The way in which atoms join together.

• A. Reactivity
• B. Bonding
• C. Isotopes
• D. Crystallization

Answer: (B)

Bonding is the way atoms join together. It happens when atoms reach a more stable electron arrangement by sharing or transferring electrons, creating connections that hold atoms in molecules or solids. Covalent bonds form when atoms share electrons, as in water. Ionic bonds form when electrons are transferred from a metal to a nonmetal, producing charged ions that attract each other, like in table salt. Metallic bonds involve a pool of electrons shared among many atoms in a metal.

This directly captures what the question asks—the mechanism by which atoms connect. The other terms describe related ideas but not the joining itself: reactivity is about how readily atoms react, isotopes are different versions of an element with varying neutrons, and crystallization is a process by which a solid lattice forms, which involves bonding but isn’t the basic act of joining atoms together.