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Chapter 3 - Chemical Bonding So far we have discussed the...

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Chemical Bonding So far we have discussed the “nuts and bolts” of describing chemical reactions and the fundamental laws that must be adhered to (conservation of mass, balanced equations). But we have said nothing about why reactions occur, what compounds are formed and what they look like. This information requires us to consider the structure of atoms which in turn will help us to understand the nature and formation of chemical bonds. In this semester you will learn about the chemical bond from the classical description , next semester a deeper mathematical understanding from quantum mechanics .
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For example, P(C6H5)3 How can we describe the bonds that form between the elements that comprise a molecule Why do molecules adopt the shapes that they do?
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The Chemical Bond – the Classical Model Chemical bonds form by sharing or transferring electrons between atom In a simple chemical bond between two elements, A and B, there are two limiting cases: 1. A covalent bond where the electrons are shared almost equally (written A-B) 2. An ionic bond where electrons are transferred from one atom to another (e.g. A+ B-) One of the most important factors in determining if a bond is best described as covalent or ionic is the relative electronegativity of the elements involved
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The Periodic Table The chemical properties of the elements are periodic functions of the atomic number (number of protons in the nucleus – in a neutral element this is the same as the number of electrons) The periodic table can help us to rationalize the behavior and bonding properties of different elements
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The Physical Structure of Atoms Negatively charged electrons surrounding a positively charged but very small nucleu The nucleus, which contains protons (positively charged) and neutrons (neutral) Essentially all of the mass of the atom is located in the nucleus The chemical properties of each element are fundamentally determined by the number of electrons and protons.
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The Shell Model of the Atom As a result of quantum mechanics the electrons are arranged in shells around the nucleus. The number of electrons that can be accommodated in the first few shells is 2, 8, and 18. Electrons Nucleus Nitrogen Sodium Many chemical properties of the elements can be rationalized by the shell model
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The Shell Model of the Atom The electrons fill up the shells in order of increasing “distance” from the nucleus (1st shell can accommodate 2 electrons, second shell 8 electrons etc.) 2nd shell 1st shell Nitrogen Sodium 3rd shell
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Support for the Shell Model Variation in ionization energies , X(g) X+ +e- shows a repeatable periodic trend when considered in the context of the shell model and the periodic table 2 8 8 18 18
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IE1 = X X+ + e- IE2 = X+ X2+ + e- etc. Variation in Successive Ionization Energies Also Supports the Idea of Electrons in Shells
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Core and Valence Electrons Valence Level Electron Core Level Electrons Nitrogen Sodium Electrons in inner shells are termed core level electrons and do not participate significantly in chemical reactions or bonding Electrons in the outermost (or partially filled) shell are termed valence level electrons and are involved in forming chemical bonds
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