Chemical element: what it is, how is its classification and the periodic table

A chemical element is understood as a pure substance composed only of identical or same type atoms. Unlike mixtures and compounds, elements cannot be decomposed into simpler substances through ordinary chemical processes, as stated in basic chemistry texts such as that of Sánchez, García and Balderas.⁽¹⁾. This essential property makes them the fundamental components of matter. In the periodic table, each element is identified with a specific symbol and is distinguished by its atomic number, which represents the number of protons present in the nucleus and defines its properties.

How many chemical elements are there?

The number of chemical elements has progressively increased. There are 118 known chemical elements, of which 90 are natural and the rest synthesized in the laboratory.⁽²⁾. The most recent ones are usually radioactive, with a short half-life and difficult stability.

The classification of elements is based on their location in the periodic table and their physicochemical properties. Generally, they are divided into metals, non-metals and metalloids, while noble gases and halogens form particular groups.

Metals

Metals are the largest family on the periodic table, located to the left of the zigzag line (except hydrogen). They are distinguished by their brightness, conductivity, malleability and ductility, in addition to forming cations easily. They include alkali, alkaline earth, transition, lanthanides and actinides, with great importance in construction, cables, electronics and catalysis. For example: gold, silver, iron, copper, aluminum and mercury.

Non-metals

Non-metals are found to the right of the periodic table and are characterized by being poor conductors, fragile in the solid state, and present in various physical states. They form anions by gaining electrons, have low density and melting points, generate acid oxides and oxyacids, and are essential for life by integrating organic and inorganic compounds. For example: oxygen, nitrogen, sulfur and carbon.

Metalloids

Metalloids, located on the zigzag line of the periodic table, have intermediate properties between metals and non-metals. They include boron, silicon, germanium, arsenic, antimony, tellurium and polonium. They are hard, brittle, with a semi-metallic shine and intermediate conductivity. They stand out as semiconductors, silicon being fundamental in integrated circuits and modern electronic devices. For example: silicon, boron and arsenic.

noble gases

The noble gases, located in group 18, are monatomic, colorless, diamagnetic and very unreactive due to their stable electronic configurations. Generally, they do not form compounds, although some have been synthesized with xenon and krypton. They have relevant applications: neon is used in illuminated advertisements and helium, light and non-flammable, in hot air balloons. For example: helium, neon, argon, krypton, xenon and radon.

Halogens

Halogens, group 17 of the periodic table, are highly reactive nonmetals whose name means “salt generators.” They form diatomic molecules and halides by joining with metals. Its reactivity decreases with increasing atomic mass. They are found as natural salts and are applied in disinfection, oxidation, drugs and polymers. For example: fluorine, chlorine, bromine, iodine, and astatine.

The periodic table organizes the elements according to their atomic number and electronic structure. Created by Mendeleev in 1869 from the atomic mass, it allowed us to anticipate unknown elements. In its modern version, the elements are arranged in periods (rows) and groups (columns), where they share chemical properties due to having the same number of valence electrons.

• Periods: The table has seven periods. In the long periods (4 and 5) the transition elements are located, while periods 6 and 7 include the lanthanides and actinides.
• Groups: there are 18 groups or families. The alkali metals (1), alkaline earth metals (2), halogens (17) and noble gases (18) stand out. Each period begins with an alkali metal and ends with a noble gas, showing a transition from metallic to non-metallic character.
• Periodic properties: in each period the atomic radius, electronegativity and ionization energy vary systematically, increasing the non-metallic character.

• Industry and technology: metals such as iron, copper and aluminum are applied in construction, machinery and cables; Chromium is used in chrome plating and vanadium in stronger steel alloys.
• Health and everyday chemistry: halogens such as chlorine and iodine are essential in disinfection and nutrition; Oxygen and carbon are vital in respiration and biochemistry.
• Advanced technology: metalloids such as silicon and germanium stand out as semiconductors in circuits, sensors and optical fibers.
• Noble gases: neon illuminates advertisements and helium is used in balloons and cryogenic refrigeration.
• Energy and defense: actinides such as uranium and plutonium support nuclear energy and military applications.

The main difference is that the element contains a single type of atom, while the compound integrates different atoms of different chemical elements chemically bonded.

A chemical element is a pure substance made up of atoms of the same type, indivisible into simpler substances through common chemical processes; Examples are iron, oxygen or gold. On the other hand, a chemical compound results from the chemical union of two or more elements in fixed proportions, acquiring properties different from the original ones; for example, water and sodium chloride. The compounds have a defined chemical formula and are not easily separated by physical methods.

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