The atomic number (symbol Z) of a chemical element is the number of protons in the atomic nucleus of every atom of that element. IUPAC Gold Book - Atomic Number The atomic number is the fundamental property that uniquely identifies an element. In an electrically neutral atom, the atomic number is also equal to the number of electrons orbiting the nucleus.

History

The modern concept of the atomic number emerged from the development of the periodic table. In 1869, Dmitri Mendeleev arranged the known elements in order of increasing atomic weight, which generally correlated with their chemical properties. However, this ordering resulted in some inconsistencies. For example, the atomic weight of tellurium (127.60) is greater than that of iodine (126.90), but Mendeleev placed tellurium before iodine to align them with elements of similar properties.

The physical basis for ordering the elements was not understood until the early 20th century. In 1913, amateur physicist Antonius van den Broek was the first to propose that the position of an element in the periodic table corresponds to the charge of its atomic nucleus. This idea was experimentally confirmed that same year by the English physicist Henry Moseley.

Moseley conducted experiments using X-ray spectroscopy to study various elements. He discovered a systematic mathematical relationship between the wavelength of the emitted X-rays and a fundamental integer associated with each element, which he identified as the nuclear charge. October 1913: Henry Moseley and the Atomic Number This integer was the atomic number. Moseley's work demonstrated that the atomic number, not the atomic weight, was the correct organizing principle for the periodic table. This resolved the discrepancies in Mendeleev's table, such as the tellurium-iodine pair, and correctly predicted the existence of then-unknown elements.

Properties and Significance

Chemical Identity

The atomic number is the defining characteristic of a chemical element. All atoms with the same atomic number exhibit the same chemical properties and belong to the same element. For example, any atom containing exactly 6 protons in its nucleus is an atom of carbon, which has the atomic number Z=6.

Relationship to Mass Number and Isotopes

While the atomic number (Z) specifies the number of protons, the mass number (A) represents the total number of protons and neutrons in an atom's nucleus. Atoms of the same element (same Z) can have different numbers of neutrons. These variants are known as isotopes. For example, Carbon-12 (6 protons, 6 neutrons) and Carbon-14 (6 protons, 8 neutrons) are both isotopes of carbon. Their atomic number is 6, but their mass numbers are 12 and 14, respectively. LibreTexts Chemistry - Atomic Structure

Charge Neutrality and Ions

In a neutral atom, the number of negatively charged electrons is equal to the number of positively charged protons, so the net electric charge is zero. When an atom gains or loses electrons, it becomes an ion with a net negative or positive charge. However, this process does not alter the number of protons in the nucleus, and thus the atom's chemical identity (its atomic number) remains unchanged.

Notation

The standard notation for a specific nuclide indicates the mass number (A) as a superscript and the atomic number (Z) as a subscript, both preceding the element's symbol (X):

^A_ZX

For example, the most common isotope of uranium is written as ²³⁸₉₂U. Because the atomic number is uniquely determined by the element symbol (e.g., U always means Z=92), the subscript is often omitted, as in ²³⁸U.

Modern Elements

The concept of the atomic number extends to all known elements, including those synthesized in laboratories. The periodic table is organized strictly by increasing atomic number, from hydrogen (Z=1) to the heaviest known element, Oganesson (Z=118). The synthesis of new elements involves creating nuclei with even higher proton counts, thereby expanding the periodic table to higher atomic numbers. Britannica - Atomic Number