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400 BCE
Democritus
Atomic theory
Democritus stated the “ atomic theory" states that matter is just a mixture of atoms These are elements that are immutable, eternal and infinitely small and, therefore, imperceptible to the senses. -
400 BCE
Democritus Atomic theory
Atoms are indivisible, homogeneous and eternal. The difference between atoms lies only in shape and size, but not in their internal qualities.- The properties of matter change according to the grouping of atoms. In this way, according to this theory, there cannot be an ad infinitum division of things, given the infinite nature of atoms. -
400 BCE
Democritus Atomic model
His atomic model was solid and it says that all atoms are different in mass, size, shape, and position -
John Dalton
John Dalton was an English chemist, physicist, and meteorologist. He is best known for introducing the atomic theory into chemistry, and for his research into color blindness, sometimes referred to as Daltonism in his honor. -
John Dalton Atomic Model
John Dalton proposed that all matter is composed of very small things which he called atoms. This was not a completely new concept as the ancient Greeks had proposed that all matter is composed of small, indivisible, cannot be divided objects. When Dalton proposed his model electrons and the nucleus were unknown. -
Thomson
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 1904 Thomson model was disproved by Hans Geige´s and Ernest
Marsden's 1909 gold foil experiment. -
Thomson Atomic model
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Thomson's Experiment
Thomson discovered the electron by experimenting with a Crookes, or cathode ray, tube. He demonstrated that cathode rays were negatively charged. Thomson realized that the accepted model of an atom did not account for negatively or positively charged particles. -
Rutherford
Rutherford model, also called nuclear atom or planetary model of the atom, description of the structure of atoms proposed (1911) by the New Zealand-born physicist Ernest Rutherford.
Rutherford’s atomic model became known as the nuclear model. In the nuclear atom, the protons and neutrons, which comprise nearly all of the mass of the atom, are located in the nucleus at the center of the atom. -
Rutherford The Gold Foil Experiment
In 1911, Rutherford and colleagues Hans Geiger and Ernest Marsden, which would fully transform the accepted atom model, launched a series of pioneering experiments.
A radioactive element emitting alpha particles was guided to a thin sheet of gold foil surrounded by a screen to detect the deflected particles. -
Rutherford Nuclear Model
The main contribution of the model to the current understanding of the atom:
He reasoned that most of the atom was empty space. He concluded that all of the positive charge and the majority of the mass of the atom must be concentrated in a very small space in the atom’s interior, which he called the nucleus.
How was the model wrong?
He couldn't explain why negatively charged electrons remain in orbit when they should instantly fall into the positively charged nucleus. -
Bohr
Bohr Model, also known as the Solar System Model, was proposed by Niels Bohr in 1915. He countered Rutherford’s statement by claiming that if the electrons moved in strict orbits, they would collapse due to gravity. -
Bohr Atomic Model
He discovered that electrons traveled in several orbitals with different proximity from the nucleus, called energy levels. He also noted that the amount of valence electrons affected the properties of the element. This explained why atoms emitted light in fixed wavelengths.
This model was disproven by Werner Heisenberg, with his principle of uncertainty. This stated that contrary to the Bohr model, we cannot be sure of an atom’s radius or orbits. -
Schrödinger
The Electron Cloud Model, or Quantum Mechanical Model, proposed by Erwin Schrödinger in 1926, was an advance on the Bohr model. -
Schrödinger Atomic Model
Schrödinger used mathematical equations to determine the likelihood of finding an electron in a certain position. The major difference was rather than being treated as particles like in Bohr’s model, Schrödinger’s model allowed the electron to occupy a 3-dimensional space. He also incorporated James Chadwick’s discovery of the neutron into the model.
