Periodic Table

• In 1864, Newlands arranged the known 56 elements in the order of increasing atomic masses. 
• He observed that the properties of every eighth element are similar to the properties of the first element.
• Based on this observation, he proposed the Law of Octaves for the classification of elements.
• Law of Octaves: When the elements are arranged in the increasing order of their atomic masses, the properties of every eighth element are similar to the first.
  
  
  
  

1. Hydrogen resembles alkali metals as well as halogens. So, a correct position could not be assigned to hydrogen in the periodic table.
2. The position of isotopes could not be explained. Isotopes are atoms of the same element having similar chemical properties but different atomic masses. If the elements are arranged according to atomic masses, the isotopes should be placed in different groups of the periodic table.
3. At certain places, an element of higher atomic mass has been placed before an element of lower atomic mass.
   For example: Cobalt (Co=58.93) is placed before nickel (Ni=58.71).
4. Some elements placed in the same sub-group had different properties.
   For example: Manganese is placed with the halogens which totally differ in their properties.

• It is a table that classifies all the known elements in accordance with their properties in such a way that elements with similar properties are grouped together in the same vertical column and dissimilar elements are separated from one another. 
• The 115 known elements are arranged in the periodic table in the order of their increasing atomic number.
• The vertical columns are called Groups.
• The horizontal rows are called Periods.

• Dobereiner’s Triads
  In 1817, Dobereiner classified elements with similar chemical properties into groups of three called triads. He noted that the atomic weight of the middle element in a triad is the arithmetic mean of the other two elements.
  This is called Dobereiner’s Law of Triads.
  
  
  
  
• Newland’s Law of Octaves
  In 1864, Newland arranged elements in the increasing order of their atomic weights and noticed that the eighth element was similar in physical and chemical properties to the first element, just like the eight notes in music. This relationship is called the Law of Octaves.
  
  
• Lother Meyer’s Law
  In 1869, Lother Meyer showed that the atomic volume is a periodic function of the atomic weight.
  
  
• Mendeleev’s Periodic Law
  In 1869, Mendeleev stated the law of chemical periodicity.
  The physical and chemical properties of elements are a periodic function of their atomic weights, i.e. when the elements are arranged in the increasing order of their atomic weights, elements with similar properties are repeated after certain regular intervals.
  This is called Mendeleev’s Periodic Law.
  
  

Certain anomalies of Mendeleev’s table:

1. Position of hydrogen 
2. Position of rare earth elements
3. Position of isotopes 
4. Position of noble gases
5. Anomalous pairs of elements
6. Cause of periodicity

• Mosley’s Modern Periodic Law
  The physical and chemical properties of elements are a periodic function of their atomic numbers. 

• The vertical columns are called groups and consist of eighteen groups numbered from 1 to 18.
• Elements having the same number of valence electrons are present in the same group.
• Elements present in the same group show the same chemical properties.
• Group 1 contains alkali metals and these elements contain 1 electron in their outermost shell.
• Group 2 contains alkaline earth metals and these elements contain 2 electrons in their outermost shell.
• Groups 3 to 12 have two outermost shells incomplete.
• Groups 13 to 18 contain 3 to 8 electrons in their outermost shell.
• Group 18 elements have a complete outermost shell. So, they are called noble elements or noble gases.
• Hydrogen has been placed at the top of group 1, because the electronic configuration of hydrogen is similar to alkali metals.
• Metals are present on the left hand side of the periodic table, whereas non-metals are present on the right hand side of the periodic table.
• Elements present in the groups 1 and 2 on the left hand side and groups 13 to 17 on the right hand side are called normal elements.
• Elements present in groups 3 to 12 in the middle of the periodic table are called transition elements.
• Elements placed at the bottom of the periodic table are called inner transition elements and contain two series of elements, lanthanides and actinides.

• The horizontal rows in the Modern Periodic Table are called periods.
• The Modern Periodic Table consists of seven periods and are numbered from 1 to 7.
• In each period, a new shell starts filling up. The period number is also the number of the shell which starts filling up in it.
• The elements in a period have consecutive atomic numbers, and the number of elements in each period is given below:

1. The first period contains 2 elements and is called a very short period.
2. The second and third periods contain 8 elements and are called short periods.
3. The fourth and fifth periods are long periods and contain 18 elements each.
4. The sixth period is the longest and contains 32 elements.
5. The seventh period is an incomplete period.