The first model of atom proposed by J.J Thomson in 1898, described that the positive charge of the atom is uniformly distributed throughout the atom and the negative electrons are embedded in it like seeds in a watermelon. This model was also known as the plum pudding model of the atom. However, subsequent studies on atoms discussed in this chapter showed that the distribution of protons and electrons in an atom is very different from that proposed in this model. In the early nineteenth century, it was also established each element is associated with a characteristic spectrum of radiation. Since hydrogen is the simplest of elements, we shall consider its spectrum in this chapter. We have provided a concise CBSE class 12 Physics revision for chapter 12 Atoms with important points to remember.