Unlike sodium and chlorine, which become more stable by transferring electrons (forming ionic bonds), some atoms become more stable by sharing electrons, forming covalent bonds.

Here's why:

* Ionic bonds: Sodium (Na) has one valence electron, while chlorine (Cl) has seven. Sodium readily gives up its electron to become a positively charged ion (Na+), and chlorine readily gains an electron to become a negatively charged ion (Cl-). These opposite charges attract, forming a stable ionic bond.

* Covalent bonds: Atoms like oxygen (O), carbon (C), and hydrogen (H) have a strong tendency to share their valence electrons with other atoms to achieve a stable electron configuration. This sharing results in a covalent bond, where two atoms share one or more pairs of electrons. This shared electron pair is attracted to the nuclei of both atoms, holding them together.

Examples of Covalent Bonds:

* Water (H₂O): Two hydrogen atoms share their electrons with an oxygen atom, resulting in two covalent bonds.

* Methane (CH₄): One carbon atom shares its four electrons with four hydrogen atoms, forming four covalent bonds.

Key takeaway: Covalent bonding is a key concept in chemistry and is responsible for the formation of a wide variety of molecules, including those that make up living organisms.