Theory - To study the Modern Periodic Table, Chemistry, Science | Science Class 9 PDF Download

Objective:

To study the modern periodic table.

Theory:

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.

Modern periodic law: The Modern periodic law states “The chemical and physical properties of elements are a periodic function of their atomic numbers”. Modern periodic table (Figure 1) 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 outer 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.

Related Terms:

1. Atomic number: Atomic number of an element is the number of protons found in the nucleus of the atom of that element.
2. Mass number: Mass number of an element is the total number of protons and neutrons in the nucleus of the atom of that element.
3. Atomic mass: The atomic mass is the mass of an atomic particle, sub-atomic particle, or molecule.
4. Electronic configuration: The electronic configuration is the distribution of electrons of an atom or molecule in atomic or molecular orbitals.
5. Melting point: The temperature at which an element changes its state from solid to liquid is called as the melting point of the element.
6. Boiling point: The temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid and the liquid changes into a vapor is known as the boiling point of that element.
    

Classification of elements:

1. Group 1 on extreme left position contains alkali metals (Li, Na, K, Rb, Cs and Fr).
2. The alkaline earth metals are metallic elements found in the group 2 of the periodic table. 
3. 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. With a few minor exceptions, the electronic structure of transition metal atoms can be written as [ ]ns²(n-1)d^m, where the inner d orbital has more energy than the valence-shell s orbital.
4. Group 18 on extreme right side position contain noble gases ( He, Ne, Ar, Kr, Xe and Rn ). Their outermost shells contain 8 electrons except He as its outermost shell is K shell and it can hold only a maximum of 2 electrons. 
5. Inner transition elements:
   1. 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. However, for convenience sake, they are shown separately below the main periodic table.
   2. 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. They are also shown separately below the main periodic table along with lanthanides.

Periodic Trends in periodic table:

Figure 2 illustrates the variation of physical and chemical properties of elements along group and periods in the modern periodic table.