The idea that atoms are indivisible was a concept proposed by the ancient Greek philosopher Democritus around 400 BCE. He hypothesized that matter consists of tiny, indivisible particles called atoms. However, our understanding of atoms has evolved significantly since then.
In modern physics, atoms are no longer considered indivisible particles. Instead, atoms are composed of subatomic particles, primarily protons, neutrons, and electrons. The protons and neutrons reside in the nucleus at the center of the atom, while the electrons orbit around the nucleus.
Protons and neutrons are themselves composed of smaller particles called quarks. Protons consist of two up quarks and one down quark, while neutrons consist of one up quark and two down quarks. These quarks are bound together by the strong nuclear force.
Electrons, on the other hand, are elementary particles and are not composed of smaller constituents. They are considered fundamental particles in the Standard Model of particle physics.
The arrangement and interactions of protons, neutrons, and electrons within an atom give rise to its chemical and physical properties. The behavior of atoms and the way they combine to form molecules can be understood through quantum mechanics, which provides a mathematical description of the behavior of particles at the atomic and subatomic scale.
So, while the ancient notion of atoms as indivisible particles was a useful concept at the time, our current understanding of atoms recognizes them as composite structures made up of protons, neutrons, and electrons, which in turn consist of even smaller particles and fundamental entities.