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Aristotle
Aristotle, disagreed with Democritus's ideas. He belived that you would never end up with a particle that could not be cut. He had such a strong influence in the community, that most people belived in his theory over Democritus's. -
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Democritus 440 BCE
Democritus, a Greek philosopher thought that you could eventually end up with a particle that could not be cut. He named this particle an atom. The word atomos in Greek means "not able to be divided." He said that atoms were small, hard particles, and made of a single material formed into different shapes and sizes. -
John Dalton
John Dalton, a British chemist and schoolteacher, came up with a theory. This theory called the atomic theory proved that:
1. All substances are made up of atoms. They are small particles that cannot be created, divided and destroyed.
2. Atoms of the same element are exactly alike, and atoms of different elements are different.
3. Atoms join with other atoms to create new substances. -
J.J. Thomson
In 1897, a British scientist named J.J. Thompson showed that there was a mistake in Dalton's theory. Thomson discovered that there are small particles inside of atoms. This means that atoms can be divided in even smaller parts. He experimented with a cathode-ray tube and discovered that a positively chared plate attracted the beam. He concluded that negatively charged particles are present in every kind of atom. These negative charged particles are called electrons. -
Ernest Rutherford
In 1909, a former student of Thomson named Ernest Rutherford decided to test 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. He put a special kind of coating behind the foil. The coating glowed when it hit by the positively charged particles. Some of the particles were deflected, and some even bounced straight back. This stated that atoms were much more dense than he expected. -
Ernest Rutherford
In 1911 Rutherford revised this atomic theory by making 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. He reasoned that positively charged particles that passed close by the nucleus were pushed away by the positive charges in the nucleus. From these he calculated that the diameter of the nucleus was 100,000 times smaller than the diameter of the gold atom. -
Schrodinger and Heisenberg
They 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 found. This area is called the electron cloud. -
Neils Bohr
In 1913, a Danish scientist who worked with Rutherford, studied the way that atoms react to light. His results led him to propse that electrons move around the nucleus in certain paths, or energy levels. In his model, electrons can jump from a path in one level to a path in another level.
