Periodic Classification of Elements

 Elements are the simple substances which can not be converted into simpler substances by any physical or chemicalmeans. 

                                              118 elements are known to human being. The elements are classified into metal, non-metals and metalloids. This classification of elements is based on their properties.

👉 Metalloids :-

                         The elements which have the properties of both metals as well as non-                                  metals  are  called as  metalloids.

                                       Examples of metalloids are boron, silicon, germanium, arsenic,                                     antimony, polonium etc.

👉 Dobereiners Triads :

       A group of three elements having the similar chemical properties are called as triads.   He made made triads of the elements which were known at that trime.

                         💢    Lithium (Li). Sodium (Na), Potassium (K)

                         💢   Chlorine (Cl), Bromine (Br), Iodine (I)

                         💢   Sulphur (S), Selenium (Se), Tellurium (Te)

                         💢   Calcium (Ca), Strontium (Sr), Barium (Ba)

👉 Dobereiners Law Of Triads :

                This law states that in a triad, when the elements are arranged in the increasing order of their atomic masses, the mean of the atomic masses of the first & the last element is approximately equal to the atomic mass of the middle element.

Element	Li	Na	K
Atomic     mass             Number	7	23	39

According to Dobereiners Law,

Sum of atomic mass of Li and K = 7 + 39 = 46

Mean of atomic masses of Li and K = 46/2 = 23

Thus it is proved that, the arithmetic mean of atomic masses of the first and the last element is approximately equal to the atomic mass of the middle element.

Demerits of Dobereiner's Law

  👉 Dobereiner could arrange only some triads from the known  elements.

 👉  Other triads did not obey Dobereiner's law.

Newlands' Law of Octaves

This law states that,when the elements are arranged in the increasing order of their atomic masses the properties of the eighth element are similar to the first.

               It is found that sodium is the eighth element from lithium and both of them have similar properties.

The following table will illustrate the example.

     

H	Li	Be	B	C	N	O
F	Na	Mg	Al	Si	P	S
Cl	K	Ca	Cr	Ti	Mn	Fe

 

 

Demerits of Newlands’ law of octaves

·         👉 Newlands’ could arrange elements only up to calcium out of 56 elements.

·         👉 After calcium the law was not obeyed.

·         👉 He could arrange only 56 elements known at that time.

·         👉 He did not include inert elements because they were not discovered.

·         👉 He placed elements of different properties in the same position.

Mendeleev’s periodic table

Dmitri Mendeleev (Russian Scientist) developed the periodic table of elements during 1869 to 1872 A.D. It was the most important step in the classification of elements. Mendeleev considered atomic mass number as the fundamental property of elements.

§             Mendeleev’s periodic law:

“Properties of elements are the periodic functions of their atomic masses.”

 

§            Mendeleev’s periodic Table:

“The tabular arrangement of elements on the basis of Mendeleev’s periodic law is called as Mendeleev’s periodic Table.”

Features of Mendeleev’s periodic Table

·         The horizontal rows in the periodic table are called as periods. There are 7 periods in Mendeleev’s periodic Table.

·         The vertical columns in the periodic table are called as groups.

·         The eight groups are numbered from I to VIII. Groups from I to VIII are further sub-divided into A & B groups.

      Merits of Mendeleev’s periodic table

·                    🔘 Mendeleev arranged the elements in the increasing order of their atomic masses.

·                 🔘  He was the first person who successfully classified all known elements at that time.

·                  🔘 He kept some blank places in his table for those elements that were yet to be discovered.

·                 🔘 He also predicted the elements like eka - boron, eka - silicon and eka - aluminum.

·                🔘 When noble gases were discovered, they were placed in a new group without disturbing the                     position of others.

·                 Atomic masses of some elements were revised to give them proper place in periodic table.

Predicted elements	Actual Element Discovered Later
Eka-boron	Scandium
Eka-Aluminum	Gallium
Eka-Silicon	Germanium

   

 

 

 

     Demerits of Mendeleev’s periodic table

·              💬 Mendeleev could not assign a fixed position for hydrogen because the properties of hydrogen                   resembles with both alkali metals and halogens.

·              💬 Isotopes of the same elements have different atomic masses. They cannot be placed separately in             the periodic table.

·               💬 Some elements with higher atomic masses were placed before those having lower atomic masses.

      Example: Cobalt (58.95) was placed before Nickel (58.72)

·                💬 Some elements were placed in the same sub group had different properties.

     Example: Manganese is placed with halogens.

Modern periodic law

In 1938, the English scientist Henry moseley revealed that atomic number is a more fundamental property of an element than its atomic mass. Depending upon this fact he stated the modern periodic law as follows

"Properties of elements are a periodic function of their atomic numbers"

Modern periodic table

In the modern periodic table, the elements are arranged in the order of their increasing atomic numbers. The modern periodic table is also called the long form of the periodic table.

"The tabular arrangement of elements on the basis of modern periodic law is called as modern periodic table."

The relation between the atomic number of an element and its electronic configuration is clearly seen in the modern periodic table.

Structure of the modern periodic table

* in the modern periodic table from the elements are arranged in the order of their increasing atomic numbers.

* The horizontal rows in the modern periodic table are called as periods. There are 7 periods in the MPT.

* The vertical columns in the modern periodic table are called as groups. There are 18 groups in the MPT.

* The arrangement of the periods and groups results into formation of boxes.

* Each box represents the place for one element.

* At the bottom of the periodic table there are two rows which are called as lanthanide series and actinide series respectively.

* The periodic table is divided into four blocks namely s -block, p- block, d -block and f- block.

* A zig-zag line in the modern periodic table separates metals from the non-metals.

*Zig Zag line in the MPT represents metalloids.

*All the metals lie on the left side of the zig-zag line while all the non-metals lie on the right side.

Modern periodic table and electronic configuration of elements

The elements belonging to the same period differ slightly in their properties while the elements belonging to the same group show similarity and graduation in their properties. these characteristics of the periods and groups in the MPT are due to the electronic configuration of the elements. The position of the element in the group and in the period is decided by its electronic configuration.

1. Groups and electronic configuration

* The vertical columns in the periodic table are called as groups.

* There are 18 groups in the MPT and their numbered from 1 to 18.

* Group number indicates the number of valence electrons present in the outermost shell of the atom of an element.

                                       Group number = Number of valence electrons

* While going from top to bottom within any group one electronic shell gets added at a time.

* All the elements belonging to the same group have same valency.

* All the elements present in the same group have similar chemical properties.

Example : The elements beryllium, magnesium, calcium, strontium, barium and radium belongs to the group 2 so number of electrons in their outermost orbit are 2.

2. Periods and electronic configuration

* The horizontal rows in the modern periodic table are called as periods. There are 7 periods in the MPT.

* The period number indicates the number of cells in the atom of an element.

                             Period number = Number of Shells

* In a period while moving from left to right, the atomic number increases by one at the same time  the number of valence electrons also increases by one.

* In a period, there is radiation in properties of elements.

* The elements with the same number of shells belongs to the same period.

Example : Lithium, beryllium, boron, carbon, nitrogen, Oxygen, fluorine and neon has two shells in their atom so they belongs to second period.

* In a period, while going from left to right, the  atomic number increases by 1 at the same time atomic radius gradually decreases.

Periodic trends in the modern periodic table

The properties which show gradual variation in a group and in a period and are repeated after a certain interval of atomic number are called as periodic properties or trends.

The periodic properties are as follows:

1. Valency

2. Atomic size

3. Metallic- non Metallic character

1. Valency : 

The capacity of an element to combine with other element is called as valency.

* The valency of an element is determined by the number of  electrons present in the outermost shell.

* Elements belonging to the same group have same valency.

* In a period valency increases from 1 to 4 and then decreases from 4 to 0.

2. Atomic size :

* Atomic size is determined using atomic radius.

* Atomic radius is the distance between the nucleus and the outermost shell of an atom.

* In a period, atomic radius decreases from left to right.

   ( Explanation : In a period, while going from left to right the atomic number increases by 1 at the              same time the positive charge on the nucleus also increases by one unit. So the one more additional         electron gets added to the same outermost shell. So electrons experience greater pull from the                 nucleus.)

* In a group from top to bottom atomic radius increases because new shells are added to every next element.

 (Explanation : The distance between the outermost electron and the nucleus goes on increasing. As a result of this the atomic sizeincreases in spite of the increased nuclear charge.)

Note : atomic radius is expressed in picometre (pm)

3. Metallic -  non metallic character :

* Metals have a tendency to donate electrons so they are electropositive.

* Non metals have a tendency to accept the electrons so they are electronegative.

* In a period,  while moving from left to right metallic character decreases and non metallic character       increases.

* In a group,  from top to bottom metallic character increases because atomic size increases.

Zig-zag line in the modern periodic table (Borderline elements)

* A zig-zag line in the modern periodic table represents metalloids.

* This line in the modern periodic table separates metals from nonmetals.

* The elements which have the properties of both metals and nonmetals are called as metalloids.

Gradation in halogen family :

* The group 17 of modern periodic table contains members of halogen family.

* The general formula of halogen family is X2.

* A gradation is observed in their physical state down the group.

* The elements fluorine and chlorine are gases, bromine is a liquid while iodine is a solid.