Atoms rarely exist in isolation; they combine to form compounds, which are substances made from two or more different elements chemically bonded together. The force holding these atoms together is known as a chemical bond, resulting from interactions involving the outermost electrons (valence electrons). Understanding how these connections form reveals the underlying principles that govern the structure and properties of all matter.
Why Atoms Form Connections
Atoms form chemical connections primarily to achieve a state of lower energy and greater stability. This drive is rooted in the tendency of atoms to complete their outermost electron shell, known as the valence shell. For most atoms, this stable configuration means having eight electrons in the valence shell, a principle called the octet rule. Noble gases, such as Neon and Argon, already possess this full outer shell, which is why they are chemically inert.
Unstable atoms interact with others to gain, lose, or share valence electrons until a full shell is achieved. The energy of the system is minimized when this stable electron configuration is reached, which is the driving force for bond formation. The type of interaction—whether electrons are shared, transferred, or pooled—defines the different categories of chemical bonds.
The Mechanism of Sharing Electrons
Covalent bonding, which typically occurs between nonmetal atoms, is a common way atoms achieve stability through the mutual sharing of valence electrons. The shared electrons orbit the nuclei of both bonded atoms, effectively completing the outer shell for each. The attractive force arises from the simultaneous attraction of the shared, negatively charged electrons to the positively charged nuclei of the atoms involved.
A single covalent bond involves sharing one pair of electrons, while atoms can also share two or three pairs to form double or triple bonds. The increase in shared electron pairs results in a progressively shorter and stronger bond between the atoms. When the atoms in a covalent bond are identical, the electron pair is shared equally, resulting in a nonpolar covalent bond.
If the two bonded atoms are different, one atom may exert a stronger pull on the shared electrons, a property called electronegativity. This unequal sharing creates a polar covalent bond, shifting the electron density toward the more electronegative atom. The atom that attracts the electrons more strongly acquires a partial negative charge, while the other atom gains a partial positive charge. This charge separation is a fundamental characteristic of many molecules, influencing how they interact with other substances.
The Mechanism of Electron Transfer
Ionic bonding involves the complete transfer of valence electrons from one atom to another to achieve stability. This process usually takes place between a metal atom and a nonmetal atom, which have large differences in electronegativity. The metal atom loses electrons to become a positively charged ion (cation), while the nonmetal atom gains electrons to become a negatively charged ion (anion).
Once these ions are formed, they are held together by a powerful electrostatic attraction—the force between opposite electrical charges. This strong, non-directional attraction is the ionic bond itself, resulting from the full charge separation during electron transfer.
This attraction causes the ions to arrange themselves into a highly ordered, repeating structure called a crystal lattice. Forming the lattice maximizes the attractive forces between oppositely charged particles and contributes significantly to the overall stability of the ionic compound. For example, in table salt (sodium chloride), the ions are locked into a rigid structure by these pervasive electrostatic forces.
Holding Metal Atoms Together
Metal atoms are held together by a unique connection called metallic bonding. Metals have relatively low electronegativity and easily lose their valence electrons, but no nonmetal atoms are present to accept them. Instead, the valence electrons from all the metal atoms become delocalized, meaning they are not associated with any single atom.
This creates a structure visualized as a lattice of positively charged metal ions submerged in a “sea” of mobile valence electrons. The bond results from the collective attraction between the positive metal ions and the surrounding pool of negative electrons. This freely moving electron sea is responsible for characteristic properties like excellent electrical and thermal conductivity, and explains why metals are malleable and ductile.