Atoms are the building blocks of all matter and are made up of three subatomic particles - protons, neutrons, and electrons. According to CBSE Class 9 Science notes, protons are positively charged, neutrons are neutral, and electrons have a negative charge. Protons and neutrons are located in the nucleus, while electrons are located in shells surrounding the nucleus. The number of protons, neutrons, and electrons in an atom determines the identity of that atom.