Periodic System Theory

Periodic System


The current Periodic System is based on the one which was proposed by Mendeleiev with the inherent corrections to Moseley’s law, as it is said, used as sort order atomic number.

It consists in eighteen vertical rows, called groups or families, and seven horizontal rows, called periods.


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All elements of a group have identical configuration of the outermost electron shell (valence shell). These electrons are called valence electrons and are responsible for its chemical properties and the type of link that will present.


Within the periods, or horizontal rows, we can distinguish:

  1. Shorter periods. Periods are 1, 2 and 3, and consisting respectively of 2, 8 and 8 elements each.
  2. Periods media as the 4th and 5th, with 18 elements each.
  3. Longer periods. So called 6th and 7th periods, each of which contains 32 elements, this is due to the lanthanide and actinide groups called.

For the purposes of electronic configuration it is divided into four zones called s, p, d and f, because in them it will be placing the electron differentiator.

The Periodic Table is classified as:

  1. Metals and non-metals. The first is located on the left side of the table, while the latter are in the right. In between them, there are lied metalloids except hydrogen.
  2. Representative elements. Are the elements belonging to the groups 1st, 2nd, 13th, 14th, 15th, 16th, 17th and 18th. They are characterized in that they complete s and p sublevels of the last layer.
  3. Transition elements. Are the elements belonging to the groups 3rd, 4th, 5th, 6th, 7th, 8th, 9th, 10th, 11th and 12th. They are characterized in that they complete the sublevel d, which is located in the penultimate layer.
  4. Elements of internal transition. Are the lanthanides and actinides completing the 4f and 5f sublevel respectively.



Physical and chemical properties of the elements are repeated throughout the periodic table.


-Atomic radius and ionic radius.


The atomic radius is usually defined as half the distance between the two cores of two adjacent atoms in a solid metal or, in the case of covalent substances, from the distance between the cores of identical atoms in a molecule.

  • In the same group or family the radius increases when we go down. This is because, when we pass from one group to the next one, the number of electrons shells.
  • In the same period, the radius increases to the left. Now the number of layers is the same, but when we move to the left the atomic number Z decreases, this translates into a lower attraction on the negatively charged.

In the case of ionic radius should be noted that:

  • The radius of a positive ion, cation, is less than its neutral element.
  • The radius of a negative ion, anion, is greater than its element.
  • Within the same period decreased cationic radius to the right, however the anionic radius increase. In the same group, both cationic and anionic radius increase when we go

-Ionization energy.

We define ionization energy: The minimum energy required to remove an electron from a neutral atom in its gaseous state and in its ground state.

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  • Within the same group the ionization energy increases upward because the peripheral electrons are in a lower electron shell and thus be more strongly attracted.
  • In the same period, in general, the ionization energy increases to the right. With increasing atomic number Z increases the attraction of the nucleus on the electron.


-Electron affinity.

It is defined as: The energy released (sometimes absorbed) when a gaseous neutral atom accepts an electron to form a negative ion.

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In the Periodic Table, the electron affinity varies, absolute value, in the same way that the ionization energy, being more difficult to establish a simple reason that can justify this variation.


-Electronegativity and metallic character.

The electronegativity is defined as: The ability that a given atom has to draw to itself the pair (or pairs) of electrons shared a covalent linkage element as defined electronegativity.


Electronegativity is a property closely related to two of those seen before. Thus, elements with high values of ionization energies and electron affinities, have high values of electronegativity therefore variation in the periodic table is similar.


On the other hand, the electronegativity serves to reflect the varying nature of the metallic elements. Thus, high electronegativity elements are not metals; and conversely, those with low electronegativity, be metals.


Therefore, the metallic character in the Periodic System vary inversely with as does the electronegativity.





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