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Period: 460 BCE to 370 BCE
Democritus
Democritus was an Ancient Greek pre-Socratic philosopher primarily remembered today for his formulation of an atomic theory of the universe. -
400 BCE
Democritus atomic model
Democritus’s model stated that matter consists of invisible particles called atoms and a void (empty space). He stated that atoms are indestructible and unchangeable. His atomic model was solid, and stated all atoms differ in size, shape, mass, position and arrangement, with a void exists between them. -
340 BCE
Democritus atomic model
Aristotle rejected Democritus atomic theory and didn't think atoms move through empty spaces because he didn't think that empty spaces existed. He believed that all matter consisted of four elements. Earth, Air, Water, and Fire. -
Period: to
John Dalton
John Dalton was a chemist, physicist, and meteorologist. Mostly known for the modern formulations of the atomic theory, he discovered one of the fundamental laws of chemistry, which is the law of multiple proportions. -
Dalton's atomic model
First atomic model and the beginning of the atomic theory. He hypothesized that the atom was a solid sphere that could not be divided into smaller particles. His discoveries made modern chemistry and physics possible. -
Atomic Model
Most scientists still agree with Dalton's statements about his explanation of atoms, but this model is no longer used since we now know that atoms have smaller particles within them, this are protons, neutrons, and electrons. Despite this Daltons had a really big part in understanding the atom, and what they are. -
Dalton's First Postulates
-All matter consists of atoms.
-Atoms cannot be created or destroyed.
-Atoms of different elements may combine with each other in whole-number ratios to form compound atoms.
-Atoms of the same element can combine in more than one ratio to form two or more compounds. -
Period: to
Joseph John Thomson
(18 December 1856 - 30 August 1940) England. He was a british physicist, credited with the discovery of the electron, which is the first sub-atomic particle discovered. -
Period: to
Niels Bohr
Niels Bohr was a Danish physicist. He grew up in the most favorable environment since his father and brother dedicated to the scientific studies. In 1913, Niels Bohr proposed his atomic model also which he recognized as the planetary model. -
Period: to
Erwin Schrödinger
Erwin Schrödinger was born in Vienna, Austria. He had a wide variety of interests not only in the scientific aspect. In 1926, he proposed his quantum mechanical model of the atom which up to today is the latest atomic model. -
Discovery of the Electron by Thomson
In 1897, Thomson showed that cathode rays were composed of some kind negative charged particles, which he calculated to have bodies smaller than atoms. Which were later called "electrons". -
Thomson's atomic model
The famous plum-pudding model was proposed by Thomson in 1904. This model was the first to include the electrons (since he descovered them). In this model, the electrons are surrounded by a soup of positive charge to balance the electrons' negative charges, like negatively charged "plums" surrounded by positively charged "pudding". -
Thomson's atomic model
In 1909, this model was disapproved by the 1909 gold foil experiment performed by Hans Geiger and Ernest Marsden, the "gold foil experiment". This gold foil experiment suggests that there is a very small nucleus of the atom that contains a very high positive charge (in the case of the experiment, the gold is enough to balance the collective negative charge of about 100 electrons, leading to Rutherford's model. -
RutherFord's Model
Proposed by the New-Zeeland born physist, Ernest Rutherford, where the electrons orbit around a massive nucleus cosisiting only of protons. This model is also called "planetary model" as it looks like the solar system. The nucleus occupies only a very small part of the atom, so it is mostly empty space. -
Nuclear Atom
A radioactive source capable of emitting alpha particles was enclosed within a protective lead shield. The radiation was focused into a narrow beam after passing through a slit in a lead screen. A thin section of gold foil was placed in front of the slit, and a screen coated with zinc sulfide to render it fluorescent served as a counter to detect alpha particles. -
Nuclear Atom
As each alpha particle struck the fluorescent screen, it would produce a burst of light called a scintillation, which was visible through a viewing microscope attached to the back of the screen. The screen itself was movable, allowing Rutherford and his associates to determine whether or not any alpha particles were being deflected by the gold foil. -
Nuclear Atom
Most alpha particles were observed to pass straight through the gold foil, which implied that atoms are composed of large amounts of open space. Some alpha particles were deflected slightly, suggesting interactions with other positively charged particles within the atom. Still other alpha particles were scattered at large angles, while a very few even bounced back toward the source. -
Nuclear Atom
Rutherford famously said later, “It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” Only a positively charged and relatively heavy target particle, such as the proposed nucleus, could account for such strong repulsion. The negative electrons that balanced electrically the positive nuclear charge were regarded as traveling in circular orbits about the nucleus. -
Nuclear Atom
The electrostatic force of attraction between electrons and nucleus was likened to the gravitational force of attraction between the revolving planets and the Sun. Most of this planetary atom was open space and offered no resistance to the passage of the alpha particles. -
Nuclear Atom
Contributions: He discovered the nucleus, how was it wrong? He couldn't explain why negatively charged electrons remain in orbit when they should instantly fall into the positively charged nucleus. This problem would be solved by Niels Bohr in 1913. -
Bohr's Atomic Model
Bohr used the hydrogen emission spectra to propose his new atomic model in which he says that electrons orbit at fixed distances from the nucleus. He referred to his model as a planetary model of the atom. Bohr proposed that when an electron goes down in the "ladder" it loses energy, as well as when an electron goes up it gains energy. -
Planetary Model
According to the Bohr model, the electrons are around the nucleus of the atom in very specific "paths" called orbits. Whenever an electron is in any of these orbits, its energy is fixed. If an orbit is closest to the nucleus, then it means that its energy is lower. The orbits that are farther away from the nucleus have a greater energy level. -
Planetary Model
The electron can't be in any of the spaces between the orbits. We can compare Bohr's atomic model to a ladder. Whenever you climb up a ladder or you go down, you can be hanging where there isn't a rung, you need to step in one so you can be stable. When an electron moves up the "ladder" its energy is increasing, if an electron goes down, its energy is decreasing. -
Planetary Model
How was this model wrong? Its main problem was that this worked with atoms that had only one electron however, this couldn't properly be applied to multi-electron atoms. This model isn't useful when it comes to energy sub-levels(s,p,d,f). -
Planetary Model
Main Contribution to the current atomic model. Niels Bohr discovered that electrons go in separated orbits. He also said that the number of electrons that travel in the last orbit or in the one that is farther away from the nucleus, would determine the characteristics or properties of the element. -
Schrödinger's Atomic Model
In 1926, Erwin Schrödinger took Niels Bohr's atomic model proposed in 1913 one step further. Schrödinger used mathematical equations so we could describe the likelihood of finding an electron in any position. This model is known as the quantum-mechanical model. -
Quantum-Mechanical Model
In this model, the exact position of an electron is not determined, instead it predicts the location of an electron. This model can be pictured as an electron cloud, the nucleus is in the center and the electrons are at its surroundings in the form of a cloud. -
Quantum-Mechanical Model
Schrödinger used only one quantum number to describe the electrons' location. In this model, the only important quantum number is n, the only important thing to know is the electron's size. A three dimensional space was allowed to be used by the electron, therefore three coordinates or three quantum numbers in this case are required. These quantum numbers were the principal(n), angular(l) and magnetic(m). Schrödinger replaced Bohr's electron location ideas with an uncertainty factor. -
Quantum-Mechanical Model
How was this model wrong? Until today, some scientists have said this model is wrong but not many have supported therefore, we can't really say what is wrong until another scientist comes up with another approved atomic model. -
Quantum-Mechanical Model
Main contribution of the model: This model is the latest model and it took the past models and theories to explain and have a better understanding of the atomic model.
