INTRODUCTION

• In 1913 Henry Moseley showed that atomic number of an element is a more fundamental property than its atomic mass. The modern periodic law can be stated as “properties of elements are a periodic function of their atomic number”.
  
  
• The atomic number gives us the number of protons in the nucleus of an atom and this number increases by one in going from one element to the next. Elements when arranged in order of increasing atomic number Z, lead to the classification of elements known as ‘The modern periodic table”.
  
  
• The discovery of periodic table is an important turning point in the development of chemistry. The elements are arranged in the periodic table in such a way that their chemical and physical properties can be predicted. Mendeleev gave shape to the modern periodic table. In this chapter we can study about electronic configuration of atoms in an element, their respective blocks and the characteristics of elements such as electronegativity, ionization energy and chemical reactivity.