Learning of 17th July

Learning Objective:

• Mendeleev and Monsey's Periodic Table

Development of the Periodic Table

Need for Periodic Classification of Elements:

• Easier to classify and study the known elements.
• Organize the known information about elements.
• Patterns and trends in the behavior could be correlated.

Lavoisier(Father of Modern Chemistry) divided elements into two main types known as metals and non metals.

Dobereiner's Triad:

• In 1817, Johann Wolfgang Dobereiner's, classified elements into groups based on their properties.
• Basis of arrangement atomic mass.
  
• Atomic mass is the sum of the number of protons and neutrons in an element.

Law of Triads:

• Dobereiner's Law of Triads sates that when three element in a triad are listed in the increasing order of their atomic masses, the atomic mass of the middle element will roughly be the average of the atomic masses of the other two elements.

Limitations of  Triad:

• All known elements could not be classified into groups of triads on the basis of their properties.
• Not all groups obeyed the Law o Triads.
• For example, nitrogen family does not obey the Law of Triads.

Newlands Law of Octaves:

• In 866, John Newlands arranged the known elements in an increasing order of their atomic mass.
• Newland's Law of Octaves:
• "When elements are arranged in order in order of increasing atomic masses, there is a repetition of the properties of every eighth element as compared to the given element."
• Based on atomic mass

Limitations:

• Not applicable throughout throughout the arrangement. Only till Calcium.
• Only space for 56 elements
• Cobalt and Nickel are in same slot - no explanation
• Elements placed in the same column as fluorine and chlorine, which have completely different properties.

Mendeleev's Periodic Table:

• Dmitri Ivanovich Mendeleev - "Properties of elements are the periodic functions of their atomic mass." publish Periodic table in 872.

Features:

• arranged 63 elements known during that time - increasing atomic mass
• Similar characteristic elements present in groups and periods
• Left gap for the discovery of new elements
• Late discoveries elements could fit without disturbing others

Drawbacks:

• -Isotopes - on element got different positions

Today's Periodic Table:

• Today's periodic table is based on Henry Moseley who just edited the Mendeleev's periodic table.
• He changed from atomic mass to atomic number.

- Mirage Chemistry