Hydrogen Bond vs Covalent Bond
 

Chemical bonds hold atoms and molecules together. Bonds are important in determining the chemical and physical behavior of molecules and atoms. As proposed by the American chemist G.N.Lewis, atoms are stable when they contain eight electrons in their valence shell. Most of the atoms have less than eight electrons in their valence shells (except the noble gases in the group 18 of the periodic table); therefore, they are not stable. These atoms tend to react with each other to become stable. Thus, each atom can achieve a noble gas electronic configuration. Covalent bond is one such chemical bond that connects atoms in chemical compounds. Hydrogen bonds are intermolecular attractions between molecules.

Hydrogen Bonds

When hydrogen is attached to an electronegative atom like fluorine, oxygen or nitrogen, a polar bonding will result. Because of the electronegativity, the electrons in the bond will be more attracted to the electronegative atom than to the hydrogen atom. Therefore, hydrogen atom will get a partial positive charge, whereas the more electronegative atom will get a partial negative charge. When two molecules having this charge separation are close by, there will be an attraction force between hydrogen and the negatively charged atom. This attraction is known as hydrogen bonding. Hydrogen bonds are relatively stronger than other dipole interactions, and they determine the molecular behavior. For example, water molecules have intermolecular hydrogen bonding. One water molecule can form four hydrogen bonds with another water molecule. Since oxygen has two lone pairs, it can form two hydrogen bonds with positively charged hydrogen.  Then the two water molecules can be known as a dimer. Each water molecule can bond with four other molecules because of the hydrogen bonding capability. This result in a higher boiling point for water, even though a water molecule has a low molecular weight. Therefore, the energy needed to break the hydrogen bonds when they are going to the gaseous phase is high. Further, hydrogen bonds determine the crystal structure of ice. The unique arrangement of ice lattice helps it to float on water, hence protects the aquatic life in the winter period. Other than this, hydrogen bonding plays a vital role in biological systems. The three-dimensional structure of proteins and DNA are solely based on hydrogen bonds. Hydrogen bonds can be destroyed by heating and mechanical forces.

Covalent Bonds

When two atoms having similar or very low electronegativity difference react together, they form a covalent bond by sharing electrons. Both atoms can obtain the noble gas electronic configuration by sharing electrons in this way.  Molecule is the product resulted by the formation of covalent bonds between atoms. For example, when the same atoms are joined to form molecules like Cl2, H2, or P4, each atom is bonded to another by a covalent bond. Methane molecule (CH4) also has covalent bonds between carbon and hydrogen atoms. Methane is an example for a molecule having covalent bonds between atoms with very low electronegativity difference.

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