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QUESTION FROM THE CLASSROOM
By Bob Becker

Q A

Was it easy for people to accept the idea that matter is made  of atoms when this idea was first introduced in the early 19th century? How did this idea appear anyway?

The idea that matter is made of very small units that cannot be divided further first appeared in Greece in the 5th century B.C. Philosophers named Leucippus and Democritus werethe first to propose that matter was made of small indivisible particles called atoms. But their idea was not studied scientifically until the early 19th century. In 1805, an English chemist andphysicist named John Dalton did scientific experiments that led him to suggest that matter was made of atoms. He revived an idea lost through the centuries and came up with what is known as the atomictheory. It was not widely accepted at first, and there were serious objections to it from some very learned and wellrespected scientists. This theory stated the following: • All matter is made up of tinyatoms—way too small to be seen, but there nonetheless. • All atoms of a given element are identical in every way—size, mass, and other properties. • Atoms cannot be created or destroyed, or even brokeninto smaller pieces. • Different atoms combine in various ratios to form compounds. • In a chemical reaction, atoms are simply separated, combined, and rearranged into new compounds.

To come upwith the atomic theory, Dalton relied on two laws: (1) the law of conservation of mass, and (2) the law of definite proportions. These laws may have seemed counterintuitive at the time, but they areobvious to people who are familiar with the atomic theory. The law of conservation of mass, formulated in 1789 by a French chemist named Antoine Lavoisier, states that in a chemical reaction, the totalmass of all the reactants is the same as the total mass of all the products.
N2(g) 3H2(g)

container, and no atoms were created or destroyed by the reaction. The atoms that were there in the...