ATOMIC MODELS DEVELOPMENT

Aristotle's Theory

400

Aristotle did not believe in the atomic theory and he taught so otherwise. He thought that all materials on Earth were not made of atoms, but of the four elements, Earth, Fire, Water, and Air. He believed all substances were made of small amounts of these four elements of matter. Most people followed Aristotle’s idea, causing Democritus’ idea- which was that all substances on Earth where made of small particles called atoms- to be over looked 2000 years.

Democritus's Theory

460

For Democritus;
1.All matter consists of invisible particles called atoms.
2. Atoms are indestructible.
3. Atoms are solid but invisible.
4. Atoms are homogenous.
5. Atoms differ in size, shape, mass, position, and arrangement.
->Solids are made of small, pointy atoms.
->Liquids are made of large, round atoms.
->Oils are made of very fine, small atoms that can easily slip past each other.

John Dalton

Oct 30, 1803

Modern scholarship has identified four basic ideas in Dalton's chemical atomic theory. 1) chemical elements are made of atoms
2) the atoms of an element are identical in their masses
3) atoms of different elements have different masses
4) atoms only combine in small, whole number ratios such as 1:1, 1:2, 2:3 and so on.

J.J. Thomson

Oct 29, 1897

J.J. Thomson discovered electrons and noticed that an atom can be divided. Also, he concluded atoms are made of positive cores and negatively charged particles within it. He developed the Plum Pudding Model before the atomic nucleus was discovered. This model shows that the electrons are surrounded by a "pudding" of positive charges to balance the negative charges. Today, J.J. Thomson's discoveries have helped people to have a better understanding of the atom and its generic makeup.

Hantaro Nagaoka

Oct 29, 1904

Hantaro Nagaoka reviewed J.J Thomson's atomic model and disagreed with it, saying that electrons couldn't be located in the positively charged atom. He modified Thomson's model, stating that the negatively charged electrons were actually located outside the atom, and that they orbit around it.

Ernest Marsden

Nov 1, 1909

While still an undergraduate he conducted the famous Geiger–Marsden experiment, called the gold foil experiment, together with Hans Geiger under Rutherford's supervision. This experiment led to Rutherford's new theory for the structure of the atom, with a centralised concentration of mass and positive charge surrounded by empty space and a sea of orbiting negatively charged electrons. He discovered the atomic existence of nucleus

Ernest Rutherford

Oct 31, 1911

Rutherford was forced to discard the Plum Pudding model and reasoned that the only way the alpha particles could be deflected backwards was if most of the mass in an atom was concentrated in a nucleus. He thus developed the planetary model of the atom which put all the protons in the nucleus and the electrons orbited around the nucleus like planets around the sun.

Niels Bohr

Oct 31, 1913

Niels Bohr atomic model is a planetary model and it has an atom consisting of small nucleus which is positively charged orbited by electrons which are negatively charged. Niels Bohr atomic model is more similar to the solar system plants are orbiting the sun except that the orbits are planner. Mathematically, the gravitational force in a solar system and the attractive electrical force between the positively charged nucleus and negatively charged electrons in an atom are of the same form.

Henry Moseley

Nov 1, 1913

Henry Moseley was an outstandingly skilled experimental physicist. In 1913 he used self-built equipment to prove that every element’s identity is uniquely determined by the number of protons it has. His discovery enabled him to predict confidently the existence of four new chemical elements, all of which were found.

Louis de Brogile

Oct 31, 1924

He believed that electrons can act like both particles and waves, just like light. He also said that waves produced by electrons contained in the orbit around the nucleus, set up a standing wave of a certain energy, frequency, and wavelength. He discovered that electrons can act like waves which helped explain some of the things electrons do that we had never been able to explain before. Today, quantum physics is applied in lasers, computers, and microscopes.

Erwin Schrödinger

Oct 31, 1926

In Niels Bohr's theory of the atom, the electrons absorb and emit radiation of fixed wavelengths when jumping between the fixed orbits around a nucleus. The theory gave a good description of the spectrum from the hydrogen atom, but must be further developed for more complicated atoms and molecules. Assuming that matter, e.g. electrons, could be regarded both as particles and as waves, Erwin Schrödinger formulated in 1926 a wave-equation that accurately gave the energy levels of atoms.

James Chadwick

Oct 31, 1932

James Chadwick discovered the neutron using evidence collected by Irene Joliot-Curie, who discovered that when beryllium was bombarded with positively charged alpha particles a beam with a high penetrating power was created. James Chadwick discoverd that this beam was not deflected by either electric or magnetic fields, meaning it contained neutral particles- neutrons.

William Conrad Roentgen

Oct 28, 1985

While he was working in the lab when he noticed a strange fluorescence coming from a nearby table. Upon further observation he found that it originated from a partially evacuated Hittof-Crookes tube, covered in opaque black paper which he was using to study cathode rays. He concluded that the fluorescence, which penetrated the opaque black paper, must have been caused by rays. This phenomenon was later coined x-rays and though the phenomenon of x-rays is not the same as radioactivity.