What are the building blocks of molecules?

The correct choice is atoms, as they are the fundamental units that make up molecules. Atoms consist of a nucleus containing protons and neutrons, surrounded by a cloud of electrons. When two or more atoms bond together, they form molecules. This process involves interactions between the outer electrons of the atoms, which can either share or transfer electrons, leading to various types of chemical bonds such as covalent or ionic bonds.

While molecules themselves are composed of atoms, they do not serve as the basic units in a way that leads to the formation of other structures; rather, they are the products of atoms coming together. Electrons and protons, while essential components of atoms, do not individually constitute the building blocks of molecules. Electrons are primarily involved in bonding situations, and protons are part of the atomic nucleus, affecting the identity of the element but not acting as building blocks for molecules on their own. Therefore, understanding the role of atoms is crucial, as they are the essential particles that combine to form all types of substances, including gases, liquids, and solids in both organic and inorganic chemistry.