Ions and Valency PDF

. ION An atom whose outermost orbit (shell) is completely filled with maximum number of electrons that it can accommodate, does not need any more electron(s). Its electronic configuration is complete or stable. The atom having stable electronic configuration can exist independently and the particle...

. ION An atom whose outermost orbit (shell) is completely filled with maximum number of electrons that it can accommodate, does not need any more electron(s). Its electronic configuration is complete or stable. The atom having stable electronic configuration can exist independently and the particle that can exist independently is called molecule (When an atom gains electron(s) from some other atom, it becomes negatively charged (). This is because the number of electrons increases there than the number of protons in the atom. The charged particle (atom or group of atoms) is called ion. The positively charged atom or group of atoms is termed as cation. An atom whose outermost orbit (shell) is not completely filled with maximum number of electrons that it can accommodate has an incomplete or unstable electronic configuration. The atoms with unstable electronic configuration tend to have a completely filled outermost orbit (attain stable electronic configuration). They can do so either by gaining electron(s) or by losing electron(s). When an atom loses electron(s), it becomes positively charged (). This is because the number of electrons decreases there than the number of protons in the atom. VALENCY Atoms except noble gases have unstable electronic configuration and they cannot exist independently. In order to get their electronic configuration stable, they tend to gain electron(s) or lose electron(s). For this tendency, they combine with other atoms and form molecules or formula units which can exist independently. The combining capacity of atoms with other atoms is called valency. Hydrogen atom can donate one electron or gains one electron to get its electronic configuration stable. As It can donate one electron, it has one unit capacity to be combined with another atom. Hence, its valecy is one (1). Valency of an element is also defined as the number of hydrogen atoms that can combine or displace with one atom of that element. 72 3.2.1 Chemical Formula A symbolic representation of one molecule of an element or a compound is called chemical formula. The chemical formula of a substance shows the number and kinds of atoms present in its one molecule or formula unit. Symbols show the kinds of atoms and subscript numbers show the number of atoms present. For example H is the chemical formula of 2 hydrogen. It shows that each molecule of hydrogen gas contains two hydrogen atoms. Similarly H O is the chemical formula of water. It 2 shows that each molecule of water contains two atoms of hydrogen and one atom of oxygen chemically combined together. For example; valency of chlorine is 1, as it combines with one hydrogen atom to form hydrogen chloride (HCl). Valency of oxygen is 2, as it combines with two hydrogen atoms to form water (H O). 2 Valency of nitrogen is 3, as it combines three hydrogen atoms to form ammonia (NH ). Similarly, 3 valency of carbon is 4, as it combines with four hydrogen atoms to form methane (CH ). 4 Valency of an ion or radical is equal to the magnitude of charge on it. For example, valency of sodium + -- 2+ ion(Na ) is 1+, valency of chloride ion (Cl ) is 1--. Valency of calcium ion (Ca ) is 2+ and valency of oxide ion 2-- (O ) is 2--. H 2 H2O Symbol Subscription How to write a formula For ionic compounds oppositely charged ions give shape to the formula. Following is the procedure for writing a chemical formula. 1. Write the symbols of oppositely charged ions side by side, the positive ion to the left and negative ion to the right. 2. Cross the valencies to the lower right of each ion. If valencies are equal, don\'t cross, as they cancel each other. Example: Write down the formulae of sodium sulphate and aluminium chloride. Step 1: According to rule 1 write down the positive ion on the left hand side and negative ion on the right hand side. + Na --2 SO4 3+ Al -- Cl 73 Step 2: Cross the valencies to set the formula. Hence the formulae of sodium sulphate and aluminium chloride are: + Na --2 SO4 3+ Al -- Cl Na₂SO₄ Sodium sulphate AlCl3 Aluminium chloride 3 CHEMICAL BOND Atoms attain stability by having 2 or 8 electrons in the outermost shell like noble gases. For this tendency atoms combine with other atoms forming chemical bonds. A chemical bond is a force of attraction between atoms that holds them together in a substance. Chemical bonds are formed in three ways: 1. By donating valence shell electrons to other atoms. 2. By gaining electrons from the valence shell of other atoms. 3. By mutual sharing of valence shell electrons between the atoms. Types of chemical bond Chemical bonds are of two types: Ÿ Ionic bond Ÿ Covalent bond 6.3.1 ionic Bond A chemical bond formed by complete transfer of electron from one atom to another atom is called ionic bond (Figure 6.4). This bond is formed when one atom loses electron from its outermost shell and the other atom gains this electron in its outermost shell. The atom losing electrons forms a positively charged ion (cation) and the atom gaining electron forms a negatively charged ion (anion). + Na -- Cl + Na Cl Sodium atom Chlorine atom Sodium chloride Figure 6.4 Ionic bond between sodium and chloride ions Na + Na + -- 1e Sodium atom Sodium ion Electron Cl -- + Cl -- e Chlorine atom Electron Chloride ion + Na + NaCl -- Cl Sodium ion Chloride ion Sodium chloride Ionic bonds are formed between metallic and non-metallic atoms. This is because, metals have a tendency to lose their outermost shell electrons to form positively charged ions (cations). Whereas, non-metals have a tendency to gain electrons in their outermost shell to form negatively charged ions (anions). Cations and anions are oppositely charged ions having electrostatic force of attraction between them making/forming ionic bond 2 Covalent Bond A chemical bond formed by mutual sharing of electrons between bonded atoms is called covalent bond. Covalent bonds are of three types: 1. Single covalent bond 2. Double covalent bond 3. Triple covalent bond 1. Single covalent bond A covalent bond formed by mutual sharing of one electron pair between bonded atoms is called single covalent bond. It is denoted by one dot pair or cross pair or dot and cross pair or one short line between the bonded atoms. Example: Hydrogen atom has one electron in its outermost shell. Two hydrogen atoms combine through a single covalent bond in which each atom equally contributes one electron in a mutually shared electron pair (Figure 6.5). 75 Other examples of single covalent bond are Cl , HCl, H O, etc. 2 2 2. Double covalent bond A covalent bond formed by mutual sharing of two electron pairs between bonded atoms is called double covalent bond. It is denoted by two dot pairs or cross pairs or dot and cross pairs or two short lines between the bonded atoms. Example: Oxygen atom has six electrons in its outermost shell. Two oxygen atoms combine through a double covalent bond in which each atom equally contributes two electrons forming two mutually shared electron pairs (Figure 6.6). H H H H or H -- H Figure 6.5 Single covalent bond + Other example of double covalent bond is CO. 2 3. Triple covalent bond A covalent bond formed by mutual sharing of three electron pairs between bonded atoms is called triple covalent bond. It is denoted by three dot pairs or cross pairs or dot and cross pairs or three short lines between the bonded atoms. Example: Nitrogen atom has five electrons in its outermost shell. Two nitrogen atoms combine through a Figure 6.6 Double covalent bond O + O O O or O = O 76 triple covalent bond in which each atom equally contributes three electrons forming three mutually shared electron pairs (Figure 6.7). + 2 Nitrogen atoms N (2, 5) N (2, 5) N N or N ≡ N (2, 8) (2, 8) The charged particle (atom or group of atoms) is called ion. The positively charged atom or group of atoms is termed as cation. The negatively charged atom or group of atoms is termed as anion. When an atom or group of atoms gains or loses electrons, it acquires charge. The charged particle is called ion. The combining capacity of an element is called valency. A symbolic representation of one molecule of an element or a compound is called its formula. A chemical bond is a force of attraction between atoms that holds them together in a substance. A chemical bond formed by complete transfer of electron from one atom to another atom is called ionic bond. A covalent bond formed by mutual sharing of one electron pair between bonded atoms is called single covalent bond. A covalent bond formed by mutual sharing of two electron pairs between bonded atoms is called double covalent bond. A covalent bond formed by mutual sharing of three electron pairs between bonded atoms is called triple covalent bond.