An atom is divisible!

Krishna: During Dalton’s time many thought that atom was indivisible. The atom as an ultimate and therefore indivisible particle of matter was a venerable and a viable scientific notion for many years. That is not to say that the indestructibility of the atom was universally accepted by scientists in the 19th century. The divisibility and impermanence of atoms accumulated in the late 19th and early 20th centuries.

Dalton's idea of the atom was to be shown inadequate in other ways, as scientists found ways to probe matter at ever smaller scales.

The celebrated characterization of cathode rays by J. J. Thomson in 1897 identified a universal constituent of atoms, the electron. The discovery of radioactivity by Henri Becquerel in 1896 inaugurated the study of the atomic nucleus, even though strong evidence for the nuclear atom would have to wait another 15 years.

Now it is very clear that atom is divisible. It can be divided into many sub-atomic particles.

In physics, a subatomic particle is a particle smaller than an atom. According to the Standard Model of particle physics, a subatomic particle can be either a composite particle, which is composed of other particles (for example, a proton, neutron, or meson), or an elementary particle, which is not composed of other particles (for example, an electron, photon, or muon). Particle physics and nuclear physics study these particles and how they interact. (1)

The elementary particles of the Standard Model are:[2]

Six "flavors" of quarks: up, down, strange, charm, bottom, and top;

Six types of leptons: electron, electron neutrino, muon, muon neutrino, tau, tau neutrino;

Twelve gauge bosons (force carriers): the photon of electromagnetism, the three W and Z bosons of the weak force, and the eight gluons of the strong force;

The Higgs boson.

The Standard Model classification of particles

All of these have now been discovered by experiments, with the latest being the top quark (1995), tau neutrino (2000), and Higgs boson (2012).

Various extensions of the Standard Model predict the existence of an elementary graviton particle and many other elementary particles, but none have been discovered so far.

The work is continuing and who knows what more is waiting to be discovered?

Footnotes:

1. Subatomic particle
2. http://Cottingham, W.N.; Greenwood, D.A. (2007). An introduction to the standard model of particle physics. Cambridge University Press. p. 1. ISBN 978-0-521-85249-4.