By Chris Deziel Updated Mar 24, 2022
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The periodic table catalogues all known elements, each defined by an atom’s nucleus—composed of protons and neutrons—and its surrounding electrons. When atoms interact, they rearrange their outer‑shell electrons to achieve a more stable configuration. This rearrangement gives rise to ionic structures or covalent molecules—both of which form the building blocks of compounds that drive the chemistry of our universe.
Atoms join in either ionic lattices or covalent bonds, producing compounds that range from simple salts to complex organic molecules.
Atoms seek a stable “octet” (eight electrons) in their outermost shell—except for the first shell, which can hold only two. An atom that has a few vacant spots will try to gain or share electrons to fill them, while an atom that carries a few extra electrons will often lose them. Noble gases (helium, neon, argon, etc.) already possess a full outer shell, so they remain inert and do not form compounds with each other or with other elements.
Ionic compounds: An atom that has a single valence electron will donate it to a partner that needs one. The donor becomes a positively charged ion, the recipient a negatively charged ion, and electrostatic attraction pulls them into a crystalline lattice. This is not a discrete molecule but a compound, as exemplified by common table salt, sodium chloride (NaCl). Learn more about ionic bonding.
Covalent bonding: Atoms with one to four extra or missing valence electrons share pairs of electrons to complete their octets. These shared pairs form strong covalent bonds, and the resulting structures are typically molecules. Water (H₂O), formed when an oxygen atom shares one electron pair with each of two hydrogen atoms, is a classic example. Covalent molecules often exhibit lower melting and boiling points than ionic counterparts. Explore covalent bonds.
Metals tend to lose electrons, forming ions that assemble into solid lattices, whereas non‑metals usually form covalent molecules that are gases or liquids at room temperature.
All inorganic and organic molecules composed of more than one element qualify as compounds. Examples include hydrogen chloride (HCl), methane (CH₄), carbon dioxide (CO₂), and the complex sugar sucrose (C₁₂H₂₂O₁₁). Carbon, with four valence electrons, uniquely balances donating and accepting electrons, making it the backbone of organic chemistry.
In contrast, molecules consisting solely of one element—such as nitrogen gas (N₂), oxygen gas (O₂), or ozone (O₃)—are not compounds because they involve only a single element. They are, however, still distinct molecular entities.
For deeper insights into the periodic table and element interactions, consult the NIST Atomic Spectra Database and the Wikipedia entry on the periodic table.
For visual reference, here’s a depiction of how water molecules bond via covalent interactions: Water molecule illustration.
