Chemical bonding I.pdf

CHEMICAL BONDING I Dr. Alya A. Arabi Department of Biochemistry College of Medicine and Health Sciences 1 Chemical bond energy ➢ It is the energy released when the bond is formed, or equally the energy required (in kcal/mol) to break the bond. (SI is kJ; 1 kcal = 4.184 kJ). ➢ Larger bond energy me...

CHEMICAL BONDING I Dr. Alya A. Arabi Department of Biochemistry College of Medicine and Health Sciences 1 Chemical bond energy ➢ It is the energy released when the bond is formed, or equally the energy required (in kcal/mol) to break the bond. (SI is kJ; 1 kcal = 4.184 kJ). ➢ Larger bond energy means a stronger bond. 2 https://www.clutchprep.com/chemistry/practice-problems/87833/there-are-two-main-types-of-covalent-bondbreakage-in-homolytic-breakage-as-in-f 3 Types of Bonds Electrons are shared Electron(s) are transferred 4 Bond Polarity and Electronegativity • Differences in EN produce bond polarity • Nonpolar Covalent Bonds: atoms with similar EN • Polar Covalent Bonds: Difference in EN of atoms < 2 units • Ionic Bonds: Difference in EN > 2 5 Covalent bonds ➢ Typical bond form between two non-metal elements ➢Each pair of electrons shared produces one covalent bond ➢ The electrons involved in the bond are called the bonding electrons or bond pair ➢ Electrons not involved in the bond are called the non-bonding electrons or the lone pairs 6 Covalent bonds in Amino Acids 7 http://www.bbc.co.uk/ Covalent bonds in the Bases of DNA 8 O=O, O2 oo o O xx o x oo O x xx Two oxygen atoms oo O xx o x O o x oo xx One oxygen molecule 9 CH4 (Methane) 10 Strength of covalent bonds The stability of a molecule is a function of the strength of the covalent bonds holding the atoms together. The strength of covalent bond: - is measured by the bond energy - depends on the degree of atomic orbital overlap 11 Classification of covalent bonds (I) Classification based on the number of electron pairs shared between two atoms: ➢ Single (H, F) e.g. H2 ➢ Double (C, O, N, P) e.g. O2 ➢ Triple (C, N) e.g. N2 1- Sigma bonds (s-bonds) 2- Pi bonds (p-bonds) 12 Polar bonds Characteristics of the polar bonds: - they are more reactive than non-polar bonds - they may increase the water solubility of molecules (like dissolves like) - they may affect the shape of molecules C-C and H-H are examples of non-polar bonds C=O, C-N and O-H are examples of polar bonds. 13 IONIC BOND • It only occurs when atoms are able to lose or gain one or two electrons. • It is usually a metal that donates the electron and a non-metal that receives it. • Electrons are transferred from one atom to another. • The transfer of electrons between atoms leads to a change in the charge on the atom • Cation: atom with positive charge • Anion: atom with negative charge 14 Ionic bond • Positive and negative charges balance one another and the compound is neutral overall •Example: 15 Coordinate covalent bond Coordinate covalent bond is a covalent bond formed when the shared pair of electrons come from only one of the atoms involved. After it is formed, it is indistinguishable from an ordinary covalent bond. A compound formed by coordinate bonds is called a coordination compound. Example: ammonium (NH4+) H3N: + H+ NH4+ 16