Bonding Theories. Why Do Atoms Bond? Principles of Chemistry: Chapter 9 Chemical bonding. A Molecular Approach, 1 st Ed.

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1 Principles of Chemistry: A Molecular Approach, 1 st Ed. Nivaldo Tro Chapter 9 Chemical bonding Bonding Theories Explain how and why atoms attach together why some combination of atoms are stable and others are not why is water H 2 O, not HO or H 3 O Lewis theory-one of the simplest bonding theories, developed by G.N. Lewis ( ) Using Lewis theory, we can draw Lewis structures e- dot structures that allow us to predict many properties of molecules i.e. molecular stability, shape, size and polarity Why Do Atoms Bond? Chemical bonds form because they lower the potential energy between the charged particles that compose atoms Coulomb s law- The potential energy (E) between two charged particles q 1 and q 2 separated by a distance r is directly proportional to the product of the charges inversely proportional to the distance between charges

2 Chemical Bonding A chemical bond forms when the potential energy of the bonded atoms is less than the potential energy of the separate atoms To calculate, you need to consider the following interactions: nucleus-to-nucleus repulsions electron-to-electron repulsions nucleus-to-electron attractions Types of Bonding Three types depending on the kind of atoms involved Ionic Bond- formed by the transfer of electrons from metal atom to non-metallic atom Covalent Bond- formed by sharing of electrons between two nonmetals Metallic Bond- electron sea model-all of the atoms in a metal lattice pool their valence e -. The positively charged metal atoms are then attracted to the sea of e- holding the metal together Types of Bonding

3 Types of Bonding Ionic Bonds Ionic bond involves the transfer of electrons from one atom (usually a metal) to another (usually a nonmetal) The number of electrons lost or gained by an atom is determined by its need to be isoelectronic with a noble gas (ns 2 np 6 ) Eight electrons in the valence shell Octet rule- When atoms bond, they tend to gain, lose, or share electrons to result in eight valence electrons Group 1A and 2A metals are very reactive, lose their outer electron to have an octet Halogens, reactive non metals gain electrons to have an octet Covalent Bonds Nonmetals have relatively high ionization energies, difficult to remove electrons from them When nonmetals bond together, it is better in terms of potential energy for the atoms to share valence e - lowest E when the electrons are between the nuclei Shared e - s hold the atoms together by attracting nuclei of both atoms

4 Metallic Bonding The simplest theory of metallic bonding-metal atoms releasing their valence electrons to be shared by all atoms/ions in the metal Bonding results from the attraction of metal cations for the delocalized electrons Representing Valence Electrons with Dots Valence electrons play a fundamental role in chemical bonding as they are held most loosely Valence electrons- For main group atoms the electrons in the outermost principal energy level The group # in P.T = # of valence e - (except for He) Lewis Electron-Dot Symbols A Lewis electron-dot symbol- A symbol in which the valence shell electrons of an atom or ion are represented by dots placed around the letter symbol of the element Put one electron on each open side of the symbol then pair the rest of electron till you have eight electrons e.g. Ne Lewis structures for period 2 elements

5 Lewis Symbols of Ions Cations have Lewis symbols without valence electrons electrons lost in the cation formation Anions have Lewis symbols with eight valence electrons. electrons gained in the anion formation Lewis Bonding Theory Lewis Bonding Theory-Atoms bond because it results in a more stable electron configuration. more stable = lower potential energy no attempt to quantify the energy, as the calculation is extremely complex Uses simple octet rule Eight electrons in v.shell like noble gases (ns 2 np 6 ) Some exceptions to the rule Li and Be and B try to achieve the He electron arrangement Expanded octets for elements in period 3 or below using empty valence d orbitals Lewis Theory and Ionic Bonding Lewis symbols can be used to represent the transfer of electrons from metal atom to nonmetal atom, resulting in ions that are attracted to each other and therefore bond. For example transfer of electron from Li to F

6 Lewis Theory Predictions for Ionic Bonding Lewis theory predicts the number of electrons that a metal atom should lose or a nonmetal atom should gain in order to attain a stable electron arrangement the octet rule This allows us to predict the formulas of the ionic compounds that result It also allows us to predict the relative strengths of the resulting ionic bond from Coulomb s law Predicting Ionic Formulas Using Lewis Symbols Electrons are transferred until the metal loses all its valence electrons and the nonmetal has an octet. Numbers of atoms are adjusted so the electron transfer comes out even. Li 2 O Example: Draw Lewis electron-dot formula for MgO and CaCl 2.

7 Lattice Energy: The rest of the Story The ionization energy of the metal is endothermic Na(s) Na + (g) + 1 e - H = +603 kj/mol The electron affinity of the nonmetal is exothermic. ½ Cl 2 (g) + 1 e - Cl - (g) H = -227 kj/mol Generally, IE of the metal > EA of the nonmetal therefore, the formation of the ionic compound should be endothermic. But the heat of formation of most ionic compounds is exothermic and generally large. Why? Na(s) + ½ Cl 2 (g) NaCl(s) H f = -410 kj/mol Energetics of Ionic Bond Formation The extra energy that is released comes from the formation of a structure in which every cation is surrounded by anions, and vice versa. Electrostatic attraction is nondirectional! no direct anion cation pair Therefore, there is no ionic molecule. The chemical formula is an empirical formula, simply giving the ratio of ions based on charge balance. Lattice Energy The ions are arranged in a pattern called a crystal lattice. maximizes the attractions between cations and anions, leading to the most stable arrangement.

8 Lattice Energy Lattice energy ( H lattice )-The energy associated with forming a crystalline lattice of alternating cations and anions from the separated gaseous ions, always exothermic measure indirectly, can be calculated from knowledge of other processes Lattice energy depends directly on size of charges and inversely on distance between ions Trends in Lattice Energy Ion Size Lattice energies become less exothermic with increasing ionic radii The force of attraction between charged particles is inversely proportional to the distance between them Larger ions mean that the center of positive charge (nucleus of the cation) is farther away from negative charge (electrons of the anion) larger ion weaker attraction smaller lattice energy Trends in Lattice Energy Ion Size As we move down the column in P.T ionic radii increases, ions cannot get as close to each other and release less energy on lattice formation

9 Trends in Lattice Energy Ion Charge The force of attraction between oppositely charged particles is directly proportional to the product of the charges Larger charge means the ions are more strongly attracted and more exothermic lattice energy Of the two factors, ion charge and ion size, ion charge is generally more important. Lattice Energy = kj/mol Lattice Energy = kj/mol Properties of Ionic Compounds high melting points, hard and brittle solids greater than 300 C All are crystalline solids at room temperature. Solids do not conduct electricity, but the liquid state does. Many are soluble in water and the solution conducts electricity. Ionic Bonding Model vs. Reality In NaCl(s), the ions are stuck in position and not allowed to move to the charged rods. In NaCl(aq), the ions are separated and allowed to move to the charged rods.

10 Covalent Bonding: Lewis Structures Lewis Theory provides us with a very simple and useful model for covalent bonding Covalent bond is formed between neighbouring atoms by sharing some of their valence electrons in order to attain octets (duet for H 2 ) The shared electrons would then count toward each atom s octet. + : H. H H H H: H Covalent Bonding Bond length-the distance between the nuclei at minimum potential energy Bond dissociation energy-energy required to separate the two bonded atoms in a molecule Covalent Bonding Bonding and Lone Pair Electrons Bonding pairs- Electrons that are shared by atoms Can be represented by dashes Nonbonding pairs or Lone pairs- Electrons that are not shared by atoms but belong to a particular atom are called. bonding pair H..Cl: + : H Cl: lone pair

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