Which of the following is not a constituent particle of an atom?

Protons and neutrons together from the sun or nucleus, at the centre of the system. The electrons orbit this nucleus, such as planets. Atoms are made up of protons, neutrons and electrons.

• If atoms are impossibly smallest, these subatomic particles are even more, so that the constituent particles of an atom are electrons, protons and neutrons.
• Photon is a tiny particle or bundle of electromagnetic radiation. It is not a constituent particle of an atom.