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The Atom What is an atom? Philosophical question: Is matter continuous and infinitely divisible, or is it divisible only until a basic, invisible particle that cannot be divided further is reached? Who discovered the atom? Democritus (400 BC) first suggests the idea of the atom. Atom is a Greek word meaning “indivisible”. Aristotle, however, did not believe in atoms. He thought all matter was continuous. Democritus does not get credit for discovering the atom because he had no scientific evidence to back it up. Jump up to the 1700s By the 1700s, all chemists accepted that an element could not be broken down further by ordinary chemical means. It was also clear that elements could join to form compounds. 1790s In the 1790s, with the help of improved balances, scientists began to study chemical reactions further. They came up with 3 laws. Law of conservation of mass States that matter can not be created nor destroyed. During a chemical reaction, the mass of the reactants will be the same as the mass of the products. Law of definite proportions States that it does not matter the size of the sample but elements always combine in the same ratio by mass when forming a compound Example: NaCl is always 39.34% sodium and 60.66% Cl no matter the size of the sample. Law of multiple proportions States that if two or more different compounds are composed of the same elements, they always combine in small whole number ratios. Example: The element carbon and oxygen can combine to form two different compounds, carbon monoxide and carbon dioxide. In carbon monoxide (CO) the ratio is 1:1. In carbon dioxide (CO2) the ratio is 1:2. Dalton’s Atomic Theory In 1808, John Dalton proposed an explanation to the three laws by coming up with the atomic theory of matter. Came up with the following Atomic Theory of Matter: – All matter is made up of atoms. – Atoms can not be divided. – Atoms of a given element are identical. – Atoms can combine to form compounds.