Hydrogen is a unique atom, because it has only two spots in its outermost electron level. Hydrogen’s valence number is one, because it has only one valence electron and needs only one shared electron to fill its energy levels. This means it can bond with many elements.

The bond joining two hydrogen atoms in a hydrogen gas molecule is a classic covalent bond. Because the hydrogen atoms are identical, neither can take the electron from the other to complete its electron shell and form an ionic bond. As a result, the two hydrogen atoms share the two electrons in a covalent bond.

It must be a covalent bond because Hydrogen is already very stable and would be unfavorable if it was taken off completely (in an ionic bond). HF is not an ionic bond because by definition, ionic bonds are electron sharing between a metal and nonmetal.

Metallic bonds

ionic

Bonds between carbon and other elements such as oxygen and nitrogen are polar. The polarity of a bond depends on the electronegativities of the bonded atoms. Large differences between the electronegativities of the bonded atoms increase the polarity of bonds.

To determine the polarity of a covalent bond using numerical means, find the difference between the electronegativity of the atoms; if the result is between 0.4 and 1.7, then, generally, the bond is polar covalent.

Step 2: Identify each bond as either polar or nonpolar. (If the difference in electronegativity for the atoms in a bond is greater than 0.4, we consider the bond polar. If the difference in electronegativity is less than 0.4, the bond is essentially nonpolar.) If there are no polar bonds, the molecule is nonpolar.

Bonds that are partly ionic are called polar covalent bonds. Nonpolar covalent bonds, with equal sharing of the bond electrons, arise when the electronegativities of the two atoms are equal.