• In the formation of ionic bond the atoms of electro positive elements lose their valence electrons to atoms of electro negative elements so that both of them can attain octet in their valence shells.
• The electrostatic attractive force that keeps cation and anion together to form a new electrically neutral entity is called an ionic bond.
• Ionic compounds are often crystalline solids with high melting points.
• A chemical bond that formed by sharing of valence-shell electrons between the atoms so that both of them can attain octet or duplet in their valence shells is called covalent bond.
• A single covalent bond is formed when two atoms share a pair of electrons
• Each shared pair of electrons is equivalent to one covalent bond.
• Electrons are not always shared equally between the atoms in a covalent bond. This leads to bond polarity
• To explain the bond angles in the molecules through covalent bonds the valence – shell – electron – pair repulsion theory (VSEPRT) is proposed.
• To explain the strengths of covalent bonds valence band theory is discussed.

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Reflections on concepts

1. List the factors that determine the type of bond that will be formed between two atoms?
2. Explain the difference between the valence electrons and the co-valency of an element.
3. A chemical compound has the following Lewis notation:
   a) How many valence electrons does element Y have?
   b) What is the valency of element Y?
   c) What is the valency of element X?
   d) How many covalent bonds are there in the molecule?
   e) To which groups the elements X and Y belong?
4. Why do only valence electrons involve in bond formation? Why not electron of inner shells?Explain.
5. How bond energies and bond lengths of molecule help us in predicting their chemical properties? Explain with examples.
6. Draw simple diagrams to show how electrons are arranged in the following covalent molecules:
   a) Calcium oxide
   (b) Water
   (c) Chlorine
7. Represent the molecule H2 O using Lewis notation.
8. Represent each of the following atoms using Lewis notation:
   (a) beryllium
   (b) calcium
   (c) lithium
   (d) bromine gas
   (e) calcium chloride
   (f) carbon dioxide
9. What is octet rule? How do you appreciate role of the ‘octet rule’ in explaining the chemicalproperties of elements?
10. What is hybridisation? Explain the formation of the following molecules using hybridisation
    a) Be Cl ₂ b) BF₃

apge no:174
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