Atoms combine to form compounds through chemical reactions, specifically by sharing or transferring electrons in a process known as bonding.

Atoms are the fundamental units of matter and serve as the building blocks of elements. Each atom is composed of protons, neutrons, and electrons. The protons and neutrons reside in the nucleus at the center of the atom, while the electrons orbit in shells around the nucleus. The outermost shell of an atom is referred to as the valence shell, and the electrons located in this shell are termed valence electrons. These valence electrons are crucial in the formation of compounds.

Compounds are created when two or more atoms interact through chemical reactions. This interaction occurs when the valence electrons of one atom engage with the valence electrons of another atom. There are two primary types of chemical bonds that atoms can form to create compounds: covalent bonds and ionic bonds.

In a covalent bond, atoms share one or more pairs of valence electrons. This type of bonding typically occurs between non-metal atoms. For instance, in a water molecule (H$_2$O), the oxygen atom shares a pair of electrons with each of the two hydrogen atoms.

Conversely, in an ionic bond, one atom transfers one or more of its valence electrons to another atom. This transfer results in the formation of ions, which are atoms or groups of atoms that carry a net electric charge. Ionic bonds generally occur between metal and non-metal atoms. For example, in sodium chloride (NaCl), the sodium atom donates one of its valence electrons to the chlorine atom, resulting in the creation of positively charged sodium ions and negatively charged chloride ions.

In both types of bonding, the involved atoms achieve a stable electron configuration, typically characterized by a full outer shell of electrons. This stability is often explained by the “octet rule,” which states that atoms are most stable when their outer shell contains eight electrons. The formation of compounds through chemical bonding is a foundational concept in chemistry, and it underlies the structure and properties of all matter.