Dalton's Atomic Theory is a scientific framework proposed by John Dalton in the early 19th century. It states that all matter is composed of tiny, indivisible particles called atoms. Dalton suggested that atoms of the same element are identical in mass and properties, while atoms of different elements differ. Additionally, Dalton's theory posits that atoms combine in simple whole-number ratios to form compounds. Chemical reactions involve the rearrangement of these atoms, but the atoms themselves remain unchanged. This foundational theory laid the groundwork for modern chemistry and our understanding of matter.