A free original English–Hindi lesson on atomic structure and selected periodic patterns. Separate element identity, isotope and charge; distinguish atomic number, mass number, actual isotope mass and average atomic mass. Use introductory shell arrangements and compare stated main-group examples without treating a trend as exceptionless. Four worked cases, labelled representations, an analogy, a quick reference and five explained questions support particle accounting and interpretation. Understanding average mass is core; weighted-average arithmetic is optional and excluded from compulsory review. Shell capacity is distinguished from filling order. These selected foundations are not an exhaustive chemistry syllabus or a claim about RRB subtopic weightage.