Atomic number
Atomic number
The atomic number of an element is equal to the number of protons in the nucleus of its atom. For example, the nucleus of an oxygen atom contains eight protons and eight neutrons. Oxygen’s atomic number is, therefore, eight. 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.
The atomic number of an element can be read directly from any periodic table. It is always the smaller whole number found in association with an element’s symbol in the table. 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. Accordingly the atomic number is often omitted from a nuclear symbol, as in16O, where the superscript represents the atomic mass (a attribute than does vary with isotopes of an element).
The concept of atomic number evolved from the historic research of Henry Gwyn-Jeffreys Moseley in the 1910s. Moseley bombarded a number of chemical elements with x rays and observed the pattern formed by the reflected rays. He discovered that the wavelength of the reflected x rays decreased in a regular predictable pattern with increasing atomic mass. 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.
Moseley’s discovery made possible a new understanding of the periodic law first proposed by Dmitri Mendeleev in the late 1850s. Mendeleev had said that the properties of elements vary in a regular, predictable pattern when the elements are arranged according to their atomic masses. Although he was
essentially 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.
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. The number of electrons in an atom, in turn, is determined by the nuclear charge. Thus, the number of protons in a nucleus (or, the nuclear charge, or the atomic number) determines the chemical properties of an element.
See also Element, chemical.
Atomic Number
Atomic number
Atomic number is defined as the number of protons in the nucleus of an atom . This concept was historically important because it provided a theoretical basis for the periodic law. Dmitri Mendeleev's discovery of the periodic law in the late 1860s was a remarkable accomplishment. It provided a keyorganizing concept for the chemical sciences. One problem that remained in Mendeleev's final analysis was the inversion of certain elements in his periodic table . In three places, elements arranged according to their chemical properties, as dictated by Mendeleev's law, are out of sequence according to their atomic weights.
The solution to this problem did not appear for nearly half a century. Then, it evolved out of research with x rays, discovered in 1895 by Wilhelm Röntgen. Roentgen's discovery of this new form of electromagnetic radiation had inspired a spate of new research projects aimed at learning more about x rays themselves and about their effects on matter. Charles Grover Barkla, a physicist at the Universities of London and Cambridge, initiated one line of x-ray research. Beginning in 1903, he analyzed the way in which x rays were scattered by gasses, in general, and by elements, in particular. He found that the higher an element was located in the periodic table, the more penetrating the rays it produced. He concluded that the x-ray pattern he observed for an element was associated with the number of electrons in the atoms of that element.
Barkla's work was brought to fruition only a few years later by the English physicist H.G.J. Moseley. In 1913, Moseley found that the x-ray spectra for the elements changed in a simple and regular way as one moved up the periodic table. Moseley, like Barkla, attributed this change to the number of electrons in the atoms of each element and, thus, to the total positive charge on the nucleus of each atom. (Because atoms are electrically neutral, the total number of positive charges on the nucleus must be equal to the total number of negatively charged electrons.)
Moseley devised the concept of atomic number and assigned atomic numbers to the elements in such a way as to reflect the regular, integral, linear relationship of their x-ray spectra. It soon came to be understood that the atomic number of an atom is equal to the number of protons in the atom's nucleus.
Moseley's discovery was an important contribution to the understanding of Mendeleev's periodic law. Mendeleev's law was a purely empirical discovery. It was based on properties that could be observed in a laboratory. Moseley's discovery provided a theoretical basis for the law. It showed that chemical properties were related to atomic structure (number of electrons and nuclear charge) in a regular and predictable way.
Arranging the periodic table by means of atomic number also resolved some of the problems remaining from Mendeleev's original work. For example, elements that appeared to be out of place when arranged according to their atomic weights appeared in their correct order when arranged according to their atomic numbers.
See also Atomic mass and weight; Atomic theory; Bohr model; Chemical bonds and physical properties; Geochemistry
Atomic Number
Atomic number
The atomic number of an element is equal to the number of protons in the nucleus of its atom. For example, the nucleus of an oxygen atom contains eight protons and eight neutrons. Oxygen's atomic number is, therefore, eight. 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.
The atomic number of an element can be read directly from any periodic table . It is always the smaller whole number found in association with an element's symbol in the table. In nuclear chemistry , an element's atomic number is written to the left and below the element's symbol; since an element's atomic number can always be determined simply by knowing its symbol, however, the former is often omitted from a nuclear symbol, as in 16O, where the superscript represents the atomic mass .
The concept of atomic number evolved from the historic research of Henry Gwyn-Jeffreys Moseley in the 1910s. Moseley bombarded a number of chemical elements with x rays and observed the pattern formed by the reflected rays. He discovered that the wavelength of the reflected x rays decreases in a regular predictable pattern with increasing atomic mass. 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.
Moseley's discovery made possible a new understanding of the periodic law first proposed by Dmitri Mendeleev in the late 1850s. Mendeleev had said that the properties of the elements vary in a regular, predictable pattern when the elements are arranged according to their atomic masses. Although he was essentially correct, the periodic table constructed on this basis had three major flaws. Certain pairs of elements (tellurium and iodine constitute one example) appear to be misplaced when arranged according to their masses.
When atomic number, rather than atomic mass, is used to construct a periodic table, these problems disappear. The reason is that an element's chemical properties depend on the number and arrangement of electrons in its atoms . The number of electrons in an atom, in turn, is determined by the nuclear charge. It is obvious, then, that the number of protons in a nucleus (or, the nuclear charge, or the atomic number) determines the chemical properties of an element.
See also Element, chemical.