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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 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 elements are based on this idea. The International Union of Pure and Applied Chemistry (IUPAC) approves the names and symbols for the elements. The modern symbol of an element is made up of one or two letters of the English or Latin name of that element. As a rule, the first letter of a symbol is always written as a capital letter and the second as a small letter. The modern representation of atoms is more convenient and meaningful than the classical representation.  To conclude, the symbols of the various elements are significant as: They represent distinct elements. They represent single atoms of the elements.

Modern Representation of Atoms

Table mentioned below shows the modern representation of atoms:







Iron (from Latin: ferrum)




Lead (from Latin: plumbum)














Copper (from Latin: cuprum)


Potassium (from Latin: kalium)






Gold (from Latin: aurum)


Silver (from Latin: argentum)




Sodium (from Latin: natrium)







Did You Know?


The International Union of Pure and Applied Chemistry (IUPAC) is an international non-governmental organisation established in 1919. 

IUPAC nomenclature, in general, is a system of naming chemical compounds and describing the science of chemistry.

Some man-made elements are given temporary three-letter symbols.

Atomic Mass  Every atom has some characteristic mass of its own and this is known as atomic mass. All the atoms of an element have the same atomic mass. Atoms of different elements have different atomic masses. Determination of atomic mass:       It is difficult to determine the mass of an individual atom; so, the mass of an atom is       ascertained in relation to the mass of C-12 atomic mass unit (amu) or unified atomic mass unit (u).


In simple words, Atomic mass is a term which gives the total mass of protons and neutrons in an atom. Also, the atomic mass is measured with respect to mass of 1/12 the mass of one carbon atom.

Atomic Mass 

The atomic masses of some common elements are given in the following table.

Elements Atomic masses (u) Elements Atomic masses (u) Hydrogen 1 Chlorine 35.5 Helium 4 Potassium 39 Carbon 12 Calcium 40 Nitrogen 14 Argon 40 Oxygen 16 Iron 56 Fluorine 19 Copper 63.5 Neon 20 Zinc 65 Sodium 23 Bromine 80 Magnesium 24 Silver 108 Sulphur 32 Gold 197

Atomic Mass Unit

Atomic mass unit (1 u) is defined as exactly one twelfth the mass of an atom of carbon-12. Atomic mass unit is only number and it has no units. On the basis of above unit, the atomic mass of carbon atom is 12 amu.

Relative atomic mass or atomic weight

Definition with respect to hydrogen:

It is the ratio of mass of one atom of an element to the mass of an atom of hydrogen taken as unity.

Definition with respect to carbon:

It is the rat…

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