Covalent bonds

A covalent bond forms when two atoms share a pair of electrons, allowing each atom to complete its valence shell. Covalent bonding typically occurs between nonmetals, whose similar electronegativities favor sharing over the outright electron transfer that produces ionic bonds. The shared pair is attracted to both nuclei, and that mutual attraction is what holds the atoms together as a molecule.

Atoms can share more than one pair. A single bond shares one pair (as in H₂), a double bond shares two pairs (as in O₂ or the C=O of carbonyl groups), and a triple bond shares three pairs (as in N₂). More shared pairs mean a shorter, stronger bond: triple bonds are the shortest and strongest of the three, though the second and third bonds (pi bonds) are individually weaker than the first (sigma bond) and are where much of organic reactivity happens.

Sharing is not always equal. When bonded atoms differ in electronegativity, the shared electrons sit closer to the more electronegative atom, creating a polar covalent bond with partial charges — the O–H bonds of water are the classic example, and molecular polarity underlies hydrogen bonding and many solubility behaviors. When electronegativities are similar, as in C–H bonds or diatomic elements, the bond is nonpolar. A coordinate covalent bond arises when one atom donates both electrons of the shared pair.

The MCAT tests covalent bonding within the nature of molecules and intermolecular interactions, including polarity and electronegativity trends from the periodic table, while USMLE Step 1 draws on covalent bonds in biochemistry — for example, enzymes that form covalent intermediates and drugs that bind their targets covalently.