<P> In the early 1800s, John Dalton used the concept of atoms to explain why elements always react in ratios of small whole numbers (the law of multiple proportions). For instance, there are two types of tin oxide: one is 88.1% tin and 11.9% oxygen and the other is 78.7% tin and 21.3% oxygen (tin (II) oxide and tin dioxide respectively). This means that 100g of tin will combine either with 13.5 g or 27g of oxygen . 13.5 and 27 form a ratio of 1: 2, a ratio of small whole numbers . This common pattern in chemistry suggested to Dalton that elements react in whole number multiples of discrete units--in other words, atoms . In the case of tin oxides, one tin atom will combine with either one or two oxygen atoms . </P> <P> Dalton also believed atomic theory could explain why water absorbs different gases in different proportions . For example, he found that water absorbs carbon dioxide far better than it absorbs nitrogen . Dalton hypothesized this was due to the differences between the masses and configurations of the gases' respective particles, and carbon dioxide molecules (CO) are heavier and larger than nitrogen molecules (N). </P> <P> In 1827, botanist Robert Brown used a microscope to look at dust grains floating in water and discovered that they moved about erratically, a phenomenon that became known as "Brownian motion". This was thought to be caused by water molecules knocking the grains about . In 1905, Albert Einstein proved the reality of these molecules and their motions by producing the first Statistical physics analysis of Brownian motion . French physicist Jean Perrin used Einstein's work to experimentally determine the mass and dimensions of atoms, thereby conclusively verifying Dalton's atomic theory . </P> <P> The physicist J.J. Thomson measured the mass of cathode rays, showing they were made of particles, but were around 1800 times lighter than the lightest atom, hydrogen . Therefore, they were not atoms, but a new particle, the first subatomic particle to be discovered, which he originally called "corpuscle" but was later named electron, after particles postulated by George Johnstone Stoney in 1874 . He also showed they were identical to particles given off by photoelectric and radioactive materials . It was quickly recognized that they are the particles that carry electric currents in metal wires, and carry the negative electric charge within atoms . Thomson was given the 1906 Nobel Prize in Physics for this work . Thus he overturned the belief that atoms are the indivisible, ultimate particles of matter . Thomson also incorrectly postulated that the low mass, negatively charged electrons were distributed throughout the atom in a uniform sea of positive charge . This became known as the plum pudding model . </P>

A collection of similar atoms make up an