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chemical bonding

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Chemical Bonding 1. Electronic configuration : The arrangement of electrons around the nucleus of an element in various energy levels is called electronic configuration. 2. Electronic configuration of the noble gases : With the exception of helium, all noble gases have eight electrons in their valence shells. Helium has only 2 electrons in its valence shell. 3. Chemical inactivity of the noble gases : The atoms of all noble gases are in minimum state of energy. They have an octet structure, i.e., 8 electrons in the valence shell. Exception is helium, which has a duplet structure, i.e., it has only two electrons in its only one shell, which is also its valence shell. 4. Formation of ions : All atoms of the elements have a tendency to attain the minimum state of energy, i.e., to attain an octet or duplet structure. This is achieved either by donating electrons from their valence shell or accepting electrons in the valence shell, such that the participating atoms acquire the electronic structure of the nearest noble gas. However, due to the donation (loss) of electrons the residual atom gets positively charged and is called cation. Conversely, due to the acceptance of electrons, the residual atom gets negatively charged and is called anion. 5. Ionic compound : The compound formed by an actual exchange of electron/electrons from the valence shell of metals to the valence shell of non-metals is called an ionic compound. 6. Ionic combination or electrovalent combination : When the participating metal atoms actually donate electrons from their valence shells, such that these electron/electrons are actually accepted by the atoms of non-metals, an ionic combination or electrovalent combination takes place. 1 Key Points 7. Ionic bond : The electrostatic bond formed between the two atoms (usually a metal and a non-metal), such that metal atom donates all its electrons from the valence shell and the non-metal atom/atoms accepts these donated electrons, with the formation of cations and anions is called an ionic bond. 8. When is ionic bond formed? (i) Higher difference in the values of electronegativity between the reacting atoms lead to the formation of ionic bond. (ii) Higher difference in the values of ionisation potential between the reacting atoms leads to the formation of ionic bond. (iii) When the participating atoms in a chemical reaction are metals and non-metals, an ionic bond is formed. 9. Strength of ionic bond : The magnitude of electrostatic force of attraction between the positively charged ions and the negatively charged ions in an ionic compound is called strength of ionic bonds. 10. Factors controlling strength of ionic bond : 1. More the difference in the electronegativity of participating atoms, more is the strength of ionic bond. 2. More the difference in the atomic radii of participating atoms, more is the strength of ionic bond. 11. Characteristics of ionic compounds. (i) They are generally crystalline solids. (ii) They have a high melting point and boiling point. (iii) They are generally soluble in water. (iv) Their density is generally more than the density of water. (v) In fused state or aqueous solution, they are good conductors of electricity. 12. Covalent bond : A bond formed between two non-metallic elements by sharing electron pair/pairs from the valence shell of reacting atoms, such that both the atoms acquire the electronic configuration of the nearest noble gas is called a covalent bond. 2 Key Points 13. When is a covalent bond formed? (i) If the participating atoms are non-metals, a covalent bond is formed. (ii) Low difference in the values of electronegativity between the reacting atoms, leads to the formation of covalent bond. (iii) Low difference in the values of ionisation potential between the reacting atoms, leads to the formation of covalent bonds. 14. Non-polar covalent compound : A covalent compound in which shared pairs of electrons are equally distributed between two atoms is called a non-polar covalent bond. For example, methane (CH4) and silicon hydride (SiH4) are non-polar covalent compounds. 15. Polar covalent compounds : A covalent compound in which shared pair of electrons are unequally distributed between the atoms is called a polar covalent compound. For example, HCl, HF, HBr are the examples of polar covalent compounds. 16. Characteristics of covalent compounds : (i) They are generally gaseous or volatile liquids or soft solids. (ii) They have a low boiling point and melting point. (iii) They are generally insoluble in water, but soluble in organic compounds. (iv) They have density which is generally less than that of water. (v) They are bad conductors of electricity. 17. When is a non-polar covalent bond formed? A non-polar covalent bond is formed (i) Between similar atoms (ii) Dissimilar atoms, so that the molecule of compound is symmetrical and electrically neutral. 18. When is polar covalent compound formed? A polar covalent compound is formed between (i) Dissimilar atoms having different electronegativity (ii) Dissimilar atoms having different atomic radii, such that their molecule is asymmetrical and electrically charged. 3 Key Points 19. Van der Waals forces : Very weak forces existing between the molecules of non-polar covalent compounds are called van der waals forces. 20. Intermolecular forces : Weak electrostatic forces between the molecules of polar covalent compounds are intermolecular forces. 21. Lone pair of electrons : The unshared pair/pairs of electrons around an atom in the middle of a molecule of a polar covalent compound is called lone pair/pairs of electrons. For example, nitrogen atom in case of the molecule of ammonia has one lone pair of electrons. Similarly, oxygen atom in case of the molecule of water has two lone pairs of electrons. 22. Coordinate covalent bond and coordinate valency : A bond formed between an ion and an atom of a polar covalent compound, having a lone pair/pairs of electrons, such that the ions accept the lone pair is called coordinate covalent bond and the valency is called coordinate valency. For example, in the formation of a lone pair of electron of nitrogen in ammonia molecule is accepted by the hydrogen ion. 23. Oxidation : When an atom or an ion loses electron/electrons, the oxidation takes place. Examples : Na e Na+ [oxidation of sodium atom takes place] Fe2+ e Fe3+ [oxidation of Fe2+ ion takes place] 24. Reduction : When an atom or an ion gains electron/electrons, the reduction takes place. Examples : Cl + e Cl [reduction of chlorine atom takes place] Cu2+ + 2e Cu [reduction of Cu2+ ion takes place] 25. Oxidising agent : The atom or the ion which gains an electron/electrons is an oxidising agent. Examples : S + 2e S2 [sulphur atom is an oxidising agent] Fe3+ + e Fe2+ [Fe3+ ion is an oxidising agent] 4 Key Points 26. Reducing agent : The atom or the ion which loses an electron/electrons is a reducing agent. Examples : Al 3e Al3+ [aluminium atom is a reducing agent] Fe2+ e Fe3+ [Fe2+ ion is a reducing agent] 27. Redox Reaction : A chemical reaction in which loss of electrons and the gain of electrons, takes place simultaneously is called redox reaction. 5 Key Points

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