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Atoms and Molecules

Introduction to the concepts of Atoms and Molecules

Atoms: An Overview 

When we talk about atoms, two questions usually strike our mind... 

Let us go through this lesson to find the answers to these questions. We will also learn how to represent different atoms in symbolic forms. So, in short, we are going to study:

  • Size of an atom
  • Representation of atoms
  • Atomic mass

Size of an Atom 

  • The size of an isolated atom cannot be measured; however, we can estimate its size by assuming that its radius is half the distance between adjacent atoms in a solid.
  • Atoms are very small in size. They are so small that it is not possible to see them even under a powerful optical microscope.
  • The size of an atom is indicated by its radius, called the atomic radius .
  • Since an atom is very small, we need a very small unit for reporting the atomic radius; thus, the radius of an atom is often expressed in nanometre .




Surfaces of Silicon Atoms

Atoms cannot be seen with the naked eye, but the use of modern techniques has enabled us to see the surfaces of atoms. The magnified image of the surfaces of silicon atoms is shown in the following figure.


Size of an Atom 

Hydrogen atom is the smallest of all atoms. The given figure shows the atomic radii of some elements.

Classical Representation of Atoms

  • A large number of elements are known to us today. It would be cumbersome to refer to them by their names all the time in our studies. For the sake of convenience, we need symbols that represent these elements. Toward the end of the nineteenth century, scientists felt this need to assign standard characteristic symbols to the elements. 
  • John Dalton was the first scientist to use symbols to represent different elements. Dalton’s proposed symbols for some elements are shown in given figure.

  • Each symbol proposed by Dalton represents an atom of the respective element. For example, if someone wanted to represent two hydrogen atoms, then he would have to draw the symbol of hydrogen atom twice as shown.

Modern Representation of Atoms

  • Many of the symbols proposed by Dalton were difficult to draw and remember. Therefore, an alternative method of representing elements was required.
  • Another scientist named Jӧns Jacob Berzelius suggested that letters of the alphabet can be used as symbols to represent the elements. The modern symbols of el…

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