What is the current periodic classification?
- History of the Periodic Table of Elements
- Principle of the current periodic classification
- Presentation of the current periodic table
- Periodic classification and electronic structure
- The element families
History of the Periodic Table of Elements
How were the elements classified before the appearance of the periodic table?
If the study of chemical elements has been of interest to scientists for centuries, it is at the beginning of the nineteenth century that the study of these elements and their classification will make great progress.
It is the German chemist Johann Wolfgang Döbereiner who imagines the first grouping the chemical elements between them.
He thinks that some elements have chemical properties that can be comparable and tries to associate them by triad. In this type of classification, several elements can be grouped into a triad.
In the middle of the 19th century, Alexandre-Émile Béguyer de Chancourtois ranked for the first time the elements according to their atomic mass. Geologist by training, he uses unusual terms that make his discovery little relayed.
John Alexander Reina Newlands first published a periodic table of the elements and supplemented it with his assumption of the law of octaves: the chemical properties of the elements are repeated every 8 times.
But it’s really another scientist who will revolutionize the classification of elements …
Dmitri Ivanovich Mendeleyev: the father of the periodic table
Mendeleev, the father of the current periodic table.
It was in 1869 that Mendeleyev presented his classification of elements, inspired by his predecessors. He then dictates the key points of his theory:
- The properties of the elements are periodic when they are arranged according to their atomic mass
- Elements with similar properties have near atomic masses
- The arrangement of the elements corresponds roughly to their valences
- The most common elements have small atomic masses
- The character of the element is a function of the importance of the atomic mass,
- Other elements will be added
- The atomic mass of an element can sometimes be modified
- Characteristic properties of elements can be considered by studying their atomic mass.
This periodic table had 63 elements at the time.
Did you know ? There is a board game to easily learn the periodic table of the elements. This is a remake of the famous naval battle with the elements that Mendeleev would surely have been proud of!
Principle of the current periodic classification
The periodic table of the elements.
The current periodic table has been reworked and is based on slightly different criteria than Mendeleyev’s table:
- The elements are no longer classified according to their atomic mass but by increasing atomic number.
- On the same column are placed elements having on their outer layer the same number of electrons.
This last criterion is comparable to that used by Mendeleyev because the reactivity and properties of an element depend on the number of electrons on the outer layer.
The current classification has 7 lines called periods as well as 18 columns called families .
Presentation of the current periodic table
The last update of the current classification dates from 2016. It is the International Union of Pure and Applied Chemistry (IUPAC), which represents the recognized authority for chemical rules and nomenclatures, which set the characteristics of the table. of periodic classification.
The table has 118 elements.
Did you know ? Of the 118 elements, 83 are called primordial, they are older than the Earth.
The last 24 elements are, as for them synthetic, they do not exist in the natural state on Earth and are man-made. Other elements can also be found in the Universe following supernova destruction for example.
Periodic classification and electronic structure
According to the criteria of the classification it is easy to find the configuration of an element from its location.
Example of carbon :
- It is located on the second period so its first layer K is complete and the second layer L corresponds to its outer layer.
- It is in the fourth family so its outer layer has 4 electrons so
its electronic configuration is: (K) 2 (L) 4 .
Conversely, the electronic configuration of an element makes it easy to place it in the classification.
Magnesium Example :
Magnesium has the electronic configuration (K) 2 (L) 8 (M) 2 .
- Its outer layer is the third layer so magnesium is in the third period.
- Its outer layer M has two electrons so it is located in the second column.
The element families
In the current classification all elements of the same family have the same number of electrons on their outer layer which gives them similar chemical properties: they react with the same type of compound, form comparable molecules and ions.
There are several large families:
- The first column corresponds to the alkali metal family : it includes lithium, sodium, potassium, rubidium, cesium and francium.
- All elements in this column have an outer layer with an electron. They all form cations by losing an electron (Li + , Na + , K + etc …). Alkali metals are bright, soft and highly reactive metals. Because of their high reactivity, these metals must be kept in mineral oil: they are not found as such in nature. Sodium (Na +) is the most common alkali metal in nature.
Sodium, an alkali metal.
- The second column corresponds to the alkaline earth metal family : it includes beryllium, magnesium, calcium, strontium, barium and radium. All elements of this family have 2 electrons on their outer layer. They all form cations by losing two electrons (Be 2+ , Mg 2+ , Ca 2+ etc). Alkaline earth metals have a characteristic silvery luster.
Did you know ? The term alkaline earth comes from the word Earth, with reference to the particularity of these metals to resist fire.
- The family of lanthanides : comprises 15 elements. The lanthanid name comes from the leader of the elements composing this family: the lanthanide. These are bright metals that dull quickly in the open air.
- This family contains elements such as lanthanum, cerium, gadolinium or terbium.
- The actinide family is a chemical family with 15 elements as well. These are heavy metals that derive their name from their leader, actinium. Actinides are all radioactive. They are soft metals with silvery highlights. Actinides include uranium, plutonium or thorium.
Did you know ? One of the actinides is called Mendelevium. This element, of atomic number 101 has been named thus in tribute to Dmitri Mendeleïev.
- The family of transition metals : this family includes many elements with a particular electronic configuration. These are elements that have a wide variety of colors.
- These are all metals driving electricity. This family includes metals such as rhodium, palladium, copper, platinum or titanium.
- The family of poor metals : they are soft or fragile metals. They tend to form covalent bonds and possess an amphoteric acidic basic character. This family contains aluminum, zinc, tin, mercury or lead.
- The family of metalloids : it is a family where the elements that compose it have particular characteristics. They look like metals, they are fragile and do not conduct electricity.
- Among them are boron, silicon, arsenic or antimony.
- The family of non-metals : these are elements that are found to be very good thermal and electrical insulators. Non-metals represent almost the entire constitution of living beings: among them are carbon, nitrogen, oxygen, sulfur, chlorine …
- The penultimate column is that of halogens : it includes fluorine, chlorine, bromine, iodine and astate. To each of these elements an electron is missing so that the last layer is complete and when they turn into ion it gains this electron and forms the ions F – , Cl – , Br – , I – and At – .
- The last column corresponds to the family of rare gases, also called noble gases (sometimes referred to as inert gas): it includes helium, neon, argon, krypton, xenon and radon.
- They all have a complete outer layer which makes them (according to the rules of duet and byte) very little reactive: they do not form an ion and do not participate in any molecule.
Did you know ? we also talk about a rare gas because the elements of this family are found in very small quantities in our environment (apart from helium) and they were discovered very late.
The following table shows an example of each family element of the periodic table: