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.