-
420 BCE
Ancient Greek philosophers' belief
Leucippus and Democritus were the first to propose, in the fifth century B.C., that all matter is made of tiny units called atoms. The two philosophers held that these were solid particles without internal structure, and came in a variety of shapes and sizes. So, it wasn't an atomic theory as well, but it was the beginning of the discovery of the atom. This theory was rejected by Aristotle. -
Dalton's theory (Billiard ball model)
In his atomic theory Dalton suggests 4 postulates:
1) All matter is made of atoms. Atoms are indivisible and indestructible.
2) All atoms of a given element are identical in mass and properties
3) Compounds are formed by a combination of two or more different kinds of atoms.
4) A chemical reaction is a rearrangement of atoms.
The essence of Dalton's theory remains valid. Many heretofore unexplained chemical phenomena were quickly explained by Dalton with his theory for example isotopes. -
Thomson's Model (Plum Pudding Model)
In Thomson’s model, the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons’ negative charges, like negatively charged “plums” surrounded by positively charged “pudding”. The electrons were thought to be positioned throughout the atom in rotating rings. In this model the atom was also sometimes described to have a “cloud” of positive charge. -
Rutherford's Model (Nuclear Atom)
Rutherford overturned Thomson’s model in 1911 with his well-known gold foil experiment,
In his experiment, Rutherford observed that many alpha particles were deflected at small angles while others were reflected back to the alpha source.
He demonstrated that the atom has a tiny, high- mass nucleus with positive charge and around it, negative constituents, called electrons, circulate at some distance. -
Bohr Model (Solar System Model)
Description:
- Atoms have only certain allowable energy states, the smaller the orbit the lower the atoms energy state. Electrons in this model move around the nucleus in only certain allowed circular orbits.
Main contribution:
-Proposed stable electron orbits; Explained the emission spectra of some elements.
How was wrong the model?
-Moving electrons should emit energy and collapse into the nucleus, model did not work well for heavier atoms. -
Erwin Schrödinger's model (electron cloud or quantum mechanic model)
Levels are divided into sub-levels and these are divided into orbitals (we can´t know the location of an electron).
Schrodinger's model allowed the electron to occupy three-dimensional space.
The advantage of this model is that it consists of mathematical equations known as wave functions that satisfy the requirements placed on the behavior of electrons. The disadvantage is that it is difficult to imagine a physical model of electrons as waves.
