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200
Democritus 440 B.C.
Democritus, a Greek philosopher, thought that if you cut something in half over and over again, you would eventually end up with a particle that could not be cut. He called it an atom. In Greek, the word atomos, means "not able to be divided." -
200
Aristotle 440 B.C.
Aristotle, a Greek philosopher, disagreed with Democritus's idea. Aristotle believed that you would never end up with a particle that couldn't be cut. -
Period: 200 to
the Development of Atomic Theory
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John Dalton
John Dalton, a British chemist and a school teacher, published his atomic theory in 1803. his theory states that all substances are made of atoms and that atoms are small particles that cannot be created, divided, or destroyed. Also, atoms of the same element are exactly alike, and atoms of different elements are different. Also, atoms join with other atoms to make new substances. -
J.J. Thomson
J.J. Thomson, a British scientist, discovered that there are small particles inside the atom, which means that atoms can be divided into even smaller particles. He thought that the electrons were mixed throughout the nucleus. -
Ernest Rutherford
Ernest Rutherford thought that atoms are soft blobs of matter. He expected the particles to pass right through the gold in a straight line, but some of the particles were deflected. Some even bounced straight back. -
Ernest Rutherford
In 1911, Rutherford revised his atomic theory. He proposed that in the center of an atom is a tiny, extremely dense, positively charged part called the nucleus. Like charges repel, so Rutherford reasoned that positively charged particles that passed close by the nucleus were pushed away by the postive charges in the nucleus. -
Niels Bohr
Niels Bohr, a Danish scientist, studied the way atoms react to light. He then proposed that electrons move around the nucleus in a certain paths, or energy levels. -
Erwin Schrodinger and Werner Heisenberg
Erwin Schrodinger, an Austrian physicist, and Werner Heisenberg, a German physicist, explained that electrons don't travel in definate paths. Their theory also states that there are regions inside the atom where electrons are likely to be found. These regions are called electron clouds.