Dalton’s theory

Dalton’s theory. In 1803 John Dalton formulated the law that bears his name and that summarizes the quantitative laws of chemistry (law of conservation of mass, made by Lavoisier; law of defined proportions, made by Louis Proust; law of multiple proportions, made by himself).


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  • 1 History
    • 1 Size of the atom
    • 2 Radioactivity
    • 3 Spectral lines
  • 2 Atomic nucleus
  • 3 Artificial radioactivity
  • 4 Nuclear reactions
  • 5 Particle accelerators
  • 6 Source


Although many other scientists, starting with the ancient Greeks, had already claimed that the smallest units of a substance were atoms , Dalton is considered to be one of the most significant figures in atomic theory because he turned it into something quantitative. Dalton showed that atoms bound together in defined proportions.

Research showed that atoms often form groups called molecules . Each water molecule , for example, is made up of a single oxygen (O) atom and two hydrogen (H) atoms joined by an electrical force called a chemical bond , which is why water is symbolized as HOH or H2O.

All the atoms of a certain element have the same chemical properties. Therefore, from a chemical point of view, the atom is the smallest entity to consider. The chemical properties of the elements are very different from each other; their atoms combine in very different ways to form many different chemical compounds. Some elements, such as the noble gases helium and argon , are inert; that is, they do not react with other elements except under special conditions. Unlike oxygen, whose molecules are diatomic (made up of two atoms), helium and other inert gases are monoatomic elements, with a single atom per molecule.

  • The Theory can be summarized in:

1.- Chemical elements are made up of very small and indivisible particles called atoms.

2.- All the atoms of a given chemical element are identical in their mass and other properties.

3.- The atoms of different chemical elements are different, in particular their masses are different.

4.- Atoms are indestructible and retain their identity in chemical changes.

5.- Compounds are formed when atoms of different elements combine with each other, in a simple integer relationship, forming defined entities (today called molecules).

Atom size

Curiosity about the size and mass of the atom attracted hundreds of scientists during a long period in which the lack of appropriate instruments and techniques prevented satisfactory responses.

Subsequently, many ingenious experiments were designed to determine the size and weight of the different atoms. The lightest atom, the hydrogen atom , has a diameter of approximately 10-10 m (0.0000000001 m) and a mass of around 1.7 × 10-27 kg. (The fraction of a kilogram represented by 17 preceded by 26 zeros and a decimal point).

An atom is so small that a single drop of water contains over a thousand trillion atoms.


A series of important discoveries made towards the end of the 19th century made it clear that the atom was not a solid particle of matter that could not be divided into smaller parts.

In 1895 , the German scientist Wilhelm Conrad Roentgen announced the discovery of X-rays , which can penetrate thin sheets of lead. In 1897 , the English physicist JJ Thomson discovered the electron , a particle with a mass much less than that of any atom. And, in 1896 , the French physicist Antoine Henri Becquerel verified that certain substances, such as uranium salts, generated penetrating rays of mysterious origin. The marriage of French scientists made up of Marie and Pierre Curie made an additional contribution to the understanding of these “radioactive” substances.

As a result of the investigations of the British physicist Ernest Rutherford and his contemporaries, uranium and some other heavy elements, such as thorium or radio, were shown to emit three different classes of radiation, initially called alpha (a) rays, beta rays ( b) and gamma rays (g). The first two, which were found to consist of electrically charged particles, are now called alpha and beta particles. Later it was verified that the alpha particles are helium nuclei and the beta particles are electrons. It was clear that the atom was made up of smaller parts. Gamma rays were finally identified as electromagnetic waves, similar to X-rays but with a shorter wavelength.

Spectral lines

One of the great successes of theoretical physics was the explanation of the characteristic spectral lines of numerous elements. Atoms excited by energy supplied by an external source emit light of well defined frequencies. If, for example, hydrogen gas is kept at low pressure in a glass tube and an electric current is passed through it, it emits visible reddish light. Careful examination of that light using a spectroscope shows a spectrum of lines, a series of lines of light separated by regular intervals.

Each line is the image of the spectroscope slot that is formed in a certain color. Each line has a defined wavelength and a certain associated energy. The Bohr theory allows physicists calculate these wavelengths easily. The electrons are supposed to be able to move in stable orbits within the atom. As long as an electron remains in an orbit at a constant distance from the nucleus, the atom does not radiate energy.

When the atom is excited, the electron jumps into a higher energy orbit, farther from the nucleus. When it falls back into an orbit closer to the nucleus, it emits a discrete amount of energy that corresponds to light of a certain wavelength. The electron can return to its original orbit in several intermediate steps, occupying orbits that are not completely full. Each observed line represents a certain electronic transition between higher and lower energy orbits.

In many of the heavier elements, when an atom is so excited that the internal electrons near the nucleus are affected, penetrating radiation ( X-rays ) is emitted . These electronic transitions involve very large amounts of energy.

Atomic nucleus

In 1919 , Rutherford exposed nitrogen gas to a radioactive source that emitted alpha particles. Some of these particles collided with the nuclei of the nitrogen atoms. As a result of these collisions, the nitrogen atoms were transformed into oxygen atoms. The nucleus of each transformed atom emitted a positively charged particle. These particles were found to be identical to the nuclei of hydrogen atoms. They were called protons. Subsequent investigations showed that protons are part of the nuclei of all elements.

No further data on the structure of the nucleus were known until 1932 , when the British physicist James Chadwick discovered another particle in the nucleus, the neutron., which has almost exactly the same mass as the proton but lacks an electric charge. Then it was seen that the nucleus is made up of protons and neutrons. In any given atom, the number of protons is equal to the number of electrons and, therefore, to the atomic number of the atom. Isotopes are atoms of the same element (that is, with the same number of protons) that have different numbers of neutrons. In the case of chlorine, one of the isotopes is identified by the symbol 35Cl, and its heaviest relative by 37Cl. Superscripts identify the atomic mass of the isotope, and are equal to the total number of neutrons and protons in the nucleus of the atom. Sometimes the atomic number is given as a subscript, such as} Cl.

The least stable nuclei are those that contain an odd number of neutrons and an odd number of protons; all but four of the isotopes corresponding to nuclei of this type are radioactive. The presence of a large excess of neutrons relative to the protons also reduces the stability of the nucleus; this happens with the nuclei of all the isotopes of the elements above the bismuth in the periodic table, and all of them are radioactive. Most of the known stable nuclei contain an even number of protons and an even number of neutrons.

Artificial radioactivity

Experiments carried out by French physicists Frédéric and Irène Joliot-Curie in the early 1930s demonstrated that the stable atoms of an element can be made artificially radioactive by adequately bombarding them with nuclear particles or lightning. These radioactive isotopes (radioisotopes) are produced as a result of a nuclear reaction or transformation. In such reactions, the slightly more than 270 isotopes found in nature serve as targets for nuclear projectiles. The development of “atombreakers”, or accelerators, that provide high energy to launch these projectile particles has allowed thousands of nuclear reactions to be observed.

Nuclear reactions

In 1932 , two British scientists, John D. Cockcroft and Ernest TS Walton, were the first to use artificially accelerated particles to disintegrate an atomic nucleus. They produced a proton beam accelerated to high speeds by a high-voltage device called a voltage multiplier. Those particles were then used to bombard a lithium core. In that nuclear reaction, lithium 7 (7Li) is split into two fragments, which are nuclei of helium atoms. The reaction is expressed by the equation.

Particle accelerators

Around 1930 , the American physicist Ernest O. Lawrence developed a particle accelerator called a cyclotron. This machine generates electrical forces of attraction and repulsion that accelerate the atomic particles confined in a circular orbit by the electromagnetic force of a large magnet. The particles spiral outward under the influence of these electrical and magnetic forces, reaching extremely high speeds.

Acceleration occurs in a vacuum so that the particles do not collide with air molecules. Other accelerators were developed from the cyclotron capable of providing increasingly higher energies to the particles. Since the apparatus required to generate intense magnetic forces is colossal, high-energy accelerators are huge and expensive installations.


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