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• Elements: Atoms of same kind.
• Number of elements: 98 natural , 20 artificial,  total = 118 elements.
• Classification of elements: classified on basis of physical and chemical properties.
• Dobereiner’s Triad: In triad, when arranged in increasing atomic mass, the middle element is roughly an average of atomic masses of other two elements. Atomic mass of Ca = 40.1, Atomic mass of Sr = 87.6, Atomic mass of Ba = 137.3. Therefore, 40.1 +137.32=88.7 87.6

Limitation: Valid only for three triads (Li,Na,K) (Ca, Sr, Ba) (Cl, Br, I).

• Newlands’ Law of Octaves: Every eighth element have similar properties as that of first. E.g. Lithium has similar properties like magnesium.   

Limitation:

1. True only upto Calcium.
2. Two elements were filled in same slot of column.

which had different properties.

• Mendeleev’s Periodic Table : [Refer the video to understand in detail]
1. 63 elements were arranged with increasing order of their atomic masses on based on their similar properties .
2. Element with similar physical and chemical properties occurs after fix number of elements. E.g. Lithium and Sodium have similar properties.    
3. Vertical columns are ‘groups’ while horizontal columns are called ‘periods’.
• The Periodic law :  “Properties of elements are periodic functions of atomic masses”
• Achievements: Mendeleev’s Periodic table :
1. For grouping some element similarity in properties were priority rather than atomic masses i.e. some elements higher atomic masses were placed before elements of slightly lower atomic masses. E.g  cobalt is placed before nickel.
2. Gaps were left in periodic table and prediction of those elements were given before their discovery.  
3. Inert gases were placed separately in periodic table as they have different properties than others (Valency: 0, All are gases).
• Limitations of Mendeleev’s periodic table :
1. Failed to classify hydrogen, as it fulfills property of alkali metals as well as halogens
2. Failed to classify position of isotopes for carbon and hydrogen.
3. Failed to predict elements to be discovered.  
• Modern Periodic Table: [Refer the video to understand in detail]
• Modern Periodic Law: “Properties of elements are periodic function of their atomic number”.  
• Modern Periodic Table: Table formed by elements arranged in increasing order of their atomic number.

• Position of elements:
1. 18 vertical columns known as ‘groups’ . 7 horizontal rows called ‘periods’.
2. Down the group, number of shells increases but number of valence electrons remains same.
3. Across period from left to right, number of shells remains same but number of valence electrons increases by 1 unit.
4. Maximum number of elements in a shell: 2n2; where n = shell number.
• Position of metals, non-metals and metalloids:
1. Metals are placed on left hand side of periodic table
2. Non-metals at right hand side of periodic table
3. Metalloids (Boron, Silicon, Germanium,etc) are placed along the zigzag line in periodic table.
• Trends in periodic Table: [Refer the video to understand in detail]