Chemical Bonds PDF

Inorganic Pharmacy – I BPM-111 Chemical Bonds Mst. Jannatul Mewa Lecturer, Department of Pharmacy Manarat International University What is Inorganic Chemistry? Inorganic chemistry is the subcategory of chemistry that studies the properties and reactions of inorganic eleme...

Inorganic Pharmacy – I BPM-111 Chemical Bonds Mst. Jannatul Mewa Lecturer, Department of Pharmacy Manarat International University What is Inorganic Chemistry? Inorganic chemistry is the subcategory of chemistry that studies the properties and reactions of inorganic elements and compounds that exist naturally in the earth and do not contain a carbon-hydrogen bond. Chemical Bond: Chemical bonds are forces that holds atom together to make compounds or molecules. It involves attractions between atoms and this attraction may be seen as the result of different behaviors of the outermost or valence electrons of atoms. For example ionic bond, covalent bond, hydrogen bond. Types of chemical bonding: Ionic bond: Ionic bond, also called electrovalent bond that involves the electrostatic attraction between oppositely charged ion. Ionic bonds are formed between two or more atoms by the transfer of one or more electrons between atoms. The formation of a positive ion involves ionization, i.e., removal of electron(s) from the neutral atom and that of the negative ion involves the addition of electron(s) to the neutral atom. Formation of ionic bond: Covalent bond: A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electrons are known as shared pairs or bonding pairs. Each bond is formed as a result of sharing of an electron pair between the atoms. Each combining atom contributes at least one electron to the shared pair. The combining atoms attain the outer shell noble gas configurations as a result of the sharing of electrons. (The dots represent electrons. Such structures are referred to as Lewis dot structures.) Formation of the chlorine molecule (Cl2): In chlorine an electron pair is shared between the two atoms in Cl2. This is called covalent bonding. The Cl atom with electronic configuration, [Ne]3s2 3p5, is one electron short of the argon configuration. The formation of the Cl2 molecule can be understood in terms of the sharing of a pair of electrons between the two chlorine atoms, each chlorine atom contributing one electron to the shared pair. In the process both chlorine atoms attain the outer shell octet of the nearest noble gas (i.e., argon), the dots represent electrons. When two atoms share one electron pair they are said to be joined by a single covalent bond. When two atoms share two pairs of electrons, the covalent bond between them is called a double bond. For example, in the carbon dioxide molecule, we have two double bonds between the carbon and oxygen atoms. When two atoms share three pairs of electrons, the covalent bond between them is called a triple bond. For example, in the nitrogen molecule , ethyne molecule we have triple bond. Co-ordinate covalent bond: A coordinate covalent bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom. A covalent bond is formed by two atoms sharing a pair of electrons.  NH3(g)+HCl(g)→NH4Cl(s) Ammonium ions, NH4+, are formed by the transfer of a hydrogen ion (a proton) from the hydrogen chloride molecule to the lone pair of electrons on the ammonia molecule. When the ammonium ion, NH4+, is formed, the fourth hydrogen is attached by a dative/co- ordinate covalent bond, because only the hydrogen's nucleus is transferred from the chlorine to the nitrogen. The hydrogen's electron is left behind on the chlorine to form a negative chloride ion. A coordinate bond is shown by an arrow. The arrow points from the atom donating the lone pair to the atom accepting it. Metallic bond: A metallic bond is a type of chemical bond formed between positively charged atoms in which the free electrons are shared among a lattice of cations. Metallic bonding is the main type of chemical bond that forms between metal atoms. They differ from covalent and ionic bonds because the electrons in metallic bonding are delocalized, that is, they are not shared between only two atoms. Instead, the electrons in metallic bonds float freely through the lattice of metal nuclei. This type of bonding gives metals many unique material properties, including excellent thermal and electrical conductivity, high melting points, and malleability. Dipole bond: Dipole-dipole forces are the result of the attraction between a partial positive and negative charge of two different molecules. They can occur between two ions in an ionic bond or between atoms in a covalent bond. Dipole moments arise from differences in electronegativity. The larger the difference in electronegativity, the larger the dipole moment. The dipole moment is a measure of the polarity of the molecule. Dipole bond Hydrogen Bond: Hydrogen bonding is a chemical bond between the hydrogen atom and highly electronegative atom. This is a very weak bond and strength of hydrogen bond is much less than the strength of covalent bond. Hydrogen bond is usually showed by dotted lines (……….)between two atom. For example, Hydrogen bond between two molecules of water Hydrogen Bond Types of Hydrogen Bonding There are two types of H bonds, and it is classified as the following: Intermolecular Hydrogen Bonding Intramolecular Hydrogen Bonding Intermolecular Hydrogen Bonding When hydrogen bonding takes place between different molecules of the same or different compounds, it is called intermolecular hydrogen bonding. For example, hydrogen bonding in water, alcohol, ammonia etc. Intramolecular Hydrogen Bonding The hydrogen bonding which takes place within a molecule itself is called intramolecular hydrogen bonding. It takes place in compounds containing two groups such that one group contains a hydrogen atom linked to an electronegative atom, and the other group contains a highly electronegative atom linked to a lesser electronegative atom of the other group. The bond is formed between the hydrogen atoms of one group with the more electronegative atom of the other group. Properties of Hydrogen Bonding  Solubility: Alcohols are soluble in water because of the hydrogen bonding which can take place between water and alcohol molecules.  Volatility: As the compounds involving hydrogen bonding between different molecules have a higher boiling point, they are less volatile.  Viscosity and surface tension: The substances which contain hydrogen bonding exist as associated molecules. So, their flow becomes comparatively difficult. They have higher viscosity and high surface tension.  The lower density of ice than water: In the case of solid ice, hydrogen bonding gives rise to a cage-like structure of water molecules. As a matter of fact, each water molecule is linked tetrahedral to four water molecules. The molecules are not as closely packed as they are in a liquid state. When ice melts, this case-like structure collapses, and the molecules come closer to each other. Thus for the same mass of water, the volume decreases and density increases. Therefore, ice has a lower density than water at 273 K. That is why ice floats. Bond length: Bond length is defined as the equilibrium distance between the nuclei of two bonded atoms in a molecule. Each atom of the bonded pair contributes to the bond length. Bond Angle: It is defined as the angle between the orbitals containing bonding electron pairs around the central atom in a molecule/complex ion. Bond angle is expressed in degree. It depends on electronegativity of atom. It gives some idea regarding the distribution of orbitals around the central atom in a molecule/complex ion and hence it helps us in determining its shape. For example H–O–H bond angle in water can be represented as under : Bond Angle:  NH3-1070, PH3-910, H2S-92.10, H2O-104.50 Bond energy: Bond energy (E) is defined as the amount of energy required to break apart a mole of molecules into its component atoms. It is a measure of the strength of a chemical bond. Bond energy is also known as bond enthalpy (H) or simply as bond strength. The unit of bond enthalpy is kJ mol–1. For example, the H – H bond enthalpy in hydrogen molecule is 435.8 kJ mol–1. H2 (g) → H(g) + H(g); ∆a H = 435.8 kJ mol–1 Shape of molecule: Molecular shape is the three dimensional arrangement of atoms in space around a central atom. The shape of a molecule depends on the number of pairs of electrons in the outer most shell surrounding a central atom. Molecular shapes are important in determining macroscopic properties such as melting and boiling points, and in predicting the ways in which one molecule can react with another. VSPER Theory: The VSEPR theory is used to predict the shape of the molecules from the electron pairs that surround the central atoms of the molecule. The theory was first presented by Sidgwick and Powell in 1940. The Valence Shell Electron Pair Repulsion Theory, abbreviated as VSEPR theory, is based on the premise that there is a repulsion between the pairs of valence electrons in all atoms, and the atoms will always tend to arrange themselves in a manner in which this electron pair repulsion is minimalized. This arrangement of the atom determines the geometry of the resulting molecule. The different geometries that molecules can assume in accordance with the VSEPR theory can be seen in the illustration provided below. In CH4, NH3 and H2O, the central atom undergoes sp3 hybridization-yet their bond angles are different, why? In CH4, NH3 and H2O the central atom undergoes sp3 hybridization. But their bond angles are different due to the presence of lone pair of electrons. It can be explained by VSEPR theory. According to this theory, 1. even though the hybridization is same, the repulsive force between the bond pairs and lone pairs are not same. Bond pair – Bond pair < Bond pair – Lone pair < Lone pair -Lone pair. So due to the varying repulsive force the bond pairs and lone pairs are distorted from regular geometry and organize themselves in such a way that repulsion will be minimum and stability will be maximum. 2. In case of CH4 , there are 4 bond pairs and no lone pair of electrons. So it remains in its regular geometry, i.e., tetrahedral with bond angle = 109° 28.’ 3. H2O has 2 bond pairs and 2 lone pairs. There is large repulsion between lp – lp. Again repulsion between lp – bp is more than that of 2 bond pairs. So 2 bonds are more restricted to form inverted V shape (eg;) bent shape molecule with a bond angle of 104° 35. 4. NH3 has 3 bond pairs and 1 lone pair. There is repulsion between lp – bp. So 3 bonds are. more restricted to form pyramidal shape with bond angle equal to 107° 18’. Write the Lewis dot structure of CO molecule. Solution: Step 1. Count the total number of valence electrons of carbon and oxygen atoms. The outer (valence) shell configurations of carbon and oxygen atoms are: 2s2 2p2 and 2s2 2p4, respectively. The valence electrons available are 4 + 6 =10. Step 2. The skeletal structure of CO is written as: C O Step 3. Draw a single bond (one shared electron pair) between C and O and complete the octet on O, the remaining two electrons are the lone pair on C. This does not complete the octet on carbon and hence we have to resort to multiple bonding (in this case a triple bond) between C and O atoms. This satisfies the octet rule condition for both atoms. This does not complete the octet on carbon and hence we have to resort to multiple bonding (in this case a triple bond) between C and O atoms. This satisfies the octet rule condition for both atoms. The Lewis Representation of Some Molecules