Atomic Number | Discovery, Formula, History, Properties

Definitions of Atomic Number

atomic number
  • In the symbol representing a particular nuclear or atomic species, the atomic number may be indicated as a left subscript.
  • Atoms of each element contain a characteristic number of protons. In fact, the number of protons determines what atom we are looking at (e.g., all atoms with six protons are carbon atoms); the numbers of protons in an atom is called the atomic number. 
  • Atomic Number is the number of a chemical element in the periodic system, where the elements are arranged to increase the numbers of protons in the nucleus. Accordingly, the numbers of protons, which is always equal to the number of electrons in the neutral atom, is also the atomic number. An atom of iron has 26 protons in its nucleus; therefore, the atomic number of iron is 26.
  • The modern periodic table is arranged so that all the elements have an increasing atomic number, and subsequently, increasing mass number.
  • Atomic number mainly helps in the classification and identification of an element.

Examples of Atomic Number

The atom’s atomic number is equal to the numbers of protons in the nucleus of an atom or the numbers of electrons in an electrically neutral atom.

Atomic number = number of protons

For example, in a sodium atom, there are 11 electrons and 11 protons.


The atomic Number of Na atom = Number of electrons = Number of protons = 11.

Formula of Atomic Number

Formula of Atomic Number

The symbol for an atom can be written to show its mass number at the top, and its atomic number at the bottom.

However, To calculate the numbers of subatomic particles in an atom, use its atomic number and mass number: numbers of protons = atomic number. The numbers of electrons = atomic number.


The concept of atomic numbers evolved from the historical 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. Because 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. 

History of Atomic Numbers

History of Atomic Numbers

We take the existence of atoms for granted – yet for centuries, no one believed in them. Around 2500 years ago, a Greek philosopher called Leucippus and his pupil Demokritos put forward the idea that the universe is made up of tiny indivisible particles called atoms.

Unfortunately, the great Greek philosopher Aristotle did not agree with them. As Aristotle’s views were accepted throughout Europe for almost 2000 years, the idea of atoms was shelved for centuries to come.

Atoms are the building blocks of matter. They combine in numerous patterns and form different substances. All atoms except the common form of hydrogen contain protons, neutrons, and electrons.

The Atomic numbers of an element is equal to the numbers of protons in its nucleus. In a neutral atom, the number of protons equals the number of electrons in shells that is the energy level around the nucleus.

                         Isotopes are atoms with the same atomic numbers but distinct neutron numbers, and hence distinct mass numbers. The average isotopic mass of an isotopic mixture for an element in a defined environment on Earth determines the element’s standard atomic weight.

A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes, and the average isotopic mass of an isotopic mixture for an element in a defined environment on Earth determines the element’s standard atomic weight.

Properties of Atomic Numbers

Atomic numbers defines an element’s chemical properties. The number of electrons in an atom determines bonding and other chemical properties. In a neutral atom, the atomic number, Z, is also the numbers of electrons.

These electrons are found in a cloud surrounding the nucleus, located by probability in electron shells or orbitals. Because the shell farthest from the nucleus is the valence shell. The electrons in this valence shell are involved in chemical bonding and show the behavior of the atom.

The bonding electrons influence the molecular geometry and structure of the atom. They interact with each other and with other atoms in chemical reactions. Because, the atomic numbers are unique to each atom and defines bonding, behavior, or reactivity characteristics. Therefore, every atom, with a different atomic numbers, acts differently.

The symbol of Z

The conventional symbol Z possibly comes from the German word Atomzahl (atomic numbers). However, prior to 1915, the word Zahl (simply number) was used for an element’s assigned number in the periodic table.

Atomic Numbers of Elements

Atomic Numbers of Elements

There are about ninety elements found on Earth. However, each one has a different number of protons, electrons, and neutrons. The total number of these subatomic particles held by the respective elements are responsible for their unique properties (including radioactivity).

The number of protons in the nucleus is called the atomic number. The atomic numbers of each element is unique. The combined number of protons and neutrons in an atom is called the atomic mass number.

While the atomic numbers always stays the same, some elements have atoms with different atomic mass numbers. This is because some elements have a different number of neutrons in the nucleus. Versions of an element with different atomic mass numbers are called isotopes.

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