Introduction

By the start of the twentieth century, physicists knew that every atom contains negatively charged electrons -- discovered by J.J. Thomson in 1897 -- embedded in some kind of positively charged material that makes the atom, taken as a whole, electrically neutral. What nobody yet knew was HOW that positive charge, and almost all of the atom's mass, was actually distributed inside it. ATOMS, the first of WBCHSE Semester IV Unit 8's two sub-topics (the second, Nuclei, turns to the nucleus itself), traces the chain of discovery that answered this question, in the order the syllabus lists it.

• The alpha-particle scattering experiment (Sections 1.2-1.2.1) -- the single experiment that first revealed a tiny, dense, positively charged core inside every atom -- and Rutherford's nuclear model built on it, together with the two serious problems that model immediately ran into (Section 1.3).
• Bohr's postulates and the resulting model of the hydrogen atom (Section 1.4), including the calculated radii of its allowed orbits (Section 1.4.1) and their energy levels (Section 1.5).
• The hydrogen spectrum (Section 1.6) -- the very phenomenon Bohr's model was built to explain, and the first great confirmation that it worked.
• The production of X-rays (Section 1.7), the continuous and characteristic X-ray spectra a target produces (Section 1.8), and Moseley's law (Section 1.9), which connects characteristic X-ray frequencies directly to atomic number and gave physics its first reliable way to measure Z experimentally.

Together these ideas explain everything from why a hospital X-ray machine works and why gold foil was the right choice for Geiger and Marsden's experiment, to why the periodic table is ordered by atomic number rather than atomic mass, and why hydrogen gas glows in specific colours rather than a plain white light when a current is passed through it.