the bond formation. The only electrons, which are present in the outermost shell of the atom, contribute towards the bond formation. These electrons are known as Valence Electrons. The electrons, which are present in the inner shell of the atom, are known as Core Electrons and they do not participate in the bond formation. Hence, a chemical bond is formed when there is sharing of electron density between two atoms. This sharing of electron density may be equal or unequal between atoms.
Types of Chemical Bonds
There are two types of chemical bonds, which are present in the molecules i.e. Ionic and Covalent Bonds. Ionic Bonds: The ionic bonds are formed by transferring electron (s) from one atom to another atom. There is an electrostatic force of attraction between cations and anions in an ionic bond. Covalent Bonds: The covalent bonds are formed by sharing of an electron pair (s) between two atoms. The covalent bonds are further classified as single, double, and triple bonds depending on the number of electron pairs shared within two atoms. Now, How to distinguish whether a particular chemical bond is an ionic bond or a covalent bond? Obviously, we cannot answer this question by looking at atoms involved in bond formation. We require a quantitative parameter to differentiate the ionic and covalent bonds. That parameter is Electronegativity values. Let us know, what is electronegativity?
Electronegativity
It is the property of an atom by which an atom attracts the shared electron pair towards it. The electronegativity values of the atoms are taken from the Pauling scale. If the electronegativity difference between two atoms in a chemical bond is greater than 2.0, then the chemical bond is considered an Ionic bond. However, if this difference is less than 2.0 on the Pauling scale, then there is a covalent bond.
Why is HCl Covalent in nature?
In the case of Hydrogen chloride (HCl), The electronegativity value of the hydrogen atom = 2.20 The electronegativity value of the chlorine atom = 3.16 The electronegativity difference of the H-Cl bond = 0.96 The electronegativity difference of the H-Cl bond is only 0.96 on the Pauling scale, indicating the covalent nature of the H-Cl bond. Hence, hydrogen chloride is a covalent compound. However, hydrogen chloride is not a true covalent compound. Why is it so and what are true covalent compounds?
Why is HCl not a true Covalent Compound?
A true covalent bond is formed when there is an equal sharing of the electron from each atom in the molecule such as homonuclear molecules i.e., H-H, F-F, and Cl-Cl, etc. However, the covalent character or we can say purity of covalency decreases when there are heteronuclear molecules. For example H-Cl, H-Br, and H-I. In heteronuclear molecules, atoms do not share electron (s) equally because of the difference in the electronegativity value. Hence, HCl is not a true covalent compound as the chlorine atom will attract a shared pair of electrons towards itself owing to its higher electronegativity (3.16) than the hydrogen atom (2.20).
Or we can say that the H-Cl bond in the hydrogen chloride compound is a polar covalent bond. It means that the H-Cl bond act as a dipole with a partial positive charge on the hydrogen atom and a partial negative charge on the chlorine atom. It would be quite interesting if we can calculate the partial charge on the atom. So, let us calculate the partial charge on the hydrogen and chlorine atom in hydrogen chloride. The charge is defined as the ratio of the dipole moment of the molecule (µ) and the distance between the two atoms (d) in a polar covalent bond. The formula for measurement of charge is as follows: Q = μ/d The dipole moment is measured in D (Debye) and the unit for the distance is m and hence, the charge is measured in D/m or C (Coulomb). The dipole moment of hydrogen chloride is 1.05 D and the distance between the hydrogen and chlorine atom in the hydrogen chloride is 127.4 * 10-12 m. 1 D = 3.33 × 10−30 C. m If we substitute the values in the above equation, the partial charge on the hydrogen atom comes out to be 0.027 x 10-18 C. There will be equal but opposite charge on the chlorine atom i.e., – 0.027 x 10-18 C Therefore, the value of partial charge in hydrogen chloride compound is ±0. 027 x 10−18 C. Our next step would be to estimate the percentage of electrons transferred from the hydrogen atom to the chlorine atom in hydrogen chloride, which is known Ionic character in the covalent bond. As we know the charge on one electron is 1. 6022 × 10−19 C. Percentage of Ionic Character = (Partial charge on the atom ∗ 100) / Charge on one electron = (0. 027 x 10−18 ∗ 100)/1. 6022 × 10−19 C = 17 % Hence, only 17 % electron density has been transferred from the hydrogen atom to the chlorine atom in the hydrogen chloride. It confirms that the H-Cl bond in the hydrogen chloride is a polar covalent bond, not an ionic bond. It is another criterion for distinguishing between Ionic and covalent bonds. Hence, the hydrogen chloride molecule is a covalent compound with a 17 % ionic character.
Conclusion
In this article, we have studied the covalent nature of the hydrogen chloride compound. In brief, hydrogen chloride is a covalent compound based on the electronegativity difference between two atoms of the HCl molecule. However, this covalent compound has some ionic character, which is calculated to be 17 %. It confirms the nature of the H-Cl bond in the HCl molecule as a polar covalent bond. In conclusion, the hydrogen chloride molecule is a polar covalent compound having ±0.027 x 10−18 C partial charge with a 17% ionic character.
