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John Dalton
John Dalton, a British chemist experimented with combining atoms proportions. His results suggested that elements combine in certain mass proportions because they are made of single atoms. Dalton published his atomic theory in 1803. This stated that all substances are made of atoms, 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
In 1897, a British scientist named J.J. Thomson discovered that there are small negatively charged particles called electrons inside every atom. This means that atoms can be divided into even smaller parts. Thomson experimented with a cathode-ray tube. He discovered that a positively charged plate attracted the beam which meant there are electrons in the atom. After discovering electrons Thomson made the plum - pudding model of the atom becasuse electrons are mixed in the atom. -
Ernest Rutherford 1909
In 1909, Ernest Rutherford tested Thomson’s theory. He designed an experiment to study the parts of the atom. He aimed a beam of small, positively charged particles at a thin sheet of gold foil. Rutherford put a small coating behind the foil which glowed when hit by the positively charged particles. He could then see where the particles went after hitting the foil. He agreed with Thomson saying atoms are “blobs” of matter thinking the particles would go straight through. However, some bounced ba -
Ernest Rutherford 1911
In 1911, Rutherford revised the atomic theory. He made a new model of the atom. He proposed that in the center of the atom is a tiny, extremely dense, positively charged part called the nucleus. Because like charges repel, Rutherford reasoned that positively charged particles that passed by the nucleus were pushed away by the positive charges in it. A particle that headed straight for a nucleus would be pushed almost straight back in the direction from which it came. Gold atom=100,000 bigger. -
Niels Bohr
In 1913, Niels Bohr, a Danish scientist who works with Rutherford, studied the way that atoms react to light. Bohr’s results led him to propose that electrons move around the nucleus in certain paths, or energy levels. In Bohr’s model, there are no paths between the levels. But electrons can jump from a path in one level to a part in another level. Bohr’s model was a valuable tool in predicting some atomic behavior, but the atomic theory still had room for improvement. -
Erwin Schrödinger and Werner Heisenberg
In the early 1920’s Erwin Schrödinger, an Austrian physicist, and a German physicist named Werner Heisenberg further explained the nature of electrons in the atom. For example, electrons do not travel in definite paths as Bohr suggested. In fact, the exact path of an electron cannot be predicted. According to the current theory, there are regions inside the atom where electrons are likely to be founds. These regions are called electron clouds. -
382 BC - Aristotle
Aristotle, antother Greek philosopher, disagreed with democritus's ideas. He believed that you would never end up with a particle that cold not be cut. He had such a strong influence on people's ideas that for a long time, most peple thought he was right. However, Democritus was right though. Matter is madeof particles, which we call atoms. -
440 BC - Democritus
Democritus, a Greek philosopher thought that you would eventually end up with a particle that could not be cut. He called this particle an atom. The word atom is from the Greek work atomos, meaning "not able to be divided." Democritus said that all atoms are small, hard particles. He thought that atoms were made of a single material formed into different shapes and sizes.
