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Atomic Theory


The first practical atomic theory was put forward by the English chemist and physicist John Dalton during the years 1803 to 1808. The background for this theory was the law of conservation of mass and the law of constant proportions.

The first of these laws had been formulated by the French chemist Antoine Laurent Lavoisier in 1785. It was based on the exact process of weighing and states that there is no measurable change in mass during a chemical reaction.

The second law was enunciated by the French chemist Joseph Louis Proust in 1799. It states that different samples of a substance contain its elements in the same proportions.

Atomic Theory Definition

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According to modern definition of atomic theory "atom is a particle indivisible in chemical changes." The idea of atoms was later adopted by Dalton in his famous theory known as Dalton's atomic theory.

Democritus Atomic Theory

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Democritus proposed that everything is made of atoms and that these atoms were indivisible and the same all the way through. Atoms were all made of the same "stuff" and come in different shapes and sizes. Between the atoms was a void. The closure Democritus associated with atom is thus remarkably similar to the contemporary closure.

Since Democritus has no immediate evidence for his atoms, one can reasonably presume that they provided an organizational function and a defense of other closures.

Atomic Theory Timeline

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Who discovered the atomic theory?

Atoms were thought to be smallest and so many scientist worked on atomic theory. Although Democrats in 400 BC put forward the concept of atoms. Later the modern atomic theory was put forward by John Dalton in 1803.
The list atomic theory scientists and the timeline is represented diagrammatically below.

Atomic Theory Timeline

JJ Thomson Atomic Theory

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Dalton's atomic theory was the first theory that gave us some idea about the internal structure of matter. However, a number of discoveries in the late nineteenth century revealed that the atom further consisted of subatomic particles such as electrons, protons and neutrons.

According to Dalton's atomic theory
  1. Matter is made up of extremely small particles called atoms.
  2. All the atoms present in a substance are identical in all respects such as mass and size but they differ from atoms of other substances.
  3. Atoms are indivisible and remain unchanged during chemical reactions.
  4. When atoms combine, they do so in simple numerical reactions.
  5. Atoms are indestructible.

Ernest Rutherford Atomic Theory

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According to Rutherford atomic theory
  1. The atom consists mainly of space.
  2. The mass of the atom is concentrated in the nucleus, which is a small core at the center of the atom.
  3. The nucleus has positive charges.
  4. Electrons move around the nucleus like planets orbiting the sun.
  5. The atom is neutral as it has the same number of positive and negatively charged electrons.

Erwin Schrodinger Atomic Theory

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The Schrodinger model is called the "quantum mechanical model" and is the model that we use today to explain and predict atomic behavior. Schrodinger studied de Broglie's thesis mainly with the aim of obtaining a better understanding of Einstein's theory of gas statistics.

He indicates that the electron travel randomly in a region or space around the nucleus rather than in fixed orbits. Each space has a definite energy level. Energy levels have sub levels, or orbitals within them.

Aristotle Atomic Theory

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Aristotle discussing the atomic theory says that Democritus claims that no elementary particle arises from any other. Nevertheless their common body is the source of everything,  differing from part to part in size and shape. They are differentiated in shapes but their substances is one.

Modern Atomic Theory

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What is the modern atomic theory?

Dalton suggested three main postulates of modern atomic theory.
  1. An element is composed of tiny particles called atoms. All atoms of a given element have the same chemical properties. Atoms of different elements show different properties.
  2. In as ordinary chemical reaction atoms move from one substance to another, but no atom of any element disappear or is changed into an atom of another element.
  3. Compounds are formed when atoms of two or more elements combine. In general these relative numbers can be expressed as integers or simple fractions.

Dalton's atomic theory summarize the development of atomic theory.

James Chadwick Atomic Theory

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James Chadwick established that an atom consists of subatomic particles that is electrons and protons.
  1. Electrons have negligible mass - hence the mass of the atom is - concerned in the nucleus. Therefore the atomic mass should be equal to the - mass of the protons.
  2. However in the case of helium - there are two protons in its nucleus and - hence atomic mass should be equal to - twice the mass of each proton.

Robert Millikan Atomic Theory

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Millikan supported atomic theory and in general a granular representation of nature. Millikan conducted the Oil Drop Experiment, in which he sprayed tiny oil droplets and had them fall through a small, charged hole into a closed area. The oil droplets now had a charge on them. He, then, introduced the area to an electric field.

Louis de Broglie Atomic Theory

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The new mechanics of atomic systems was taken by de Broglie. He postulated that because photons have wave and particle characteristics perhaps all forms of matter have wave as well as particle properties. According to de Broglie electrons has a dual particle - wave nature.

According to de Broglie electrons have a dual nature, particle and wave. De Broglie description waves provide a physical picture of the Bohr theory of atoms.

Antoine Lavoisier Atomic Theory

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Antoine Lavoisier developed a the modern quantitative chemistry, followed by John Dalton's work in identifying the atom as the foundation building block of all matter. He proved that in combustion reactions, it is the oxygen present in the air that combines with the combustible materials.

Antoine Lavoisier concluded that "total mass of material present before a chemical reaction remains unaltered after the chemical reaction".

Albert Einstein Atomic Theory

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Albert Einstein discovered that light could be described as a particle but the atom was discovered, contrary to definition to be composed of a nucleus of protons and neutrons and orbiting shells of electrons.

The atomic theory is closely linked with Einstein's theory of only one substance in all the universe, for it is based on the assumption that the only difference between the known elements lies in the number of electrons revolving around the proton which forms the center of each atom of the material.

Henri Becquerel Atomic Theory

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Discovery of radioactivity in 1896 by Henri Becquerel which helped lead to the development of atomic theory in the twentieth century. Chemist were not able to use their methods to determine the structure of the atom.

The discovery of radioactivity by Henri Becquerel and the work of Marie and Pierre Curie showed however that heavy elements were not stable. Becquerel also has observed that electroscope loses its charge under the effect of this radiation because the radiation induces charges in the air.

Atomic Theory of Matter

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The history of atomic theory of matter provides an excellent illustration of the power of the experimental method in dealing with old problems and in preparing the way for new discoveries.

According to atomic theory matter is neither lost nor gained during a chemical reaction.
  1. All matter is made up of indivisible and indestructible particles called atoms.
  2. All atoms of a given element are identical both in mass and in properties.
  3. Compounds form when atoms of different element combine in ratios of small whole numbers.
  4. Elements and compounds are composed of definite arrangements of atoms. Chemical change occurs when the atomic arrays are rearranged.

Henry Moseley Atomic Theory

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An English scientist Henry Moseley first determined the atomic numbers of the elements through the use of X-rays.

The number of protons in the nucleus of an atom determines the atomic number. All atoms of the same element have the same number of protons and therefore the same atomic number; atoms of different elements have different atomic numbers. Thus atomic number identifies the element.

Marie Curie Atomic Theory

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The work of Marie and Pierre curie showed that atoms are made up of smaller particles. These subatomic particles differ in mass and in electrical charge. They asserts that matter is made up of atoms.

Together they discovered two new radioactive elements. They called one polonium after Madame Curie's native country Poland. They called the second radium meaning "giver of rays". Its salts glowed in the dark.

Joseph Proust Atomic Theory

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A french chemist Joseph Proust made a great breakthrough when he observed that elements always combine in fixed proportions by weight when they form compounds, so a gram of hydrogen will always react with eight grams of oxygen to form water.

Joseph Proust states that "different samples of the same compound always contain its constituent elements in the same proportion by mass."

William Crookes Atomic Theory

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In 1903, the chemist and academic, Sir William Crookes separated protactinium from uranium and made important observations on the motion and scintillation  of decaying uranium.  Crookes had previously discovered thallium in 1861.

Atom ism had passed its philosophical and abstract beginnings with the exploitation of radioactivity and it was the predawn of the Nuclear Age.

Michael Faraday Atomic Theory

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Michael Faraday was another early worker hose major contribution to chemistry was Electrochemistry, in which a certain quantity of electricity during electrolysis or electrode position of metals was shown to be associated with certain quantities of chemical elements and fixed quantities of the elements therefore with each other, in specific ratios. These were early clues to the atomic nature of matter.
Gay Lussac's Law of combining volumes of gases his vapor density determinations and his development of the volumetric analysis of solutions can be appreciated as basic contributions to modern physics and chemistry.
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Dalton's Atomic Theory states that
  1. All substance consist of tiny particles - atoms and molecules that is matter has a discrete structure.
  2. Atoms and molecules are in continuous motion.
  3. There exist forces of interaction between the atoms and molecules.
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Niels Bohr model of the atom is very small, positively charged nucleus which contains proton and neutron surrounded by negatively charged electrons.
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Werner Heisenberg Atomic Theory

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Werner's contribution to the atomic theory was that he calculated the behavior of electrons, and subatomic particles that also make up an atom.