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Determination of the mass unit volume (which is a size called density) for different gas allows direct comparison of molecular masses of these gases. Considering the oxygen standard, with the value of 16 atomic mass units (AWU), then it is found that the helium has 4003 UMA, fluorine and sodium 19 UMA.
The periodic elements system
In the mid-nineteenth century,a few chemists believed that similarities between the chemical properties of several items, were involving a regular basis that could be demonstrated by ordering elements in a table or periodical.Russian chemist Dimitri Mendeleev proposed a map of the elements, called "periodic table", in which elements are arranged in rows and columns so that elements with similar chemical properties to be grouped.Under this arrangement, each element has a number (atomic number) starting from 1 for hydrogen to 92 for uranium. Since that time, not all chemical elements were known,so there were left blank spaces, each corresponding to an unknown element. With this table, subsequent research led to the discovery of lack elements.
Consequently, a large number of ingenious experiments were aimed at determining the size and weight of the atom. The easiest atom, the hydrogen has a diameter of 1x10^-8 cm and weight of 1.7x10^-24 g. An atom is so small that a single drop of water contains more than one million of millions billions atoms.
The fact that an atom is not the smallest particle of a substance, became evident with the discovery of radioactivity. In the year of 1896, the French physicist Antoine Henri Becquerel discovered that certain substances, like salts of uranium, emit penetrating rays of unknown origin. With only a year before, german scientist Wilhelm Conrad Roentgen announced the discovery of rays that could penetrate the layers of graphite, which are called X-rays.
French scientists Marie Curie and her husband Pierre Curie contributed to deeper understanding of radioactive substances.
As a result of researches English physicist Ernest Rutherford and his contemporaries, has proved that uranium and other heavy elements as breadwinner and radios, transmits three different types of radiation, called alpha, beta and gamma (α, β, γ).It was revealed that the first two types of rays were composed of particles of electrically charged matter, and have kept the original names. Gamma radiation have been identified as electromagnetic waves, similar to X rays, but with wavelengths less.
Knowing the nature of radioactive emissions allowed them to elucidate the mystery of physicists atom.
It was found that, far from being a solid particle of matter, the atom is more of a spatial structure. In the center of this structure is located a "heart" small kernel called. Rutherford established that the mass of atom is concentrated in the nucleus.He also found that satellites, called electrons, moving around the nucleus, on the trajectory called orbit.
To explain the structure of atom, the Danish physicist Niels Bohr in 1913 developed a theory known as "atomic theory of Bohr. He assumed that electrons are arranged in layers or quantum levels, at a considerable distance from the nucleus. This type of arrangement is called a electronic configuration . The number of these electrons is equal to the atomic number.
In Bohr's model, the electrons could only have particular energies, not amounts in-between. They would "jump" from one energy level to another, without having an amount of energy between those of the energy levels.
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