CHEMICAL BONDING. Chemical bond is the force which holds two or more atoms or ions together in a stable molecule.

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1 SN Kansagra School CHEMISTRY CHAPTER - 2 CHEMICAL BONDING STD. X Chemical bond is the force which holds two or more atoms or ions together in a stable molecule. AN ATOM An atom is the smallest unit of matter taking part in a chemical reaction and is built up of subatomic particles- protons, neutrons and electrons. Two or more atoms [metallic or non metallic] combine to form a molecule. The force which holds the atoms together as a stable molecule is a chemical bond. Types of elements- involved in chemical combination. METALS: have 1, 2 or 3 electrons in valence shell. They lose 1, 2 or 3 electrons and become positively charged ions (cations). NON METALS: have 4, 5, 6 or 7 electrons in valence shell. They gain 3, 2 or 1 electrons and become negatively charged ions (anions). Chemical combination: Atoms combine to form a molecule by two means: 1. Transfer of valence electrons form a metallic atom to a non metallic atom. 2. Sharing of valence electrons between two atoms (generally both non metallic). 1

2 LEWIS THEORY OF CHEMICAL BONDING In 1916, Lewis put forth a comprehensive theory of bonding based on the electronic concept of the atom. According to him, bonding of an atom depends chiefly on the electrons in the outermost shell. He observed that the number of electrons in the last orbits of inert gases formed a most stable grouping. Except the inert gases, all other elements have atoms with unstable and incomplete outer shell. They tend to lose or gain electrons so as to acquire an electronic configuration identical with that of the nearest inert gas in the periodic table having 8 electrons in the outermost orbit, except helium which contains 2 electrons. It is this tendency of atoms to complete and thus, stabilise their outermost ring of electrons which causes them to combine chemically. This theory has been applied to explain the three common types of bonds. (1) Electrovalent bond (2) Covalent bond (3) Coordinate bond METHODS FOR ACHIEVING CHEMICAL BONDING A stable electronic configuration in two combining atoms resulting in chemical bonding between them is achieved by: Electron transfer of valence electrons from one atom to another leading to the formation of electrovalent bond and formation of electrovalent compound. Electron sharing of pairs of electrons between two reacting atoms leading to covalent bonding and formation of covalent compound. 2

3 PERIODIC PROPERTIES WHICH AFFECT THE FORMATION OF CHEMICAL BOND For the formation of ionic bond Ionisation potential Electron affinity Electronegativity Lower the value of I.P. of a metallic atom greater the ease of formation of the cation. Higher the value of E.A. of a non metallic atom, greater the ease of formation of the anion. Larger the difference between two combining atoms, electron transfer takes place easily. For the formation of Covalent bond Ionisation potential, Electron affinity and Electronegativity Electronegativity difference High between both the atoms. Should be negligible between two combining atoms. TYPES OF CHEMICAL BONDS 1. ELECTROVALENT BOND Electrovalent or ionic bond: The chemical bond that is formed between two atoms by transfer of one or more electrons from the atom of an electropositive or metallic element to the atom of an electronegative or non metallic element is called an electrovalent or ionic bond. 1. An electrovalent bond is formed by the complete transfer of electrons from one atom to another. 2. Transfer of electrons takes place from the atom of a metallic element to the atom of a nonmetallic element. 3. Due to transfer of electrons, one atom loses electrons while other atom gains electrons, leading to the formation of charged particles called ions. 4. There exists a strong force of attraction between oppositely charged particles, resulting in the formation of an electrovalent bond. 3

4 5. The compounds formed by transfer of electrons from one atom of metallic element to another atom of a non metallic element is called an electrovalent or ionic compound. It is clear that for an ionic bond to form, there must exist an atom that readily loses one or more electrons, and another that readily accepts these electrons. Elements of group 1and 2 of the periodic table have the greatest tendency to lose electrons and get converted into cations. Elements of group 16 and 17 of the periodic table have the greatest tendency to gain electrons and get converted into anions. PROPERTIES OF IONIC COMPOUNDS 1. These compounds are obtained by the transfer of electrons from one atom to another and therefore consists of charged particles. 2. These compounds are generally soluble in water but insoluble in non-polar solvents like benzene and ether. 3. They possess high melting and boiling points. 4. They conduct electricity in molten state and in solution. 5. They are crystalline hard and brittle solids. 6. The reactions of ionic compounds are in fact the reactions of their ions. These reactions take place at a rapid speed. 4

5 FORMATION OF IONIC COMPOUNDS 1. SODIUM CHLORIDE Sodium loses an electron to resemble the configuration of neon [2, 8] While chlorine takes up that electron to resemble the configuration of argon [2, 8, 8]. Sodium forms a cation and chlorine an anion. The oppositely charged ions are held together by strong electrostatic forces resulting in the formation of the electrovalent compound, Sodium chloride. Electron dot diagram: 5

6 2. MAGNESIUM CHLORIDE Magnesium has two valence electrons which it must lose to resemble the configuration of the inert gas, neon. One chlorine atom requires one electron to form a stable octet. Hence, two chlorine atoms, each with a capacity to take up one electron are required to form the compound, magnesium chloride. Electron dot diagram: 6

7 3. CALCIUM OXIDE Calcium needs to lose electrons from its outermost shell in order to resemble argon while oxygen needs two electrons to attain the configuration of neon. Therefore, calcium atom loses two electrons which are taken up by oxygen and a stable, ionic compound, calcium oxide is formed. COVALENT BOND 1. A covalent bond is formed by the mutual sharing of electron pairs between two atoms. 2. The sharing of electron pairs take between atoms of non metallic elements. This is because both of them are deficient in electrons and both have a tendency to gain electrons. Thus, transfer of electrons is not possible and the octet is satisfied by sharing of electrons. 3. Atoms which in their outermost shells have seven, six and five electrons share one, two and three pairs of electrons respectively forming covalent molecules. 7

8 4. The sharing of electrons between atoms results in the formation of non ionised molecules. This is because electrons are not transferred from one atom to another, leading to the absence of ions. 5. The chemical compound, formed as a result of mutual sharing of electrons or electron pairs thereby establishing a covalent bond, is called a covalent or molecular compound. Covalency of an atom is the number of its electrons forming shared pairs with other atoms. Hydrogen molecule Oxygen molecule Covalent bond: The chemical bond that is formed between two combining atoms by mutual sharing of one or more electrons of atoms of non-metallic elements is called a covalent or a molecular bond. FORMATION OF COVALENT MOLECULES: 1. HYDROGEN: Each of two hydrogen atoms contributes one electron so as to have one shared pair of electrons between them. Both atoms attain stable duplet structure, resulting in the formation of a single covalent bond between them. Atomic or orbit structural diagram of hydrogen molecule formation 8

9 Electron dot diagram of hydrogen molecule formation 2. CHLORINE: Each of the two chlorine atoms contributes one electron so as to have one shared pair of electrons between them. Both atoms attain stable octet structure, resulting in the formation of a single covalent bond between them. Atomic or orbit structural diagram of Chlorine molecule formation Electron dot diagram of hydrogen molecule formation 9

10 3. OXYGEN: Each of the two oxygen atoms contributes two electrons so as to have two shared pair of electrons between them. Both atoms attain stable octet structure, resulting in the formation of a double covalent bond between them. Atomic or orbit structural diagram of oxygen molecule formation Electron dot diagram of oxygen molecule formation 10

11 4. NITROGEN: Each of the two nitrogen atoms contributes three electrons so as to have three shared pair of electrons between them. Both atoms attain stable octet structure, resulting in the formation of a triple covalent bond between them. Atomic or orbit structural diagram of Nitrogen molecule formation 5. CARBON TETRACHLORIDE: Electron dot diagram of Nitrogen molecule formation Four atoms of chlorine and one atom of carbon combine to form one molecule of carbon tetrachloride. Carbon has four valence electrons and hence requires four electrons to complete its octet. Chlorine has seven valence electrons and requires only one electron to complete its shell. Therefore, four chlorine atoms share one electron each with carbon. Thus, both, carbon and chlorine, complete their valence shells with eight electrons. 11

12 6. METHANE: Four atoms of hydrogen and one atom of carbon combine to form one molecule of methane. Carbon has four valence electrons and hence requires four electrons to complete its octet. Hydrogen has one valence electron and requires only one electron to complete its shell. Therefore, four hydrogen atoms share one electron each with carbon. Thus, carbon completes its valence shells with eight electrons and hydrogen completes its valence shell with two electrons. POLAR AND NON-POLAR COVALENT COMPOUNDS Non polar covalent compounds Covalent compounds are said to be non polar when shared pair of electrons are equally distributed between the two atoms. No charge separation takes place. The covalent molecule is symmetrical and electrically neutral. Polar covalent compounds Covalent compounds are said to be polar when shared pair of electrons are unequally distributed between the two atoms. Charge separation takes place. The atom which attracts electrons more strongly develops a slight negative charge. e.g. H 2, Cl 2, O 2, N 2, CH 4, CCl 4. e.g. H 2O, NH 3, HCl. 12

13 Coordinate bonds (Dative bonds) Coordinate bond or Dative bond is formed by sharing of two electrons between two atoms where both the electrons of the shared pair are contributed by one atom and another atom merely participates in sharing. This is a special type of covalent bond. The atom which contributes the shared pair of electrons is called the donor, while the other which simply shares the electron pair contributed by donor, is known as acceptor. The donor atom has a complete octet in the valence shell but it still has one or more unshared pairs of valence electrons called lone pairs of electrons. The donor contributes such a pair of electrons for being shared by the acceptor, which is short of two electrons in its valence shell. The bond formed in this way is generally represented by an arrow ( ) pointing from donor atom to the acceptor atom. FORMATION OF HYDRONIUM ION: A water molecule contains an oxygen atom having two lone pairs of electrons linked to two hydrogen atoms through covalent bonds. The hydrogen ion (H + ) in hydronium ion shares one lone pair of electrons of the oxygen atom of water molecule. This results in the formation ofh 3O + ion. Lewis electron-dot representation: FORMATION OF AMMONIUM ION: An Ammonia molecule contains a nitrogen atom having one lone pair of electron linked to three hydrogen atoms through covalent bonds. The hydrogen ion (H + ) in ammonium ion shares one lone pair of electrons of the nitrogen atom of ammonia molecule. This results in the formation ofnh 4 + ion. Lewis electron-dot representation: 13