3. Periodic classification of the elements

3.1 Principle of classification 

The electron configuration of the atoms of the different elements is the basis of the classification known as periodic classification of the elements. Elements are classified in a table, each in a box, in an increasing order of their charge number Z. This is why Z is called the atomic number of the element. Each horizontal row of order n in the table starts with the gradual filling of the ns sublevel (ns¹) and ends with the filling of the np level sublevel (np⁶), except in the first row , where the K shell n =1 does not have a 1p sublevel. We move from one row to the other when we start filling a new level. Elements are thus placed in columns that have the same type of valence energy sublevels.

 

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3.2 Description

The modern periodic classification uses a table containing 7 rows and 18 columns.

•  Elements of the same row have the same number of energy levels. The rows (shells) are numbered 1 through 7.
•  Elements which belong to the same column have the same number of electron in their valence sublevels. The columns are numbered 1 through 18.

 

 

 

•  The first row ( n=1) includes 2 elements : Hydrogen ( 1s¹) and Helium (1s²).
• The second row ( n=2) includes 8 elements and starts by filling 2s¹ through 2p⁶.
•  The third row ( n=3) includes 8 elements and starts by filling 3s¹ through 3p⁶.
•  The 4th and 5th rows also fill the sublevel d and thus include 10 more elements or 18 elements each.
•  The 6th row includes elements whose electron configuration ends with 4f. To reduce the width of the periodic table, 14 elements possessing similar chemical properties and located between boxes 57 and 72, are placed on the bottom of the table. With lanthanum ( Z=57), they constitute the lanthanides.
•  Similarly, elements of the 7th row, between boxes 90 and 103, constitute, along with actinium ( Z= 89), the actinides. Elements in each column constitute a family. Among the main families of the periodic table are : The family of alkali metals, the family of alkaline earth, the halogen family and the family of noble gases.

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3.3 Periodicity

A-General properties of some families

•  The elements of column 1 ( with the exception of hydrogen) constitute the family of alkali metals. The possess one valence electron and may easily lose this electron to become positive ions: They react vigorously with water and hydrogen.

•  The elements of column 2 constitute the family of the alkaline earth. Their properties are related to those of the alkali metals, thus their name. They react easily with oxygen but they don’t react with water as vigorously as the alkali metals. They possess two valence electrons that they can easily lose to become positive ions.

•  The elements of column 17 constitute the halogen family. They possess 7 valence electrons and can easily gain one electron to become negative ions. They exist as diatomic molecules and react rapidly with hydrogen to give hydrogen halides.
•  The elements of column 18 constitute the family of nobles gases. They are characterized by the saturation of their valence shell. Except for helium, which only possesses 2 electrons, all noble gases possess 8 valence electrons, called octet. They possess a very low reactivity and are thus called inert gases.

B-Ionization energy

When an atom A absorbs energy, it is excited. When the absorbed energy overcomes the attraction of an electron to the nucleus, this electron is removed from the atom which becomes a positive ion A⁺. This energy is called ionization energy of the atom (E₁). The energy required to remove one electron from the valence shell of the atom is called ionization potential.

 

 

In the periodic table:

   In the same row, the energy of the first ionization increases as the atomic number increases.

   In the same column, it decreases as the number of energy level increases. 

C-Electron affinity

Atoms may also gain electrons and become negative ions. When a negative ions is formed, the atom loses a quantity of energy called the electron affinity of this atom. It may be considered as the energy required to remove an electron from the negative ion of this atom.

 

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3.4 Importance of periodic classification

A-Metals and Non-metals

Metals are elements that have the tendency to lose electrons and become positive ions: They are electron donors.

Non-metals are elements which have a tendency to gain electrons and become negative ions: They are electron acceptors. The majority of chemical elements are metals.

 

B-The valence concept

The valence of an atom is the number of electrons lost or gained by this atom. Atoms of columns 1 and 2 have a tendency to lose 1 and 2 electrons, respectively, and therefore possess valences of 1 ad 2 respectively. Those columns 16 and 17 have a tendency to gain 2 and 1 electrons, respectively, and therefore have a valence of 2 and 1, respectively. 

 

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