Write short notes.
a. Mendeleev’s periodic law.
b. Structure of the modern periodic table.
c. Position of isotopes in the Mendeleev’s and the modern periodic table.
a. Mendeleev’s periodic law.
b. Structure of the modern periodic table.
c. Position of isotopes in the Mendeleev’s and the modern periodic table.
a. Mendeleev’s periodic law.
- According to Mendeleev’s Periodic Law, “Physical and chemical properties of elements are periodic function of their atomic masses”.
- Mendeleev classified elements according to their atomic masses and arranged these elements in increasing order of their atomic masses.
- Mendeleev classified periodic table into horizontal rows and vertical coloumns. The horizontal rows are called as periods and vertical columns are called groups. Mendeleev’s Periodic Table contains seven horizontal rows and nine vertical columns.
- The elements with similar properties comes into same group.
- Mendeleev also left gaps in his periodic table for undiscovered elements like aluminum, silicon and Boron in his periodic table and named them Eka-Aluminium, Eka-silicon and Eka-Boron.
- Mendeleev not only predicted the existence of Eka-Aluminium, Eka-silicon and Eka-Boron but also described the general physical properties of these elements.
- These elements discovered later and named as Gallium, Germanium and Scandium.
- Mendeleev's periodic table could predict the properties of several elements on the basis of their position in the periodic table.
- Mendeleev's periodic table could accomodate noble gases when they were discovered.
Demerits of Mendleev's periodic table:
- The position of isotopes could not explained.
- Wrong order of atomic masses of some elements could not be explained.
- Position of Hydrogen could not be assigned in a periodic table.
b.Structure of the modern periodic table:
Periodic Table: It is the table of chemical elements arranged in order of atomic number such that elements with similar atomic structure appear in the vertical columns.
The Modern periodic law states that The chemical and physical properties of elements are a periodic function of their atomic numbers. Modern periodic table is based on the modern periodic law.
Main features:
- Groups - There are 18 vertical columns in the periodic table. Each column is called a group. All elements in a group have similar chemical and physical properties because they have the same number of valence electrons.
- Periods - In periodic table elements are arranged in a series of rows. Elements of the same period have the same number of electron shells.
Classification of elements:
- Group 1 contains alkali metals (Li, Na, K, Rb, Cs and Fr).
- The alkaline earth metals are metallic elements found in the group 2 of the periodic table.
- Elements present in groups 3 to 12 in the middle of the periodic table are called transition elements. In the transition elements, valence electrons are present in more than one shell.
- Group 18 on extreme right side position contains noble gases ( He, Ne, Ar, Kr, Xe and Rn ). Their outermost shell contains 8 electrons except He as its outermost shell is K shell and it can hold only 2 electrons.
- Inner transition elements:
- 14 elements with atomic numbers 58 to 71 (Ce to Lu) are called lanthanides and they are placed along with the element lanthanum (La), atomic number 57 in the same position (group 3 in period 6) because of very close resemblance between them.
- 14 elements with atomic numbers 90 to 103 (Th to Lr) are called actinides and they are placed along with the element actinium (Ac), atomic number 89 in the same position (group 3 in period 7) because of very close resemblance between them.
c.Position of isotopes in the Mendeleev’s and the modern periodic table:
Isotopes: Isotopes are the atoms having same atomic number but different atomic masses.
Therefore, according to Mendeleev’s classification these should be placed at different places depending upon their atomic masses.
For example, hydrogen isotopes with atomic masses 1,2 and 3 should be placed at three places. However, isotopes have not been given separate places in the periodic table because of their similar properties. So this was drawback of Mendeleev's periodic table as he could not explained the position of isotopes.
Modern periodic table is based upon arrangement of the elements on the basis of their atomic number. So that, all the isotopes of hydrogen should be placed at same place depending upon their atomic number.
- According to Mendeleev’s Periodic Law, “Physical and chemical properties of elements are periodic function of their atomic masses”.
- Mendeleev classified elements according to their atomic masses and arranged these elements in increasing order of their atomic masses.
- Mendeleev classified periodic table into horizontal rows and vertical coloumns. The horizontal rows are called as periods and vertical columns are called groups. Mendeleev’s Periodic Table contains seven horizontal rows and nine vertical columns.
- The elements with similar properties comes into same group.
- Mendeleev also left gaps in his periodic table for undiscovered elements like aluminum, silicon and Boron in his periodic table and named them Eka-Aluminium, Eka-silicon and Eka-Boron.
- Mendeleev not only predicted the existence of Eka-Aluminium, Eka-silicon and Eka-Boron but also described the general physical properties of these elements.
- These elements discovered later and named as Gallium, Germanium and Scandium.
- Mendeleev's periodic table could predict the properties of several elements on the basis of their position in the periodic table.
- Mendeleev's periodic table could accomodate noble gases when they were discovered.
- The position of isotopes could not explained.
- Wrong order of atomic masses of some elements could not be explained.
- Position of Hydrogen could not be assigned in a periodic table.
Periodic Table: It is the table of chemical elements arranged in order of atomic number such that elements with similar atomic structure appear in the vertical columns.
The Modern periodic law states that The chemical and physical properties of elements are a periodic function of their atomic numbers. Modern periodic table is based on the modern periodic law.
Main features:
- Groups - There are 18 vertical columns in the periodic table. Each column is called a group. All elements in a group have similar chemical and physical properties because they have the same number of valence electrons.
- Periods - In periodic table elements are arranged in a series of rows. Elements of the same period have the same number of electron shells.
- Group 1 contains alkali metals (Li, Na, K, Rb, Cs and Fr).
- The alkaline earth metals are metallic elements found in the group 2 of the periodic table.
- Elements present in groups 3 to 12 in the middle of the periodic table are called transition elements. In the transition elements, valence electrons are present in more than one shell.
- Group 18 on extreme right side position contains noble gases ( He, Ne, Ar, Kr, Xe and Rn ). Their outermost shell contains 8 electrons except He as its outermost shell is K shell and it can hold only 2 electrons.
- Inner transition elements:
- 14 elements with atomic numbers 58 to 71 (Ce to Lu) are called lanthanides and they are placed along with the element lanthanum (La), atomic number 57 in the same position (group 3 in period 6) because of very close resemblance between them.
- 14 elements with atomic numbers 90 to 103 (Th to Lr) are called actinides and they are placed along with the element actinium (Ac), atomic number 89 in the same position (group 3 in period 7) because of very close resemblance between them.
c.Position of isotopes in the Mendeleev’s and the modern periodic table:
Isotopes: Isotopes are the atoms having same atomic number but different atomic masses.
Therefore, according to Mendeleev’s classification these should be placed at different places depending upon their atomic masses.
For example, hydrogen isotopes with atomic masses 1,2 and 3 should be placed at three places. However, isotopes have not been given separate places in the periodic table because of their similar properties. So this was drawback of Mendeleev's periodic table as he could not explained the position of isotopes.
Modern periodic table is based upon arrangement of the elements on the basis of their atomic number. So that, all the isotopes of hydrogen should be placed at same place depending upon their atomic number.
