Atomic number or atomic number (Z) is the number of protons in the nucleus of an atom, and at the same time it represents the total number of electrons in the atom with a neutral charge. The atomic number determines the type of chemical element on the periodic table.^[1]

When Mendeleev arranged the known chemical elements according to their similarity in chemical properties, it was noted that their arrangement according to the atomic mass had led to some differences. iodine and tellurium; If they were arranged according to the atomic mass, their place would be wrong, but when they were arranged according to the atomic number (the number of protons), this led to their chemical properties matching the arrangement.

• Hydrogen has an atomic number of 1, since its nucleus contains 1 proton.
• Helium has an atomic number of 2, and in its nucleus there are 2 protons and 2 neutrons).
• Then comes lithium in the periodic table and there are 3 protons in its nucleus (its atomic number is 3) in addition to 4 neutrons,
• Thus the atomic number of the elements increases, even the heavy ones such as lead (its atomic number is 82) and uranium (its atomic number is 92).

In addition to the atomic number Z, which is the number of protons, we also care in chemistry and physics the atomic mass A, which is the sum of the protons and neutrons in the atomic nucleus, called A the mass number.

It has been observed that the equal numbers of protons and the numbers of neutrons stabilize the nucleus, especially for the first 20 elements in the periodic table. After that, the nucleus needs more neutrons to maintain the stability of the nucleus in order to offset the increasing repulsion force between the protons (the neutrons are electrically neutral). If the nucleus is not stable, it is radioactive. We may find, among the different isotopes of an element, what is stable or unstable, for example carbon-12, which has 6 protons + 6 neutrons, is stable, while carbon-14, which has 6 protons + 8 neutrons, is radioactive and unstable.

Regardless of whether the element is radioactive or non-radioactive, its chemical properties are not affected. Changes in the spectra of the elements appeared to depend on the increasing atomic number Z. These differences were finally explained by Henry Moseley in 1913.^[2]

Moseley watched the spectral lines of excited atoms and found that they were consistent with Bohr’s model of the atom. He also saw that the frequency of the spectral lines of the different elements is directly proportional to the square of the atomic number Z.

Moseley’s experiment in 1913[عدل]

Moseley measured the wavelength of the photons emitted from the subatomic shells of the elements between aluminum (spectral lines K and L), where Z = 13 and compare it with the spectrum lines of the elements, even for gold Z =79.^[3] It was found that the frequency of the outgoing photons increases directly with the square of the atomic number Z. This brought Moseley to his law called Moseley’s law that the atomic number is directly related to the charge of the atomic nucleus calculated, i.e. to the number of protons Z. Moseley also indicated that the group of lanthanides (from element lanthanum to element lutetium) must include 15 members (no less or more), and that was at this time far from the thinking of chemists.

The atomic number is very much directly proportional to the mass number (not to be confused with) which represents the number of protons and neutrons in the nucleus of an atom. The mass number often comes after the name of the element, for example carbon-14 (which is used to calculate radiocarbon dating), uranium-235, and uranium-238.

Astronomical phenomena and physical sciences indicate that the universe first began in the form of a very hot cloud of about 75% hydrogen and helium (about 23%), with the remainder being other heavier elements. And after they came together under the influence of the gravitational forces between their atoms, stars and star clusters formed from them. When a cloud of hydrogen and helium shrinks to form a star, the temperature of the accumulating mass rises. The more the material is compressed in the ball formed, the higher the temperature. The contraction continues, accompanied by a rise in temperature, until conditions of high pressure and high temperature (about 12 million degrees Celsius) in the heart of the star are sufficient to ignite a nuclear reaction in it. This reaction is characterized by nuclear fusion in which three or four hydrogen atoms fuse together to produce helium. And from the fusion of helium with hydrogen and helium with helium, elements heavier than that gradually emerged. That is, when the nuclear fusion reaction begins in the hydrogen and helium sphere, the cluster becomes a star that shines around it.

According to the theory of the Big Bang and the theory of star formation, all the elements previously found in the periodic table were formed in the hearts of stars from the elements hydrogen and helium.

Chemical properties[عدل]

Each element has a set of chemical properties that depend on the number of electrons present in the neutral atom, and is symbolized by Z. The arrangement of these electrons follows the principles of quantum mechanics. It represents the number of electrons in each electron orbital of the element – Especially in the outer orbit – The main factor in determining the chemical behavior of that element. So only the atomic number determines the chemical properties of the element, as those properties determine the number of electrons in the outer atomic shell. The electrons in the outer shell of an atom are called valence electrons.

As we explained above, the chemical properties of an element depend on its atomic number and therefore on the number of valence electrons in its outer shell. The chemical properties of an element and its radioactive isotopes are completely identical, as the difference in atomic mass results from the increase or decrease in the number of neutrons in the nucleus of the atom, while the number of protons and the number of electrons are equal in the atom.

This is why an element can be defined as containing atoms with a certain atomic number. Many of the elements we find a mixture of isotopes, and all isotopes of the element have the same chemical properties. (see mass number)

The relationship between the number of neutrons and the atomic mass[عدل]

Atoms with the same atomic number with a different number of neutrons are called atoms

$\displaystyle N\,$

, are called isotopes of the element. All isotopes of a particular element show the same chemical properties for that element, and they behave in the same way in their chemical reactions.

The number of neutrons in the nucleus of an atom can be determined by knowing its atomic number Z and its atomic mass

$\displaystyle A\,$

.

$\displaystyle N=A-Z\,$

Where the atomic mass of an element is equal to the sum of the protons and neutrons in its nucleus:

$\displaystyle A=N+Z.\,$

Coding items[عدل]

The atomic number is associated with the name of the chemical element, that is, all atoms with the same atomic number belong to the same element; understand isotopes. The atomic number determines the arrangement of the element in the periodic table; It is usually written to the left below the element symbol.

$\displaystyle _Z^A\textSymbol.$

That is, for example, carbon has 6 protons:

$\displaystyle _\ 6^12\mathrm C$

As for 12, it is the atomic mass of the element (the sum of the protons and neutrons).

As that

$\displaystyle _\ 6^14\mathrm C$

It denotes carbon-14, which is an isotope of carbon-12, and differs from it in the presence of 8 neutrons in its nucleus, while carbon-12 has 6 neutrons in its nucleus.

new items[عدل]

The atomic numbers are relied upon when counting the elements present. In 2010, the atomic numbers of the elements were assigned from 1 to 118. It was also possible to manufacture elements with an atomic number of Z greater than that. The process of producing new elements takes place by bombarding atoms of heavy elements with ions or protons, so that the nucleus of the heavy element swallows an ion forming a new element. Its atomic number is equal to the sum of the protons in the nucleus of the heavy element and the protons gained from the ion. Usually, these manufactured heavy elements are unstable and decay within milliseconds. In general, the half-life becomes shorter with increasing atomic number, so an island of stability may exist for undiscovered isotopes of certain numbers of protons and neutrons.

The atomic number of antimatter[عدل]

Antimatter consists of antiprotons in the nucleus and antielectrons hovering around it, meaning that the antiproton has a negative charge and the antielectron has a positive charge (positron). The antiproton does not differ from the usual proton in terms of their masses, but the difference is in their opposite charges only. Likewise, for the electron and the positron, their masses are equal, but their charges are opposite. This is why it is theoretically possible Antihydrogen atom Consisting of a negatively charged proton and a positively charged positron, it was possible to prepare an antihydrogen atom in the laboratory, but for a very short period. But in nature there are no such antimatter, and it is called antimatter because it annihilates itself and annihilates the corresponding atom immediately (they are opposites), and two gamma rays are produced from them.

the reviewer[عدل]

Read also[عدل]

• match
• periodic table
• list of elements (atomic number)
• elemental synthesis
• nuclear fusion
• Thompson (unit)

• Physics portal
• Physical chemistry portal
• Chemistry portal
• Quantum Mechanics Portal

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