A chemical element, in the science of chemistry, is a pure substance whose constituent atoms have an equal number of protons in their nucleus.  This number (number of protons), represented by the symbol Z, is called the atomic number of that element. All atoms that have the same number of protons (equal atomic number) have the same chemical properties. But the atoms of an element can have different numbers (neutrons) which are called (isotopes) of that element. Sometimes, for simplicity, a chemical element is simply called an element. The chemical properties of the atoms of an element are determined by their electronic structure, which is also dependent on the number of protons in the nucleus of that atom. In the general expression of the element concept, it is said that the property of an atom distinguishes it from other properties that other atoms have. In general, the meaning of the characteristic is the number of protons of an atom in its nucleus, which forms its characteristic. This is the same characteristic that, for example, distinguishes copper from gold. All chemical elements are made of baryons. However, cosmologists believe that baryons make up only 15% of the visible matter in the universe; Therefore, the rest of the materials in the universe is dark matter, whose nature is not known, but it is not composed of the elements we know.  Among all the elements, hydrogen and helium, which were formed at the beginning of the Big Bang, are the most abundant. are the elements in the universe
A pure substance that consists of only one element's atoms is called a "simple substance". Such a substance cannot be decomposed into another substance. From this point of view, simple matter is placed against compound matter. For example, oxygen is an element. But the substance that we know as oxygen gas in nature is actually a simple diatomic substance of this element called "dioxygen" or "molecular oxygen" (O2). Ozone is another form of oxygen found in nature with the formula (O3). The relationship between dioxygen and ozone is called allotropy. In other words, dioxygen and ozone are different forms of the element oxygen. Diamond and graphite are also two allotropes for the element carbon. Other elements such as sulfur and phosphorus also have well-known and widely used allotropes. Among the known elements, few are found in simple or pure form in nature, among which copper, gold, silver, carbon and sulfur can be mentioned. Except for a few elements, the rest of the elements, including the noble gases (inert gases) and noble metals, are usually found on Earth in combination and not in pure form.
Chemical elements cannot be converted into each other by ordinary chemical reactions. The only reaction that can be used to change the number of protons in the nucleus of the atoms of an element and convert one element into another element is a nuclear reaction, which is called a nuclear transformation reaction.
So far, 118 elements have been discovered or created. Of these, 90 elements are found in nature, and the rest are made artificially and with the help of nuclear reactions in the laboratory.  Among all the elements, 81 elements have at least one stable isotope, except for element number 43 (tecnesium) and Element number 61 (promethium) all have an atomic number equal to or lower than 83. In other words, in the periodic table, the only elements that are lighter than element 84 (polonium) and do not have stable isotopes are technetium and promethium. Except for the mentioned 81 elements, the rest are radioactive isotopes (radionuclides) that turn into other elements over time and following nuclear decay.