Important notes for the Radioactivity chapter for class 10. Contains all the relevant terms and definitions in a crisp way as per the prescribed ICSE pattern. The reference taken are from Selina, Dalal, and the previous years' board papers. (last 5 years) as well as keeping in mind the MCQs prescribed as per the latest guidelines from ICSE. Students can find the definitions and the explaination of the terms in the simplest yet effective words. Strictly to be followed for revision during board exams for better results. For defining some parts, reference has been given from the books(Selina publication) wherever the author finds the book's definition better and more effective for the students preparing for their board examination. Regarding the numerical, the formula and the terms are described in detail and the same is to be referred for solving the sample papers and previous year's board's papers. Please do comment on any suggestions needed and the same will be incorporated in the next chapter.