How are atoms defined?

Atoms are defined as the basic building blocks of matter, and they consist of a nucleus that contains protons and neutrons, surrounded by electrons that orbit the nucleus. This structure is fundamental to the atomic theory and is critical for understanding chemical interactions. The nucleus provides the mass and positive charge of the atom, while the negatively charged electrons are involved in chemical bonding and reactions.

The other options do not accurately represent the definition of atoms. Atoms are indeed smaller than molecules, but they are not considered the smallest unit of a molecule since a molecule is made up of two or more atoms bonded together. Atoms are not made up solely of electrons; they also include protons and neutrons, making the first option the only complete and accurate definition. Lastly, while atoms can form chemical bonds to create molecules, the formation of bonds is a process that occurs after atoms are already defined entities.