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ELEMENTRY IDEA OF BONDING, , 2, CHAPTER, , CONTENTS,  INTRODUCTION,  BONDS,  COVALENT BOND,  BONDING IN METALS, INTRODUCTION, We know that different elements have different, atomic numbers and electronic configurations., The properties of atoms depend upon their, electronic configurations. Some atoms are more, reactive than others. Noble gas (He, Ne, Ar, Kr,, Xe and Rn) atoms are not reactive at all; they are, inert and stable. Then the question arises why, noble gases do not react to form compounds,, while other elements do so? This can be answered, by comparing the electronic configurations of, noble gases with those of other elements. Also, it, is important to understand how and why atoms, react to form molecules and compounds. Atoms, gain electrons in their outermost shells or lose, them from their outermost shells, or share, electrons with other atoms in such a way that their, outermost shells become filled to capacity. They, can do this by reacting with other atoms. As long, as the outermost shell can accommodate more, electrons, i.e., it is not full, an atom tends to, combine with other atoms in order to fill its, outermost shell. When the outermost shell is filled, to capacity, the atom becomes stable., The atoms of all other elements (elements, other than the noble gases) have in their outermost, shells less than 8 electrons, i.e., their outermost, shells are not filled to capacity. Therefore, the, atoms of these elements combine with other, atoms to achieve stable configurations like those, of the noble gases. It is the tendency on the part of, an atom to achieve a stable configuration (like, , that of the noble gases) which is responsible for, its chemical reactivity., , BONDS, We know that an atom tends to attain stability by, acquiring the electronic configuration of its, nearest noble gas. This can be achieved in anyone, of the following manners during chemical, combination:, 1. By the transfer of electron(s) from one atom, to another, 2. By the sharing of valence electrons between, the two combining atoms, There must be some kind of force which binds, the atoms together in a molecule. The attractive, force which holds together two atoms, two, molecules, two ions or a combination of these is, known as a chemical bond., The two modes of attaining the electronic, configuration of the nearest noble gas give, rise to two types of bonds-the electrovalent, bond and the covalent bond., ,  The Electrovalent Bond :, In this type of bond, valence electrons are, transferred from one atom to another. One atom, donates its excess electrons to another atom so, that both the atoms may acquire a stable noble gas, configuration. The atom which loses electron, becomes positively charged and is called the, cation. The atom which takes up the electron lost, by the first atom becomes negatively charged and, is called the anion. These two oppositely charged, ions are now held together by an electrostatic, force of attraction. This force of attraction binding, the two atoms together is known as an, electrovalent or ionic bond., Thus, the chemical bond formed between two, atoms by the transfer of one or more valence, electrons from one atom to the other is known as, an electrovalent or ionic bond. It is also called a, polar bond.

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EXAMPLES- : Combination of sodium (Na) and, chlorine (Cl) atoms to form sodium chloride, (NaCl), The atomic number of sodium is 11. So its, electronic configuration is 2, 8, 1. It has only one, electron in its outermost shell. The Na atom, transfers this electron and becomes positively, charged sodium ion (Na+)., 1e, , Na ,  [ Na ], , 2, 8,1, , 2, 8, , Thus, the electronic configuration of the Na+ ion, is the same as that of neon which is the noble gas, nearest to sodium in the periodic table., Let us consider the chlorine atom (Cl). The, atomic number of chlorine is 17. So its electronic, configuration is 2, 8, 7. It has 7 electrons in its, outermost shell. It, thus, lacks 1 electron to, acquire a stable noble gas configuration. So a, chlorine atom takes 1 electron transferred by the, sodium atom and becomes negatively charged, chloride ion (Cl–)., Cl +, , le  [ Cl ], , –, , (2, 8, 8), , (2, 8, 7), , Thus, the chloride ion (Cl–) attains the, configuration of the nearest noble gas, argon., [Valence electrons are shown by dots around the, symbol.], The two ions (Na+ and Cl–) being oppositely, charged, are now held together by electrostatic, force of attraction as Na+Cl–., , Na +, , (2, 8, 1), , Na +, , Cl  Na+ Cl–, , (2, 8, 7), , (2, 8), , (2, 8, 8), , Cl  Na+[ Cl ]– or Na+Cl–, (2, 8, 8), , The formation of sodium chloride can be shown, diagrammatically as in figure., , Sodium atoms, (Na), , Na+, , +, , Cl, , Chlorine atoms, (Cl), , +, , Note(a) In the formula of an ionic compound, the, positive ion is written first,, (b) Charges on the ions of an ionic compound are, usually not shown with the formula. So,, sodium chloride is usually expressed as NaCl,, not as Na+Cl–., Compound, , Formula, , Ions involved, , Sodium chloride, , NaCl, , Na+ and Cl–, , Magnesium chloride, , MgCl2, , Mg2+ and Cl–, , Magnesium oxide, , MgO, , Mg2+ and O2–, , Calcium chloride, , CaCl2, , Ca2+ and Cl–, , Calcium oxide, , CaO, , Ca2+ and O2–, , Ammonium chloride, , NH4Cl, , NH 4 and Cl-, , Barium chloride, , BaCl2, , Ba2+ and Cr, , Potassium nitrate, , KNO3, , K+ and NO3, , (NH4)2SO4, , NH 4 and SO 24, , Cupric sulphate, , CuSO4, , Cu2+ and SO 24, , Cupric chloride, , CuCl2, , Cu2+ and Cl–, , Ammonium sulphate, ,  Electrovalency :, When an element forms electrovalent bond, its, valency is known as electrovalency., , Electron, , Na, , The force that holds Na+ and Cl– ions together is, called an electrovalent bond. As this bond exists, between ions, it is also called an ionic bond. An, electrovalent bond is polar, i.e., the positive and, negative charges are separated. Compounds, containing such bonds are called' electrovalent, or, ionic, or polar compounds., , Cl–, , (Chloride ion), (Sodium ion), Sodium Chloride, (Na+Cl–), , The number of electrovalent or ionic bonds an, atom can form is called its electrovalency. The, electrovalency of an element is, therefore, equal, to the number of electrons lost or gained by the, atom to form an ion., Elements which lose electrons show positive, electrovalency and those which gain electrons, show negative electrovalency. For example, in the, formation of sodium chloride (Na+Cl–), the, electrovalency of sodium (Na) is +1, while that of, chlorine (Cl) is – l., Elements which lose or gain one, two, three, ... ,, etc., electrons are said to be monovalent (or

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univalent), divalent (or bivalent), trivalent, ... ,, etc., respectively., Monovalent elements : Na, CI, F, Trivalent elements : Al, B, of, , electrovalent, ,  Single covalent bond :,  A single covalent bond is formed when one pair, of electrons is shared between two atoms., , Divalent elements : Mg, Ca, Ba, O, ,  Characteristics, compounds :, , 3. Triple covalent bond, , EXAMPLES :, or, , ionic, , 1. Electrovalent compounds are made up of, positively and negatively charged ions. For, example, sodium chloride (NaCl) is made up, of Na+ and Cl– ions arranged in a definite, order in three dimensions to form crystals., 2. Electrovalent compounds have high, melting and boiling points. This is due to, the presence of strong electrostatic forces of, attraction between the positive and negative, ions. A large amount of heat energy is, required to break this force of attraction., Hence, the melting and boiling points of, electrovalent compounds are high., 3. Electrovalent compounds are usually, soluble in water but insoluble in organic, solvents such as benzene, acetone, carbon, disulphide and carbon tetrachloride., 4. Electrovalent compounds conduct electricity, in molten state and in their aqueous solutions., In solid electrovalent compounds the ions are, held together in fixed positions and cannot move., Hence, such compounds in the solid state do not, conduct electricity., When an electrovalent compound is dissolved, in water or is melted, the crystal structure breaks, down. The ions now become free to move and, can, therefore, conduct electricity., That the ionic compounds in molten state or, in solution become conductors of electricity., , THE COVALENT BOND, The chemical bond formed when two atoms share, electrons between them is known as a covalent, bond., The sharing of electrons between the two atoms, takes place in such a way that both the atoms, acquire the stable electronic configurations of, their nearest noble gases., There are three types of covalent bonds :, 1. Single covalent bond, 2. Double covalent bond, , 1. Formation of a hydrogen molecule (H2) :, A molecule of hydrogen consists of two, hydrogen atoms. Each hydrogen atom has one, electron. When two atoms of hydrogen, combine, one electron of each takes part in, sharing. Thus, two electrons (one pair of, electrons) are shared between the two atoms., H +, , H, ,  H:H, , The shared electron pair always exists, between the two atoms. The two dots between, the two H atoms represent the pair of shared, electrons. One pair of shared electrons gives a, single bond. Such a bond is represented by a, short line between the two atoms. Thus, a, hydrogen molecule may be represented as in, figure., H : H or H – H, , H : H, , Once the bond is formed, the both atoms have, a stable configuration of the noble gas, helium., 2. Formation of a methane molecule (CH4) :, A carbon atom has four electrons in its, outermost shell (valence shell). It shares its, valence electrons with those of four H atoms., Thus, an atom of carbon forms four single, covalent bonds with four H atoms., , H, |, C + 4 H  H C H or H—C—H, |, H, H, Pictorially, a methane molecule may be, represented as in Figure., H, , H, H C H, H, ,  Double covalent bond :

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A double covalent bond is formed when two pairs, of electrons are shared between the two, combining atoms. A sharing of two pairs of, electrons is shown by marking two short lines, between the symbols of the two atoms., EXAMPLES :, 1. Formation of an oxygen molecule (O2) :, An atom of oxygen contains six electrons in, its valence shell. It requires two more, electrons to attain a stable eight-electron, configuration (octet). This is achieved when, each of the two oxygen atoms shares its two, electrons with the other, resulting in the, formation of a stable oxygen molecule., , +, , O, , , , Pictorially, the oxygen molecule may be, represented as in figure., O O, , 2. Formation of an ethylene molecule (C2H4) :, In the formation of an ethylene molecule, (C2H4), each of the two C atoms combines, with two H atoms to form two single covalent, bonds. The remaining two electrons of each C, atom form a double bond between the two C, atoms., H, , H, C  C, , H, , An atom of nitrogen has five electrons in its, valence shell. It requires three more electrons, to attain the stable octet. This is achieved, when two nitrogen atoms combine together, by sharing three electrons each to form a, nitrogen molecule., Triple bond, , N + N,  N N or N  N, Pictorially, a nitrogen molecule can be, represented as in figure., , 2. Formation of an acetylene molecule (C2H2) :, , O O or O  O, , H, H, 2 C + 4 H  C C or, H, H, , 1. Formation of a nitrogen molecule (N2) :, , N N, , Double bond, , O, , EXAMPLES :, , H, , (Ethylene molecule), , Pictorially, a molecule of ethylene (C2H4), may be represented a in figure., , H, H, C C, H, H, ,  Triple covalent bond :, A triple covalent bond is formed when three pairs, of electrons (six electrons) are shared between the, two combining atoms. A triple bond is shown by, marking three short lines between the two, symbols of the atoms., , In an acetylene molecule, two C atoms, combine with two H atoms. Each C atom, shares three of its valence electrons with the, other C atom. One electron of each C atom is, shared with one electron of a H atom., Thus, in a molecule of acetylene, there is a, triple covalent bond between the two C atoms, and each C atom is joined to one H atom by a, single covalent bond. Pictorially, a molecule, of acetylene may be represented as in figure., H C C H, ,  Characteristics of covalent compounds :, 1. Covalent compounds are made up of neutral, molecules. Hence, the forces of attraction, between the molecules are weaker than those, found in ionic compounds. Therefore,, covalent compounds are usually volatile, liquids or gases., 2. The melting and the boiling points of, covalent compounds are generally low., Since covalent compounds are made up of, neutral molecules, the forces of attraction, between the molecules are very weak. So, a, comparatively small amount of heat energy is, required to break these weak intermolecular, forces of attraction. Hence, they have low, melting and boiling points., 3. Covalent compounds are insoluble in water, but soluble in organic solvents., 4. Covalent compounds do not conduct, electricity. This is because they are made up

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of neutral molecules, not ions, and do not, produce ions in the molten state or in aqueous, solutions., , BONDING IN METALS, As you know, metals are hard solids and they are, made up of atoms. It has been established that the, atoms in a metal are very closely packed together., The force that holds the atoms closely together in, a metal is known as the metallic bond., Metal atoms lose one, two or three electrons to, form positively charged ions, called cations., The electrons thus lost move freely in the metal,, i.e., these electrons become mobile, but the, cations do not leave their positions. So in a metal, lattice it is assumed that the metal ions are, immersed in a sea of electrons. Due to the, presence of mobile electrons, metals are good, conductors of heat and electricity., , POINT TO REMEMBER, A chemical bond is the force of attraction which, holds together the atoms in a molecule., An electrovalent bond is formed as a result of, complete transfer of electron(s) from one atom to, another.,  Atoms which can lose electrons and form positive, ions are said to be electropositive., Atoms which can gain electrons and form, negative ions are said to be electronegative., The total number of electrons lost or gained by an, atom to attain the stable configuration of the, nearest noble gas is known as the valency of the, atom., The number of electrovalent bonds an atom can, form is called its electrovalency., Covalent bonds are formed by the mutual sharing, of electrons between the combining atoms., Compounds containing covalent bonds are called, covalent compounds., The. total number of electrons shared by a given, atom in the formation of covalent bonds to attain, the stable configuration of its nearest noble gas is, known as its covalency., The force that holds the atoms closely together in, a metal is known as the metallic bond.

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Exercise-I, A., , Very Short Answer Type Questions, , Q.1, , What is the nature of forces existing between, ions in ionic bonds ?, , Q.2, , How many covalent bonds are there in a, molecule of nitrogen ?, , Q.3, , What are the two main types of chemical, bonds ?, , Q.4, , Give an example of a molecule with a double, bond., , Q.5, , Give an example of a molecule with a triple, bond., , Q.6, , The atomic number of sodium is 11. What is, its electrovalency ?, , Q.7, , An atom has configuration 2, 6. What will be, its covalency ?, , Q.8, , Element X has 10 protons and 10 electrons., Will it be reactive ?, , Q.9, , Which of the elements would be most stable?, 9A, 10B, 11C, , Q.10, , Three elements X, Y, and Z have the, following configurations :, (i) X = 2, 8, 1, (ii) Y = 2, 7, (iii) Z = 2, 8, 2, What type of molecule will form between the, following ?, (i) X and Y (ii) Z and Y, (iii) Y and Y, , Q.11, , An element X has the configuration . Another, element Y has the configuration . What type, of bond will be formed between X and Y ?, , Q.12, , How many covalent bonds are present in a, molecule of methane ?, , Q.13, , Give one example each of a covalent, compound containing (i) three single bonds, (ii) one double bond (iii) one triple bond, , Q.14, , Which type of bond is present in the, following molecules ?, (i) F2 (ii) O2, (iii) N2, , Q.15, , Why do covalent compounds generally not, dissolve in water ?, , B., , Short Answer Type Questions, , Q.16, Q.17, Q.18, , Q.19, Q.20, Q.21, Q.22, , Q.23, , Distinguish between ionic and covalent, compounds., Two argon atoms do not form a covalent, bond to give an argon molecule Ar2. Why ?, The elements X and Y have the following, configurations :, X, 2, 6, Y, 2, 8, 8, 2, What is the nature of the bond between X and, Y?, Distinguish between an atom and an ion., What is the role of valence electrons in the, formation of a chemical compound ?, Define electrovalency., Describe the nature of bond between the, following, (i) sodium and bromine, (ii) carbon and chlorine, (iii) hydrogen and chlorine, Draw the electron-dot structures of MgF2,, CaO, H2O and CO2., , Q.24(i) How many single bonds are there in acetylene, molecules ?, (ii) How many double bonds are there in, acetylene molecule ?, (iii) How many triple bonds are there in acetylene, molecule ?, Q.25 From the list of compounds– I2, N2, CO2,, C2H2 and O2 choose the molecule that, contains., (i) only a triple bond, (ii) only a single bond, (iii) only a double bond, (iv) two double bonds, (v) single and triple bonds

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Exercise-II, A., , Long Answer Type Questions, , Q.15, , The chloride ion has a ......... charge., , Q.1, , What is the octet rule ? Explain it with the, help of suitable examples., , Q.16, , Na+ and Cl– ions combine together to form an, ......... solid., , Q.2, , Write the electronic configuration of the, noble gases., , Q.17, , The size of the sodium ion is .......... than that, of the sodium atom., , Q.3, , Explain the formation of calcium sulphide, starting from calcium and sulphur. Draw the, diagrammatic representation of the atom, showing the electronic arrangement in, various shells., , Q.18, , Na+ ion has the same configuration as that of, ..............., , Q.19, , F– and Ne contain the same number of ...., , Q.20, , Mg2+ is a ............... cation., , Q.21, , HCl is an example of ................ compound., , Q.4, , Define cations and anions. Discuss with, suitable examples., , Q.5, , What is an electrovalent bond and how is it, formed ?, , Q.6, , Show the formation of covalent bonds in HCl,, CCl4, CH4, H2, O2 and Cl2, , Q.7, , What is the difference between ionic and, covalent compounds ?, , Q.8, , State the properties of electrovalent and, covalent compounds., , Q.9, , How will you find out which of water-soluble, compounds A and B is electrovalent ? Select, one electrovalent and one covalent compound, from the following :, canesugar, urea, calcium oxide, sodium, sulphide, hydrogen chloride gas and calcium, chloride., , Q.10, , B., , State, the, valencies, and, electronic, configurations of the elements of atomic, number 6, 7 and 8. What type of valency do, they have and why ?, , Fill in the blanks, , Q.11, , The number of electrons in the valence shell, of noble gases is ........... except helium which, has .............., , Q.12, , Noble gases exist as individual ..........., , Q.13, , The valency of Cl is 1 because it contains, ....... electron less than the stable neon gas, configuration., , Q.14, , The chlorine atom can ............... one electron, to become a chloride ion., , C., , True /False Type Questions, , Q.22, , CCl4 is a good conductor of electricity., , Q.23, , The number of valence electrons in all noble, gases except helium is 8., , Q.24, , Hydrogen tends to achieve stable duplet, arrangement., , Q.25, , Mg2+and O2– have achieved stable octet, arrangement., , Q.26, , Water contains one single covalent bond., , Q.27, , The size of Na+ is smaller than the size of Na., , Q.28, , Calcium oxide is a covalent compound., , Q.29, , A solution of magnesium chloride conducts, electricity.