-
400 BCE
“Atomic Theory of the Universe”
His theory proposed that the world was made up of very small and indivisible particles, of eternal existence, homogeneous and incomprehensible, whose only differences were shape and size. He named these particles "atoms" Democritus and his teacher Leucippus were the creators of the atom concept. they founded a philosophical current known as atomism that affirmed that all matter consisted of two things, atoms and the void. -
400 BCE
Contribution and errors of the atomic model of Democritus
Based on the writings of Democritus, the idea of the atom was taken up again, and thanks to his contributions its modern understanding was easier. As reason was used, it was not proven that the atom was real (lack of validity)
Atoms do not have different shapes, but they can have different sizes. -
400 BCE
Democritus' reasoning
Democritus knew that if you take a stone and cut it in half, each half has the same properties. He reasoned that if he kept cutting the stone into smaller and smaller pieces, he would reach a point so small that it could no longer be divided. He called these infinitesimally small bits of matter atoms, which means "indivisible," but he didn't create a representation about his athom model. -
Dalton's atomic model errors
Despite the obvious importance of Dalton's Atomic Model in the rise of modern chemistry, it should be noted that this theory has numerous shortcomings, as noted later. For example, Dalton thought that gases were monatomic substances, and that molecules were always composed of the smallest possible proportion. This led him to assume that water was made up of one hydrogen atom and one oxygen atom (HO) and to miscalculate the atomic weight of many compounds -
Dalton's atomic theory or dalton's atomic model
This model proposes a scientifically plausible explanation for most of the enigmas of chemistry in the eighteenth and nineteenth centuries. It postulates that all the matter in the world is composed of atoms, that is, that there is a finite number of fundamental particles. In addition, it maintains that simply from the combination of these particles all the complex structures of matter are possible. The direct predecessor was the Greeks of classical antiquity. -
The postulates of this model are:
Matter is made of tiny, indestructible, indivisible particles called atoms.
Atoms of the same element are always identical to each other. Atoms of different elements have different masses and properties.
Atoms don't split and cannot be created or destroyed during chemical reactions.
Atoms of different elements can combine to form compounds in different ratios and quantities.
When combined, atoms are arranged in simple relationships, described by whole numbers. -
Contribution of the model to the understanding of the atom
Dalton proposed the first fundamental model for chemistry. He allowed to solve doubts. For example, he explained the cause of fixed stoichiometric ratios in chemical reactions. The possibility of verifying many of Dalton's postulates laid the foundation for future chemistry. The great advantage of this model was that of scientifically explaining an immense set of complex facts and diverse compounds from a fairly simple combinatorial theory. -
Thompson's atomic model
Joseph John Thomson thought that the electrons were immersed in a positively charged substance that counteracted the negative charge of the electrons, since atoms are neutrally charged. Thompson called the electrons corpuscles and thought that they were arranged non-randomly, in rotating rings, although the positive part remained indefinitely. His experiments with cathode ray tubes showed that all atoms contain small, negatively charged subatomic particles called electrons. -
Main contribution of Thomson's model
Thomson's model represented an important step in the development of atomic theory as it incorporated subatomic particles and new discoveries, such as the existence of the electron, and introduced the notion of the atom as a non-inert and divisible mass. From this model, scientists assumed that atoms were made up of smaller units, and that atoms interacted with each other through many different forces. -
Thomson model errors
Thomson's model of the atom could not explain how charge is held on electrons within the atom. Nor could it explain the stability of an atom. The theory did not mention anything about the nucleus of the atom. -
Contribution of the Rutherford model
-Most of the atomic mass is concentrated in the nucleus, larger and heavier than the rest of the particles, and endowed with a positive electric charge.
-Around the nucleus and at great distances from it are electrons, with a negative electrical charge, which orbit it in circular paths.
-The sum of the positive and negative electrical charges of an atom should give zero as a result, that is, they should be equal, so that the atom is electrically neutral. -
Rutherford's atomic model errors
-It was not possible to explain how it was possible for a set of positive charges to remain united in the atomic nucleus, since they should repel each other, since they are all charges of the same sign.
-The stability of the atom could not be explained, because when considering the negatively charged electrons that revolve around the positive nucleus, at some point these electrons must lose energy and collapse against the nucleus. -
Rutherford Atomic Model
Rutherford proposed that atoms have a nucleus where the greatest percentage of their mass is found. According to this theory, this nucleus has a positive electrical charge and is orbited by particles of opposite charge and smaller size (electrons). he experimented with several thin sheets of gold that would be bombarded in the laboratory with helium nuclei (alpha particles, which have a positive charge), thus measuring the angles of deflection of the particle beam as it passed through the gold. -
Bohr's Atomic Model
Bohr creates an atomic model where the electrons will move around the nucleus in circular orbits, as in the solar system. He explain why atoms had characteristic emission spectra, in addition to adding ideas about the photoelectric effect. The atom is considered neutral since it has no electrical charge, and there are equal numbers of protons and neutrons. The electrons are distributed in an orderly manner in different orbits called levels. -
Bohr's Atomic Model
The model explained the stability of matter and chemical bond formation. He postulated that each electron rotate around the nucleus in an orbit without emitting radiant energy and this absorption or emission of energy by an atom is done by a change in orbit. If there is energy emission, the electron jumps to a lower energy orbit; but, if it is absorbed, it jumps to a higher energy one. He did not perform any experiments, he drew conclusions by joining the theories of Planck and Rutherford. -
Errors of the atomic model of Bohr
-The model worked to understand the functioning of certain types of atoms, such as hydrogen, but not for others with a more complex structure; that is, with a greater number of electrons.
-You were also wrong in assuming that the electrons were in set orbits.
-I do not take into account the wave-particle duality. -
Contribution of the atom model of Bohr
-He helped discover quantum physics and the structure of the atom.
-Bohr's model was the first to recognize the concept of quantum mechanics in the electronic structure of a hydrogen atom.
-Described the stability of atoms.
-The non-relativistic quantum Schrödinger model was added to the concept of quantum mechanics. -
Contributio of the atom model of Erwin Schrodinger
At that time, it was considered that electrons only revolved in circular orbits around the atomic nucleus, Erwin claimed that electrons could also revolve in more complex elliptical orbits and calculated relativistic effects. -
"The quantum mechanical model of the atom"
In this model the electrons were originally contemplated as a standing wave of matter whose amplitude decayed very fast when exceeding the atomic radius. Based on Young's experiment demonstrating wave-particle duality, he divided the emission of a light beam passing through a small hole through a camera. The result was that both light beams hit a wall in a dark room. There the interference pattern between the two waves became apparent, showing that light can act as both a particle and a wave. -
Errors in Schrodinger's atomic model
• It does not take into account the relativistic effects of fast electrons.
• The model also does not take into account electron spin.
• This model cannot explain why an electron in an excited quantum state can decay to a lower energy level if there is a free one.