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400 BCE
Democritus athomic theory
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. Also that they are homogenous, meaning they have no internal structure. His atomic model was solid, and stated all atoms differ in size, shape, mass, position and arrangement, with a void exists between them. -
400 BCE
Experiment of Democritus
Democritus knew that if you took a stone and cut it in half, each half had the same properties as the original stone. He reasoned that if you continued to cut the stone into smaller and smaller pieces, at some point you would reach a piece so tiny that it could no longer be divided. -
400 BCE
How was the model wrong?
Democritus knew that if you took a stone and cut it in half, each half had the same properties as the original stone. He reasoned that if you continued to cut the stone into smaller and smaller pieces, at some point you would reach a piece so tiny that it could no longer be divided. -
400
Main contribution to the current understanding
Democritus discovered the atom, the most samllest particle and every thing around us is composed of them -
Billiard ball model
Based on all his observations, Dalton proposed his model of an atom, he defined an atom to be a ball-like structure, as the concepts of atomic nucleus and electrons were unknown at the time. -
Description of the model
Dalton discovered that certain gases could only be combined in certain proportions.
These experiments built upon two theories that had emerged near the end of the 18th century which dealt with chemical reactions. The first was the law of conservation of mass.
The second was the law of definite proportions, first proven by the French chemist Joseph Louis Proust in 1799.
Studying these laws and building on them, Dalton developed his law of multiple proportions. -
Main contribution
In fact, it was this very observation that is believed to be the first time that Dalton hinted at the supposed existence of atoms. In the paper which addressed gas absorption in water.
Dalton proposed that each chemical element is composed of atoms of a single, unique type, and though they cannot be altered or destroyed by chemical means, they can combine to form more complex structures. -
What was wrong?
In Dalton's Atomic Theory the indivisibility of an atom was proved wrong: an atom can be further subdivided into protons, neutrons and electrons. However an atom is the smallest particle that takes part in chemical reactions. According to Dalton, the atoms of same element are similar in all respects. -
Atomic model of Thompson (Raising pudding)
The main characteristic of thompson´s model is that this raising pudding represents the atom. He said that the electrons were distributive randomly around a positive mass.
He defined the atom as neutral, but he didn't define a nucleus in the atom. -
JJ Thompson
His model is one of the most famous because of its comparación with a raising pudding, that is something common in our life.
Thompson´s model had some problems, starting with the absence of explication about how the electrons maintain inside the atom.
other mistake was that he didn't considerate a nucleus, something really important demonstrated by Rutherford. -
Main contribution
The nucleus was postulated as small and dense to account for the scattering of alpha particles from thin gold foil, as observed in a series of experiments performed by undergraduate Ernest Marsden under the direction of Rutherford and German physicist Hans Geiger in 1909.
The Rutherford model supplanted the “plum-pudding” atomic model of English physicist Sir J.J. Thomson, in which the electrons were embedded in a positively charged atom like plums in a pudding. -
What was wrong?
The main problem with Rutherford's model was that 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. -
Nuclear atom / planetary model of the atom
The model described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun.
Most alpha particles were observed to pass straight through the gold foil, which implied that atoms are composed of large amounts of open space. -
Mistakes with Bohr´s model
He never describe the reason of why the electrons have defined levels.
The principal mistake in his model, is that this model can only been proved in the atom of hydrogen and they electrons. because if we change of atom to a one with more electrons the model start to have some problems. -
Niels Bohr model (Solar system)
Niels Bohr made a model based on Rutherford´s model, they are really similar with little differences. The reason of they similitudes can be that Bohr was student of Rutherford and they work tighter more than one time. The main characteristic of this model is that they are defined levels where the electrons can orbit, so it cannot be in the middle of levels. He also said that the energy levels have different numbers of electrons and the energy can be absorb or emitted when they change of level. -
Electron Cloud
basically this model propose the existence of sublevels in a same level, this was improve the model of Bohr that has a little problem with the electrons. -
Schrödinger
His model has three things wrong.
the first one is that he didn't take in account the spin of electrons.
The second is that he ignore the relativistic effects of the fast electrons and finally he didn't explain the reason why an electron fall down to an inferior level.
