How are the elements classified in the periodic table?

The periodic table is a graphic representation of information on chemical elementslike the symbol or the atomic number, in columns and rows, that is, in a tabular layout. If read from left to right and top to bottom, the atomic number is increasing. The atomic mass is also increasing in this sense, with some exceptions.

The rows of the periodic table are known as periods. They are numbered from 1 to 7 in descending order; period 1 is the upper period and period 7 is the lower period. The longer the period, that is, when descending the periodic table, the number of energy levels of the atom in the ground state (unexcited) increases.

Each column of the periodic table is a cluster and there are a total of 18. The groups come together to form 4 blocks depending on the last occupied orbital: s, p, d Y F.

Structure of the periodic table

Element Types

In addition to systematic representations and descriptions based on atomic characteristics, categories or types can be established in the periodic table based on physical and chemical properties shared by a group of elements. One of the most widespread classifications has three big categories: metals, metalloids and non-metals. These categories are further divided into smaller groups:

  1. metals: alkali metals, alkaline earth metals, transition metals, post-transitional metals, lanthanides, actinides.
  2. metalloids
  3. No metals: halogens, noble gases

Alkali metals

Alkali metals include group 1 elements from Lithium (Li) to Francium (Fr). Hydrogen is in group 1 but it is not an alkali metal, in fact hydrogen shows very few metallic characteristics and is often categorized as a non-metal.

alkaline earth metals

The alkaline earth metals coincide with group 2, from beryllium (Be) to radium (Ra). They tend to have a very high melting point and their oxide compounds form very basic alkaline solutions.


The lanthanides are the group formed from the element with atomic number 57, lanthanum (La), which gives the group its name, to the element with atomic number 71, Lutetium (Lu). The valence shell of the lanthanides is 4f; Together with the actinides (5f) they form the f block.


The actinides is the group that includes from atomic number 89, Actinium (Ac), to 103, Lawrencium (Lr). The valence shell is 5f and they are all radioactive. They are rare elements, in fact only thorium (Th) and uranium (U) occur in nature in significant amounts.

transition metals

Metals or transition elements are located in the center of the periodic table, in the d block, which ranges from group 3 to group 12. They are characterized by having a partially occupied d orbital in their electronic configuration.

Post-transitional metals

The post-transitional metals, sometimes referred to simply as "other metals," are Aluminum (Al), Gallium (Ga), Indium (In), Thallium (Tl), Tin (Sn), Lead (Pb), and Bismuth (Bi). . These elements are considered metals but tend to have more moderate metallic characteristics; for example, they tend to be softer or relatively poor conductors.


Metalloids are substances with intermediate properties between metals and non-metals. They typically behave like non-metals, but may appear metallic or conduct electricity in some circumstances. The metalloid elements, also known as semimetals, are Boron (B), Silicon (Si), Germanium (Ge), Arsenic (As), Antimony (Sb), Tellurium (Te) and Polonium (Po); sometimes Astatine (At) is also included.

No metals

Under the term "non-metals" all the other elements would be included, from the halogens to the noble gases, but it is very often used for non-metallic elements that cannot be classified as halogens or as noble gases, that is, for Hydrogen (H), Carbon (C), Nitrogen (N), Phosphorus (P), Oxygen (O), Sulfur (S) and Selenium (Se).


Halogens are a type of non-metallic elements that coincide with group 17 of the periodic table, which ranges from Fluorine (F) to Astatine (At), the latter sometimes included in the metalloids. Halogens are usually very reactive elements, which is why they are commonly found in nature as part of other substances and rarely in pure form.

Noble gases

Those known as noble gases coincide with group 18. All these elements are gaseous under normal conditions of pressure and temperature, they have no color, they have no smell, and their great stability makes them worthy of the common adjective of being "chemically inert".

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