You have learnt about electron configuration of elements and periodic table in previous lessons. It was learnt that there are over 115 elements.

• How do they usually exist?

• Do they exist as a single atom or as a group of atoms?

In Class IX, we saw that several elements like oxygen, nitrogen and hydrogen exist as diatomic molecules. What is the force that is holding these constituent atoms together in molecules?

• Are there elements which exist as atoms?

• Why do some elements exist as molecules and some as atoms?

In lower classes you have also learnt about the different laws of chemical combinations. Since the formation of chemical compounds takes place as a result of combination of atoms of various elements in different ways, it raises many questions.

• Why do some elements and compounds react vigorously while others are inert?

• Why is the chemical formula for water H₂ O and for sodium chloride NaCl, why not HO₂and NaCl₂?

• Why do some atoms combine while others do not? We will try to answer such questions in this chapter.

• Are elements and compounds simply made up of separate atoms individually arranged?

• Is there any attraction between atoms?

Let us take the example of common salt, NaCl. When you shake salt from a shaker does it separate into sodium and chlorine? No! This shows that the sodium and chlorine atoms are being held together.

• What is that holding them together?