Chemistry FA-3 PDF

# Types of Hydrogen Bonding - In molecules, two types of hydrogen bonding are observed: - Intermolecular hydrogen bonding - Intramolecular hydrogen bonding ## Intermolecular Hydrogen Bonding - Hydrogen bond formed between two different polar molecules of the same substance or of different...

# Types of Hydrogen Bonding - In molecules, two types of hydrogen bonding are observed: - Intermolecular hydrogen bonding - Intramolecular hydrogen bonding ## Intermolecular Hydrogen Bonding - Hydrogen bond formed between two different polar molecules of the same substance or of different substances. - This type of bonding takes place in water, hydrogen fluoride, or ammonia. ## Intramolecular Hydrogen Bonding - Hydrogen bond formed between the atoms of the same molecule. - In this type of hydrogen bonding, only one molecule of a substance is involved and a ring formation takes place. - Other examples of compounds having intramolecular hydrogen bonding include ortho-fluorophenol (F.CH₂OH), ortho-nitroaniline [O₂N.C₆H₄(NH₂)], o-nitrophenol, and o-hydroxybenzaldehyde. ### Key Points - Hydrogen bonding is a special kind of dipole-dipole interaction. - Strength of hydrogen bond depends upon the electronegativity of the highly electronegative atom to which the hydrogen is bonded. - p-nitrophenol shows intermolecular hydrogen bonding. # Covalent Bond and Hydrogen Bond ## Chemical Bond - The electrostatic forces of attraction between a partially positively charged hydrogen atom of a polar molecule and a highly electronegative atom. ### Covalent Bond - Formed by equal sharing of electrons between atoms of different kind. - Formed by equal sharing of electrons between atoms of the same kind. - The pair of electrons are shared by only one atom or ion or molecule in the formation of a bond. ### Types - **Intermolecular H-bonding:** Example: H₂O, HF, NH₃ - **Intramolecular H-bonding:** Example: o-nitrophenol ## Introduction - The process of chemical bonding can be done by another mode of chemical combination, which is by sharing one or more electrons. - In this bond, the atoms do not acquire any charge as the electron or electrons are not lost completely. - The bond is therefore, termed as non-polar bond. - A covalent bond is a chemical bond formed between two atoms (usually non-metals) by sharing of valence electrons. - These are also known as molecular bonds. - The sharing of electrons takes place in such a way that both the atoms attain inert gas configuration. - The actual bond is formed due to the force of attraction between the nuclei of two atoms and the shared pair of electrons. ### Covalent Bond - The type of chemical bond formed by sharing of electrons with the atoms of other elements or same element is called a covalent bond. - In this bond, both the atoms obtain nearest inert gas configuration. ## Examples - Most of the elements do not occur free in nature but as compounds. - Elements that occur free are relatively unreactive. - When elements react with other elements, they form chemical bonds. - The association between atoms in a molecule by chemical forces is called chemical bond. - Chemical bonding refers to the study of these chemical bonds. ## Electronic Configurations of Noble Gases | Element | Symbol | Atomic Number | Electronic Configuration | Number of Valence Electrons | |---|---|---|---|---| | Helium | He | 2 | 2 | 2 | | Neon | Ne | 10 | 2,8 | 8 | | Argon | Ar | 18 | 2,8,8 | 8 | | Krypton | Kr | 36 | 2,8, 18,8 | 8 | | Xenon | Xe | 54 | 2, 8, 18, 18,8 | 8 | | Radon | Rn | 86 | 2, 8, 18, 32, 18,8 | 8 | ## Modes of Chemical Combination - Atoms combine together in order to complete their respective octet or duplet, so as to acquire a stable inert gas configuration. - The different modes of chemical combination are: - By transfer of electrons from one atom to another atom, that is, one atom may lose electrons and the other may gain electrons. - The bond thus formed between the two atoms is called ionic bond (or electrovalent bond). - By sharing of electrons which can occur in two ways as follows: - When the shared electrons are contributed by the two combining atoms equally, the bond formed is called covalent bond. - When electrons are contributed entirely by one of the atoms but shared by both, the bond formed is called coordinate bond. ## Lewis Symbols (Electron Dot Representation) - G.N Lewis, an American chemist, introduced simple notations to represent the valence electrons in an atom. - These notations are called Lewis symbols or electron dot symbols. - According to Lewis notations, the symbol of the element represents the whole of the atom except the valence electrons. - So, the valence shell electrons are shown as dots across around the symbol of the atom. ## Electron dot representation (Lewis symbols) of some common elements | Element | Symbol | Atomic Number | Electronic Configuration | Valence Electrons | Lewis Symbol | | --------------- | -------- | -------------- | ------------------------- | ------------------ | ------------- | | Hydrogen | H | 1 | 1 | 1 | H | | Helium | He | 2 | 2 | 2 | He | | Lithium | Li | 3 | 2,1 | 1 | Li | | Beryllium | Be | 4 | 2,2 | 2 | Be | | Boron | B | 5 | 2,3 | 3 | B | | Carbon | C | 6 | 2,4 | 4 | C | | Nitrogen | N | 7 | 2,5 | 5 | N | | Oxygen | O | 8 | 2,6 | 6 | O | | Fluorine | F | 9 | 2,7 | 7 | F | # Covalent Compounds - Covalent compounds exhibit some common and characteristic properties: - Most of the covalent compounds are gases or liquids at room temperature. - They have low melting and boiling points. - Covalent compounds are poor conductors of electricity. - These compounds are insoluble in some solvents, and are soluble in other solvents. - They are neither hard nor brittle. - Reactions of covalent compounds are slow. - They are directional in nature. ## Formation of Covalent Compounds - Formation of hydrogen chloride molecule (HCI): - The hydrogen atom has one electron in K-shell and needs one more electron to have a stable configuration. - Chlorine atom has seven electrons in its valence shell (M-shell) and hence, needs one more electron to have a stable configuration. - Formation of water molecule (H₂O): - The hydrogen atom has one electron in K-shell and hence needs one more electron to attain a stable configuration. - The oxygen atom has six electrons in its valence shell (L-shell) and hence, needs two more electrons to have a stable configuration. - Formation of carbon dioxide molecule (CO₂): - The carbon atom has four electrons in its valence shell (L-shell) and needs four more electrons to have a stable configuration. - The oxygen atom has six electrons in its valence shell (L-shell) and needs two electrons to have a stable configuration. ## Coordinate Covalent Bond - Coordinate covalent bond is a specific type of covalent bond. - In this type of bond, the pair of electrons are derived from a single atom and are shared between both the bonded atoms. - During the bond formation, the atom contributing the pair of electrons is known as donor, while the atom which does not contribute but only shares the given pair of electrons is referred to as acceptor. - Coordinate bond is represented by "→" (an arrow) starting from the donor atom and directed towards the acceptor atom. ## Examples - Formation of ammonia-boron trifluoride: - Ammonia molecule contains nitrogen atom with a lone pair of electrons. - BF₃ has boron atom with an incomplete octet (with a vacant p, orbital). - Therefore, nitrogen of ammonia donates its lone pair of electron to boron and thus, forms coordinate covalent bond. ## Types of Covalent Bonds - Covalent compounds can be classified based on polarity and number of shared pair of electrons: - **Non-polar covalent bonds:** This type of bond is formed by sharing of electrons between atoms of the same element. - **Polar covalent bonds:** This type of bond is formed by sharing of electrons between atoms of different elements. ## Properties of Covalent Compounds - Most of the covalent compounds are gases or liquids at room temperature. - They have low melting and boiling points (Diamond is an exception). - Covalent compounds are poor conductors of electricity. - These compounds are insoluble in some solvents, and are soluble in other solvents. - They are neither hard nor brittle. - Reactions of covalent compounds are slow. - They are directional in nature. # Ionic Compounds - One of the atoms (cation) has: - 1, 2, or 3 electrons in the valence shell. - Low ionization potential. - Low electronegativity and electron affinity. - Large atomic size. - The other atom (anion) has: - 5, 6, or 7 electrons in the valence shell. - High electron affinity and electronegativity. ## Properties of Ionic Compounds - These are generally crystalline solids at room temperature. - These compounds have high density. - Reactions of ionic compounds are very fast in aqueous solutions and fused state. - They are poor conductors of electricity (insulators) in the solid state. - They are soluble in solvents like water, and insoluble in organic solvents. - In molten (fused) state or in aqueous solution, the ions conduct electricity. # Hydrogen Bond - The electrostatic forces of attraction between a partially positively changed hydrogen atom of a polar molecule and a highly electronegative atom of the same molecule or of a different molecule is known as a hydrogen bond. - This weak electrostatic bond has a strength of about 10-50 kJ mol-1. - Hydrogen bonds are formed only when hydrogen is attached to a highly electronegative atom (F, O, and N). ## Coordinate Covalent Bond - The properties of coordinate covalent bond are: - A coordinate covalent bond is a strong bond with a specific directional characteristic similar to a covalent bond. - The melting and boiling points are intermediate to those of the ionic and the covalent compounds. ## Comparison of Boiling Points of NH₃ and HCI - NH₃ has a higher boiling point than HCl, even though they have nearly the same molecular weights. - NH₃ forms hydrogen bonds while HCl does not. - This is because of the smaller size of the nitrogen atom compared to the chlorine atom. ## Associated Ammonia Molecules - Define dipole moment. - The degree of polarity of a polar covalent bond or of a polar molecule is expressed in terms of dipole moment (μ). ## Factors Favourable for covalent bond formation. - Both atoms have less difference in electronegativity. - Either of the atom does not attract the shared electron pair with more than optimum force. - The number of valence electrons in the two atoms are 4, 5, 6 or 7. ## Properties of Covalent bond - Covalent compounds exhibit some common and characteristic properties: - They compounds are gases or liquids at room temperature . Some are soft solids. - They have low melting and boiling points. - Covalent compounds are poor conductors of electricity. - Most covalent compounds are insoluble in polar solvents but are soluble in other solvents. - They are neither hard nor brittle. - Reactions of covalent compounds are slow. - By restriction of covalent compounds, hard bonds have to broken and new bonds have to form by redistribution of valence electrons again. - They are directional in nature.