Atoms consist of three basic particles: protons, electrons, and neutrons. The nucleus (centre) of the atom contains the protons (positively charged) and the neutrons (no charge). The outermost regions of the atom are called electron shells and contain the electrons (negatively charged). The topics covered under this chapter include subatomic particles, Thomson’s atomic model, Rutherford’s atomic model, Bohr’s model and the quantum mechanical model of the atom. The types of questions asked in the NCERT exercise section for this chapter include Basic calculations regarding subatomic particles (protons, electrons, and neutrons), Numericals based on the relationship between wavelength and frequency, Numericals based on calculating the energy associated with electromagnetic radiation, Electron transitions to different shells, Writing electron configurations and Questions related to quantum numbers and their combinations (for electrons).