History of the Atom

Niels Bohr (h)

Bohr built on Rutherford's theory because of the discovery he made in 1913. Bohr suggested that electrons travel around the nucleus in definite paths. These paths are located at certain "levels" from the nucleus. Electrons cannot travel between paths, but they can jump from one path to another.

JJ Thomson (d)

JJ Thomson discovered more about the atom in 1897. Thomson conducted an experiment and used the cathode-ray tube to show that there are small particles inside atoms. This discovery identified an error in Dalton's atomic theory. Since the beam moved away from the negatively charged plate and toward the positively charged plate, Thomson knew that the particles must have a negative charge which we call electrons. Thomson changed the atomic theory to include the presence of electrons. Thomson proposed a model of an atom called the "plum-pudding" model, in which negative electrons are scattered throughout soft blobs of positively charged material.

Robert Millikan and James Chadwick (e&f)

JJ Thomson may have discovered the electron, but in 1909 with his oil drop experiment he calculated the mass of the electron. In 1932, James Chadwick discovered the neutron. Chadwick's discovery of the neutron led directly to the discovery of fission and ultimately to the atomic bomb.

Dalton (c)

John Dalton was a scientist who developed another theory in 1803. Scientists during his time knew that elements combined and formed compounds. Dalton hypothesized that the reason for this was because elements are made of atoms. He published his own atomic theory. It said - All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed. - Atoms of the same element are exactly alike, and atoms of different elements are different and have different properties. - Atoms join with other atoms to make new substances. His theory was mostly correct but it was revised as more was learned about the atom.

Marie Carie

Marie Carie also made a discovery that revised the atomic theory. She showed that the atom was divisible. This was significant as up to that time. It also proved that a portion of Dalton's theory that states an atom is indivisible was wrong.

Aristotle and Democritus (a&b)

One of the earliest theory of the atom was made in Greece, 440 BCE by Democritus, who was a philosopher. He philosophized that everything was made up of tiny indivisible substances. He called them "atomos" which meant indivisible. He couldn't prove his belief. Aristotle was another philosopher during this time. He disagreed with Democritus, so he developed his own theory. He believed that everything was combinations of the four elements, earth, fire, water, and air. Although Democritus's philosophy may have been more accurate, many people believed Aristotle was correct mainly because he had a better reputation and his points overcame Democritus's points.

Ernest Rutherford (g)

Rutherford is another scientist who revised the atomic theory due to his discoveries. In 1909, Rutherford conducted an experiment in which he shot a beam of positively charged particles into a sheet of gold foil. Rutherford predicted the particles would pass. through the gold and continue in a straight line. Most of the particle went through, and some didn't, so he changed the atomic theory and developed a new model of the atom. Rutherford's model says that most of the atomic mass is found in the nucleus which is tiny, extremely dense, positively charged region in the center of an atom. In Rutherford's model the atom is mostly empty space, and the electrons travel in random paths around the nucleus.

Erwin Schrodinger

Schrodinger also made a discovery that revised the atomic theory. He proved the atom says that electrons do not travel in definite paths around the nucleus. The exact path or position of moving electron cannot be predicted or determined. Instead they are in regions inside the atom were electrons are likely to be found called electron clouds. Electron clouds are regions inside an atom where electrons are likely to be found.