The atomic mass of an element is the mass of the number of particles in its nucleus given in atomic mass units. Although it is sometimes referred to as atomic weight, this is inaccurate, since weight is a measurement that includes the effect of g ravity. One atomic mass unit (amu) is 1.66 x 10- 24 grams. This is equal to one-twelfth the mass of an atom of carbon-12. This standard was adopted internationally in 1961, replacing an arbitrarily assigned value of 16.000 amu for the atomic mass of an atom of oxygen .
The instrument most often used to measure atomic mass is the mass spectrometer (spectrograph). In this instrument, atoms are vaporized and then changed to positively charged particles by knocking off electrons. These charged particles are passed through a magnetic field that causes them to be deflected different amounts, depending on the siz e of the charge and mass. The particles are eventually deposited on a detector plate where the amount of deflection can be measured and compared with the charge. Very accurate relative masses are determined in this way.
In the early years of the study of the behavior of matter, it became generally accepted that mass was conserved in physical and chemical change. By the end of the eighteenth century, studies of compounds suggested that when elements combine, the compounds that form follow the law of constant (or definite) proportio ns. This law was formally stated by the French chemist Joseph Proust (1754-1826) in 1799: the ratio of the masses of the elements in a compound will always be the same, independent of the method of preparation or the proportions that are mixed in its preparation.
Early in the nineteenth century, John Dalton (1766-1844), the English chemist who is sometimes called the father of modern chemistry, introduced the concept of atomic mass when he formulated a model of the atom as a round, solid, indivisible particle that represents the elementary unit in volved in chemical change. He postulated that all the atoms of the same element have the same atomic mass, while the atoms of a different element have a different atomic mass. He determined that compounds always have the same percentage composition by mass. His explanation was that compounds have a fixed ratio based on the number of atoms of each element in the compound. He expanded on the law of constant proportions, which deals with compounds made of any two single elements, adding to it the law of multiple proportions, to explain situations where more than one compound is formed from the same two elements. The law of multiple proportions states that the ratio of the element to itself in multiple compounds will be a small whole number, like carbon monoxide and carbon dioxide.
In 1815, the English chemist William Prout (1785-1850) observed that the atomic weights of several gases were whole number multiples of the atomic weight of hydrogen. His conclusion based on this experimental result was that every atom is made up of a number of hydrogen atoms that are somehow held together.
In 1864, the English chemist J.A.R. Newlands (1837-1898) and the German chemist Lothar Meyer (1830-1895) recognized independently that there was a connection between the atomic masses of the elements and the periodic repetition of chemical and physical properties. In 1869, the Russian chemist Dmitry Mendeleev (1834-1907) tried to arrange the 63 known elements in a pattern that would reflect similar chemical behavior. He found that the arrangement was essentially one of increasing atomic mass with three exceptions where he had to reverse the order of atomic mass in order to maintain the pattern of similar chemical behavior.
After the turn of the century, changes in the mo del of the atom led to an awareness of a relationship between the atomic mass and the number of particles in the nucleus. At first, the nuclear particles were thought to be only those that carry the positive charges of the atom, but it was soon obvious that, except for hydrogen, the weight of an atom was around twice the weight of the number of pr otons it contained. Ernest Rutherford (1871-1937) suggested in 1920 that the atom must have neutral particles (neutrons) whose mass is similar to that of the proton. This particle was identified by the English physicist James Chadwick (1891-1974) in 1932.
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