The document discusses several key concepts in atomic theory and the discovery of atomic structure: 1) Dalton's atomic theory from the early 19th century proposed that all matter is composed of tiny indivisible particles called atoms. 2) Atoms of the same element have the same mass and properties, while atoms of different elements have different masses and properties. 3) Chemical reactions involve changes in how atoms are combined, but atoms themselves are neither created nor destroyed in such reactions. 4) Experiments in the late 19th and early 20th centuries led to discoveries of the electron, proton, neutron, and the nuclear structure of atoms.

26. Symbols of Elements

28. Mass Number