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What is Organic Chemistry? Organic chemistry (a science of less than 200 years old) is the study of the compounds of carbon, which are the central substances of which all living things on this planet are made. - Proteins (blood, muscle, and skin.). - DNA (contain all the genetic information). - Enzymes (catalyze the reactions that occur in our bodies). (clothing, household items, medicines, pesticides, insecticides,....etc) 1 Atomic Structure Shell proton + electron N N + - neutron 2 Atomic Number and Atomic Mass The atomic number (Z) is the number of protons in the atom's nucleus. or the number of electrons around the nucleus. The mass number (A) is the number of protons plus neutrons (All the atoms of a given element have the same atomic number) Isotopes are atoms of the same element that have different numbers of neutrons and therefore different mass numbers The atomic mass (atomic weight) of an element is the weighted average mass in atomic mass units (amu) of an element’s naturally occurring isotopes atomic weight 3 Properties of subatomic particles Symbol Relative Charge electron e- -1 negative 0 9.11x10-28 proton p+ +1 positive 1 1.67x10-24 neutron n0 0 neutral 1 1.67x10-24 Particle Relative Mass Actual Mass (g) Is very small 4 Atomic Structure Quantum mechanics: describes electron energies and locations by a wave equation in atoms in terms of: Main or principal energy levels (shells) (n) Energy sublevels (subshells) (orbitals) 5 Principal Energy Levels (shells) (n) Contain electrons that are close in energy. Have values of n = 1, 2, 3, 4, 5, 6….. The 1st shell (n = 1) is the lowest in energy, 2nd level is higher, and so on 1<2<3<4 Maximum number of electrons = 2n2 n =1 2(1)2 = 2 n =2 2(2)2 =8 n=3 ????? Each shell contains subshells known as atomic orbitals 6 What is orbital? is a region of space where the probability of finding an electron is large. Why it is important to understand location of electrons? (Arrangment of e- creates bonds between atoms, these bonds will be involved in chemical reactions) Electron cloud has no specific boundary so we show the most probable area 7 Shapes of Atomic Orbitals Four different kinds of orbitals for electrons, denoted s, p, d, and f s orbitals: spherical p orbitals: pairs of dumbbells aligned along x, y and z axis at 90° to each other d orbitals: complex shapes f orbitals: very complex 8 p-Orbitals In each shell there are three perpendicular p orbitals, px, py, and pz, of equal energy Lobes of a p orbital are separated by region of zero electron density, a node 9 Orbitals and Shells Orbitals are grouped in shells of increasing size and energy (Sublevel energy: s<p<d<f) Different shells contain different numbers and kinds of orbitals Each orbital can be occupied by 2 electrons 10 s Number of orbitals 1 Electrons allowed 2 p 3 6 d 5 10 f 7 14 Sublevel 11 Electron Configurations Ground state electron configuration: Z electrons (Z = atomic number of the atom) are placed serial into the orbitals according to the following rules: Aufbau principle“build-up”: electrons go into lowest energy orbitals first. 1s→2s→2p→3s→3p→4s→3d 12 Pauli exclusion principle: each orbital can hold a max. of two electrons having opposite spins. (Electron spin can have only two orientations, up ↑ and down ↓) Hund’s rule: When filling sublevels other than s, electrons are placed in individual orbitals of equal energy before they are paired up. O8 1S2 2S22P4 ↑↓ ↑↓ ↑↓ ↑ ↑ 1s 2s 2px 2py 2pz 13 Electron configuration and orbital diagram Electron Configuration Li3 1S2 2S1 C6 1S2 2S22P2 N7 1S2 2S22P3 O8 1S2 2S22P4 Orbital Diagram ↑↓ ↑ 1s 2s 2px 2py 2pz ↑↓ ↑↓ ↑ ↑ 1s 2s 2px 2py 2pz ↑↓ ↑↓ ↑ ↑ ↑ 1s 2s 2px 2py 2pz ↑↓ ↑↓ ↑↓ ↑ ↑ 1s 2s 2px 2py 2pz Exercise: Give the electronic configuration for the following: Na+ Cl- Mg+2 (knowing that Na11, Mg12, Cl17) 14 Write the electronic configuration for the following elements: a) Ca 20 b) Na 40 1s2,2s2,2p6,3s2,3p6, 4s2 d) Cl 17 35 1s2,2s2,2p6,3s2,3p5 11 23 c) e) Si 28 1s2,2s2,2p6,3s2,3p2 16 1s2,2s2,2p4 1s2,2s2,2p6,3s1 14 O 8 f) B 5 11 1s2,2s2,2p1 15 Variation of atomic properties with electronic structure 1. 2. 3. Atomic and ionic size Ionization Energy (IE) Electron Affinity (EA) 16 1. Atomic and ionic size A. Atomic size, Atomic Radii Higher effective nuclear charge Electron held more tightly Atomic size decrease Electrons held less tightly Orbital size increase Atomic size increase Representation of Periodic Table 17 Size increase form top to bottom within a group? because of larger orbitals and electrons are added farther from the nucleus, there is less attraction. Size decrease form left to right within a period? Because of increasing positive nuclear charge increase the attraction of the valence electron inward 18 B. Ionic size When electrons are added to an atom (creating a negative ion) the repulsion between them increases, and causes them to push away from each other, therefore making the size of the ion larger than the neutral atom. When electrons are removed from an atom (creating a positive ion) the repulsion decrease, and the remaining electrons are drawn closer to the nucleus, therefore making the size of the ion smaller than the neutral atom. 19 2. Ionization Energy Ionization energy (IE) is the energy required to remove an electron from an atom (in the gas phase) or ion in its ground state. Mg (g) + 738 kJ ---> Mg+ (g) + e1st ionization energy X → X+ + e2nd ionization energy X+ → X2+ + e3rd ionization energy X2+ → X3+ + e- (Generally the (n+1)th ionization energy is larger than the n th ionization energy. Always, the next ionization energy involves removing an electron from an orbital closer to the nucleus. Electrons in the closer orbitals experience greater forces of electrostatic attraction; thus, their removal requires increasingly more energy.) 20 3. Electron Affinity Electron affinity (EA) is the potential energy change associated with the addition of an electron to an atom (in gas phase) or ion in its ground state. A(g) + e- ---> A-(g) O atom + electron EA = - 141 kJ E.A. = ∆E O ˗ ion 21 Summary Size, IE, and EA in periodic table Larger Atomic Size Larger Larger Ionization energy Larger Larger Electron affinity Larger 22 Chemical Bonding The octet rule Why do atoms bond together? (has less energy) more stable Atoms tend to attain electron configuration of noble gases (why?) by: 1) Losing electrons 2) Gaining electrons 3) Sharing electrons Electron configuration of noble gases is very stable, because e- have filled up their orbitals, having very high IE, thus it’s extremely difficult to break an e- away. 23 1- Ionic Bonds formed by the transfer of one or more electrons from one atom to another to create ions. It is an attractive force between oppositely charged ions and the products are called salts. Atoms involved widely differ in electronegativity. (Electronegativity measures the ability of an atom to attract electrons) 24 2- Covalent Bonds When 2 or more atoms of the same electronegativities react, a complete transfer of electrons does not occur. In these instances the atom achieve noble gas configuration by sharing electrons . Covalent bonds form between the atoms, and the products are called molecules. Covalent bond forms when two atoms approach each other closely so that a singly occupied orbital on one atom overlaps a singly occupied orbital on the other atom 25 Q. In a tabular form, compare between ionic bonds and covalent bonds. 26