Candela Rocío Barbisan
Chemical engineer
The periodic table is one in which the chemical elements are arranged according to atomic number, which determines their periodic classification based on their properties.
The periodic table as we know it today was developed in 1914 by the physicist H. Moseley. For this, several scientists previously put their eyes on the subject such as D. Mendeleiv and L. Meyer who worked on the concept of atomic mass and properties of the elements. But it was Moseley who ordered the elements according to their atomic number and found a certain correlation in the properties that these presented according to their location.
Currently, the elements have certain chemical properties based on the configuration of their valence electrons and this is repeated with a periodic trend if they are located according to their increasing atomic number (Z).
What are we talking about when we say “chemical properties”?
Among the properties we include atomic radius, electronegativity, electron affinity, ionization energy, to name a few. These properties are based on the determination of the effective nuclear charge. Well, before focusing on this term, we must understand how the Periodic Table is structured and then delve into the effective nuclear charge.
This very useful tool carries a symbology that must be interpreted, first of all, the abbreviation of the name of the elements derives from Latin. Secondly, there is also the atomic number, the number of protons in the nucleus, if the atom is neutral it will coincide with the number of electrons.
On the other hand, we also find additional information such as electronegativity, oxidation state, atomic mass and its crystalline structure.
Image of the periodic table and organization system
The Table is made up of eighteen columns that make up the different “groups” and seven rows that indicate the “periods”.
Each group has a representative name being:
– Group 1: Alkali metals
– Group 2: Alkali earth metals
– Group 3 to 12: Transition elements
– Group 13: Earth elements
– Group 14: Carbonoids
– Group 15: Nitrogenoids
– Group 16: Chalcogens
– Group 17: Halogens
– Group 18: noble gases
In addition, there are two series at the bottom that are the Lanthanides and Actinides, transition elements and note that Hydrogen is excluded from the groups.
When the elements are located according to their groups, it is observed that the entire group has the same electronic configuration in its valence electrons, this explains its behavior and its tendencies in many cases.
As for the periods, they do not have a defined name, which is why they were numbered from 1 to 7, starting in period 1, at the top, which contains only Hydrogen and Helium. Depending on the increase or decrease of the atomic number within the same period, we will see the progression of different properties and these tendencies are repeated in each one of the periods, from there it is that we speak of periodic properties. Elements of the same period have the same external energy level where the valence electrons are located and, in turn, the internal configuration of their electrons (without taking valence electrons into account) responds to that of the noble gas that they contain. precedes in the period. The period number designates the level where the valence electrons are located.
This introduction was necessary to, now yes, understand the concept of effective nuclear charge. When the electrons are located in the respective energy levels, they are attracted by the positive charges of the nucleus towards it. Well, not all electrons are attracted by the same force, the outermost electrons experience less attraction due to the shielding generated by the inner electrons, closer to the nucleus. In conclusion, the effective nuclear charge represents the certain degree of attraction experienced by the outermost electron.
Now, as we move in a period from left to right, increasing the atomic number, the effective nuclear charge experienced by the last electron increases because, although both the number of electrons and protons on the atom increases, the electrons are always incorporated at the same energy level, with which the shielding due to the internal electronic configuration does not vary, but the force of attraction towards the nucleus increases due to the increase in positive charges in it.
On the other hand, in a group when we increase the atomic number (from top to bottom) all the elements have the same external electronic configuration, so the nuclear charge does not change.
Following
References
• Chang, R. (2006). Essential principles of general chemistry, Raymond Chang. McGraw Hill. Madrid. IS.
Image: iStock, jelen80