Download Unit 4: Chemical Bonding Notes Chemical Bond—a mutual

Unit 4: Chemical Bonding Notes Chemical Bond—a mutual electrical attraction between the nuclei and valence electrons of different atoms that binds the atoms together. Chemical bonds create more stable arrangements of matter. The goal of any atom is to gain, lose, or share valence electrons creating chemical bonds to provide a more stable arrangement of matter. Cation—a positive ion. An atom that has lost electrons. (metals form cations) Oxidation—loss of electrons (becoming positively charged) Anion—a negative ion. An atom that has gained electrons. (nonmetals usually form anions) Reduction—process of gaining electrons (becoming negatively charged) Ionic Bonding—chemical bonding that results from the electrical attraction between large numbers of cations and anions. (value 1.7-­‐3.3) Covalent Bonding—bonding that results from the sharing of electron pairs. 2 Types of Covalent Bonding – Nonpolar Covalent Bonding – Polar Covalent Bonding Nonpolar Covalent Bond—a covalent bond in which the bonding electrons are shared equally, resulting in a balanced distribution of electrical charge (value 0-­‐0.3). Polar Covalent Bond—covalent bond in which the unbonded atoms have an unequal attraction for the shared electrons (value 0.3-­‐1.7). Calculating Bond Type—subtract electronegativities Electronegativity Values Image Source: http://www.science.marshall.edu/pricew/211/Chapt_sum/Chapt_8/electroneg.gif Retrieved October 23, 2014 Molecules and Molecular Compounds • A molecule is a group of atoms held together by covalent bonds (usually composed only of nonmetals). • The composition of a compound is given by its chemical formula. • A chemical formula indicates the numbers of atoms in a chemical compound using symbols and subscripts. Image Source: http://www.bbc.co.uk/staticarchive/4e1862d0b757481990a81c6eebea40179f88903a.gif Retrieved October 23, 2014 •
Diatomic Molecule—molecules containing only 2 atoms. •
7 Diatomic Elements—H2, O2, F2, Cl2, Br2, I2, and N2 Image Source: http://ouitschem.weebly.com/uploads/8/6/9/7/8697521/219940320.gif Retrieved October 23, 2014 •
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Other atoms can fill their outermost s and p sublevels by sharing electrons through covalent bonding. Such bond formation follows the octet rule: Chemical compounds tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest energy level. •
Exceptions to the octet rule include those for atoms that cannot fit eight electrons, and for those that can fit more than eight electrons. Common exceptions to the octet rule: • Phosphorous • Boron • Hydrogen •
Bond energy is the energy required to break a chemical bond and form neutral atoms. Atoms want to combine to reach stable (noble gas) electron configurations. •
Electron-­‐dot notation is an electron-­‐configuration notation in which only the valence electrons of an atom are shown, indicated by dots placed around the element’s symbol. Image Source: http://chemsite.lsrhs.net/bonding/images/al_dot.gif Retrieved October 23, 2014 •
Electron-­‐dot notation can also be used to represent molecules. Image Source: http://chemistryfunfacts.weebly.com/uploads/5/2/2/4/5224712/6338169.png?237 Retrieved October 23, 2014 •
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The pair of dots between the two symbols represents a shared electron pair (a covalent bond). Bond pair—a pair of electrons involved in bonding. Lewis Structures • An unshared pair, also called a lone pair or nonbonding pair, is a pair of electrons that is not involved in bonding. • The pair of dots representing a shared pair of electrons in a covalent bond is often replaced by a long dash. Image Source: http://www.meritnation.com/img/shared/discuss_editlive/4255118/2013_09_01_21_40_59/zgfrzyxa1167343484998808837.jpg. Retrieved October 23, 2014 •
A structural formula indicates the kind, number, and arrangement, and bonds but not the unshared pairs of the atoms in a molecule. Image Source: http://2012books.lardbucket.org/books/introduction-­‐to-­‐chemistry-­‐general-­‐organic-­‐and-­‐
biological/section_15/9015dee78ce51648b28fe5e89e114fa2.jpg Retrieved October 23, 2014 •
A single bond is a covalent bond in which one pair of electrons is shared between atoms. A single line represents a single bond. Image Source: http://upload.wikimedia.org/wikipedia/commons/thumb/c/c3/Wasserstoff.svg/220px-­‐Wasserstoff.svg.png Retrieved October 23, 2014 Multiple Covalent Bonds • A double bond is a covalent bond in which two pairs of electrons are shared between atoms. Image Source: http://0.tqn.com/y/chemistry/1/W/L/G/1/ethylene.jpg Retrieved October 23, 2014 •
A triple bond is a covalent bond in which three pairs of electrons are shared between atoms. Image Source: http://nonsibihighschool.org/intbasch9_files/image035.png Retrieved October 23, 2014 Rules for Drawing Lewis Structures 1. Determine the total number of valence electrons in the atoms. 2. Arrange atoms to form a skeleton structure. If C is present, it is the central atom. Otherwise the least electronegative atom is central (except for hydrogen which is never central). Connect the atoms by bonds. 3. Add unshared pairs of electrons so that each hydrogen atom has 2 electrons and each other nonmetal has 8 electrons (there are some exceptions) 4. Count electrons to be sure the number used equals the number available. • An ionic compound is composed of positive and negative ions that are combined so that the numbers of positive and negative charges are equal. Most ionic compounds exist as crystalline solids. Image Source: https://dr282zn36sxxg.cloudfront.net/datastreams/f-­‐
d%3Ad3e4243fa907179240bdd28c718f724a2e147e7564ac1d6f23adccb0%2BIMAGE%2BIMAGE.1 Retrieved October 23, 2014 •
The strong attraction between ions in an ionic compound gives ionic compounds some characteristic properties: • high melting points • hard but brittle • not electrical conductors in the solid state, because the ions cannot move Comparison of Ionic and Covalent Bonds Ionic Covalent (Molecular) Bonds formed by attraction between oppositely Bonds formed when electrons are shared. charged ions. High melting point—compounds usually solids at Low melting points—compounds may be solids, room temperature liquids, or gases at room temperature. Ionic Compounds—good conductors when dissolved Covalent Compounds—poor conductors when in water dissolved in water Formed between metal and nonmetal ions Formed between nonmetals Polyatomic Ions • polyatomic ion—a group of atoms with a charge. Drawing Lewis Structures for Polyatomic Ions • If ion is positive subtract electrons • If ion is negative add electrons Image Source: http://crescentok.com/staff/jaskew/isr/tigerchem/oxidation/poly1.gif Retrieved October 23, 2014 •
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Chemical bonding is different in metals than it is in ionic or covalent compounds. Metallic bonding gives metals the properties of: • Conductivity • malleability • ductility • luster The Metallic Bond Model Valence electrons are free to move about the metal. As a result these mobile electrons create what is known as a sea of electrons around the metal atoms. The chemical bonding that results from the attraction between metal atoms and the sea of electrons is called metallic bonding. Image Source: http://blogs.scientificamerican.com/lab-­‐rat/files/2012/03/metallic-­‐bonding-­‐300x185.png Retrieved October 23, 2014 Molecular Geometry & the VSEPR Theory • Molecular geometry—the three-­‐dimensional arrangement of a molecule. • VSEPR—valence-­‐shell electron-­‐pair repulsion • VSEPR theory-­‐theory used to predict the shapes of molecules VESPR Theory Number of Electron Domains Bonding Domains Nonbonding Domains Hybridization 2 2 0 sp 3 3 0 sp2 2 1 4 4 0 3 1 2 2 5 0 4 1 3 2 2 3 6 0 5 1 4 2 Electron Domain Geometry Linear Molecular Geometry General Formula (AXE Formula) Linear Trigonal Planar AX2 Trigonal planar AX3 Bent Tetrahedral AX2E AX4 Trigonal pyramidal AX3E 5 6 sp3 sp3d sp3d2 Tetrahedral Bent Trigonal bipyramidal Octahedral AX2E2 Trigonal bipyramidal Seesaw AX5 AX4E T-­‐shaped Linear Octahedral AX3E2 Square pyramidal AX5E Square planar AX4E2 AX2E3 AX6 A = Element #1; B = Element #2; E = lone pair electrons around center atom. Intermolecular Forces • Intermolecular forces-­‐-­‐forces of attraction between molecules • Higher boiling point—stronger molecular forces 1. Dipole—intermolecular force created by equal but opposite charges separated by a short distance. An arrow pointing toward the negative pole and a crossed tail at the positive pole represents a dipole. The lower case Greek letter delta (δ) may also represent the poles (δ+ for positive pole and δ-­‐ for negative pole)
For example: Image Source: http://3.bp.blogspot.com/-­‐4XwAEJYxYO4/UEXGEnF51WI/AAAAAAAABt8/bhDwaVkMvx0/s1600/HCl+polarity.png. Retrieved October 23, 2014 2. Dipole-­‐Dipole Forces—forces of attraction between polar molecules. Image Source: http://3.bp.blogspot.com/_HbJwA7v1Jss/TQojpBdry_I/AAAAAAAAAA0/AUQSTs9hASM/s1600/Dipole-­‐dipole-­‐interaction-­‐in-­‐HCl-­‐ Retrieved October 23, 2014 3. Hydrogen bonding—intermolecular force in which a hydrogen atom is bonded to a highly electronegative atom (usually O, N, or F) and attracted to an unshared pair of electrons in a nearby molecule. Image Source: http://www.elmhurst.edu/~chm/vchembook/images2/160hbondwater.gif Retrieved October 23, 2014 4. London dispersion forces—intermolecular attractions resulting from the constant motion of electrons (these forces are also known as van der Walls forces) Image Source: http://4.bp.blogspot.com/_hm25eS2tXB0/TSnr1MS5z5I/AAAAAAAAADE/2tYY0-­‐fCQgU/s1600/london.gif Retrieved October 23, 2014 Cohesion vs. Adhesion Cohesion is the property of like molecules sticking together, being mutually attractive. This property is caused by the shape and structure of its molecules creating electrical attraction. Adhesion is the tendency of different types of particles to cling to one another. Both cohesion and adhesion are the result of intermolecular attractions. Image Source: http://water.usgs.gov/edu/graphics/adhesion-­‐cohesion-­‐2.gif Retrieved October 23, 2014