![]() One problem that remained in Mendeleev's final analysis was the inversion of certain elements in his periodic table. It provided a keyorganizing concept for the chemical sciences. Dmitri Mendeleev's discovery of the periodic law in the late 1860s was a remarkable accomplishment. This concept was historically important because it provided a theoretical basis for the periodic law. Thus, the number of protons in a nucleus (or, the nuclear charge, or the atomic number) determines the chemical properties of an element.Ītomic number is defined as the number of protons in the nucleus of an atom. The number of electrons in an atom, in turn, is determined by the nuclear charge. When atomic number, rather than atomic mass, is used to construct a periodic table, these problems disappear, since an element ’s chemical properties depend on the number and arrangement of electrons in its atoms. Although he wasĮssentially correct, the periodic table constructed on this basis had a major flaw: Certain pairs of elements (tellurium and iodine constitute one example) appear to be misplaced when arranged according to their masses. Mendeleev had said that the properties of elements vary in a regular, predictable pattern when the elements are arranged according to their atomic masses. Moseley ’s discovery made possible a new understanding of the periodic law first proposed by Dmitri Mendeleev in the late 1850s. Moseley hypothesized that the regular change in wavelength from element to element was caused by an increase in the positive charge on atomic nuclei in going from one element to the next-heavier element. He discovered that the wavelength of the reflected x rays decreased in a regular predictable pattern with increasing atomic mass. Moseley bombarded a number of chemical elements with x rays and observed the pattern formed by the reflected rays. The concept of atomic number evolved from the historic research of Henry Gwyn-Jeffreys Moseley in the 1910s. Accordingly the atomic number is often omitted from a nuclear symbol, as in 16O, where the superscript represents the atomic mass (a attribute than does vary with isotopes of an element). In nuclear chemistry, an element ’s atomic number is written to the left and below the element ’s symbol The number of protons for a particular element never varies, if one changes the number of protons one is changing the element. It is always the smaller whole number found in association with an element ’s symbol in the table. The atomic number of an element can be read directly from any periodic table. Since each proton carries a single positive charge, the atomic number is also equal to the total positive charge of the atomic nucleus of an element. Oxygen ’s atomic number is, therefore, eight. For example, the nucleus of an oxygen atom contains eight protons and eight neutrons. ![]() ![]() The atomic number of an element is equal to the number of protons in the nucleus of its atom.
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