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Lecture 4 - 3.1 Chemical Bonding Lewis ...

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Unformatted text preview: 3.1 Chemical Bonding Lewis Symbols Principal Types of Chemical Bonds: Ionic and Covalent Polar Covalent Bonding and ElectronegaBvity 3.2 Naming Compounds and WriBng Formulas of Compounds Ionic Compounds Covalent Compounds 3.3 ProperBes of Ionic and Covalent Compounds Physical State MelBng and Boiling Points Structure of Compounds in the Solid State SoluBons of Ionic and Covalent Compounds Compound: two or more atoms of the same or different elements are joined Chemical bond: aKracBve forces to link two atoms (the force overcomes the repulsion of the posiBvely charged nuclei of the two atoms; bond formaBon involving valence electrons are responsible for the chemical bond) Lewis Symbols (Lewis structure) element symbol + valence electrons v༇  Octet rule: the four sides of the symbol represent four atomic orbitals v༇  v༇  v༇  v༇  v༇  Each side represents an atomic orbital, which may hold up to two electrons The number of dots used corresponds to the number of valence electrons Only valence electrons parBcipate in bonding Each unpaired dot is available to form a chemical bond Number of valence e (main group) = the group number Lewis Dot Symbols for RepresentaBve Elements O N Ne Principal Types of Chemical Bonds: Ionic and Covalent Ionic bond: a transfer of one or more electrons from one atom to another Covalent bond: aKracBve force due to sharing of electrons between atoms Polar bond: bonds have features of both types Ionic Bonding electrons are lost by a metal forming caBon, electrons are gained by a nonmetal forming anion, obey the octet rule (Nobel Gas configuraBon), caBon- anion aKracBon forming ionic bond Na + Cl à༎ NaCl Na, low ionizaBon energy, low electron affinity N a ž༉ à༎ Na + e- [Neon] + − Cl, high electron Affinity, high ionizaBon energy : Cl⋅ + e − → ': Cl : $ [Ar] . . .. .. % .. " & # EssenBal Features of Ionic Bonding ①  Atoms with low I.E. and low E.A. tend to form caBons (posiBve ions) ②  Atoms with high I.E. and high E.A. tend to form anions (negaBve ions) ③  Ion formaBon takes place by electron transfer ④  The ions are held together by the electrostaBc force of the opposite charges ⑤  Nonmetals of representaBve elements tend to form ionic bond with metals Na loses one electron, it becomes a smaller Na+ Cl gains that electron, it becomes a larger Cl- Ion pair: electrostaBc force that holds the ion pair like Na+Cl−together The ion pairs arrange in a regular 3D repeaBng array called crystal ladce Covalent Bonding H ⋅ + ⋅ H → H : H each atom aKains a noble gas configuraBon (octet) by sharing electrons Each H has two electrons around it and aKained a He configuraBon The shared electron pair is called a Covalent Bond Features of Covalent Bonds Atoms with similar tendencies to gain or lose electrons form covalent bonds Compounds containing covalent bonds are called covalent compounds or molecules The diatomic elements have completely covalent bonds (totally equal sharing) H2, N2, O2, F2, Cl2, Br2, I2 .. .. .. .. : F⋅ + ⋅ F : → : F : F : .. .. .. .. Each F atom is surrounded by 8 electrons, i.e. [Ne] configuraBon Two atoms not idenBcal to form a covalent bond: F (in HF): 2 e- shared with H and 6 lone- pair e- . O (in H2O): 4 e- shared with the 2 H atoms and 4 lone- pair e- . C (in CH4): 8 e- shared with the 4 hydrogen atoms. N (in NH3) has 6 e- shared with the 3 hydrogen atoms and 2 lone- pair e- . .. .. 2H ⋅ + ⋅ O⋅ → H : O : H .. .. 2e– from 2H 2e– for H 6e– from O 8e– for O H . .. 4H ⋅ + ⋅ C⋅ → H : C : H ⋅ ⋅⋅ H 4e– from 4H 4e– from C .. .. 3H ⋅ + ⋅ N⋅ → H : N : H . .. H 2e– for H 8e– for C Polar Covalent Bonding and ElectronegaBvity Ionic bonding involves the transfer of electrons Covalent bonding involves the sharing of electrons Polar covalent bonding - bonds made up of unequally shared electron pairs somewhat posiBvely charged somewhat negaBvely charged .. .. H⋅ + ⋅F:→ H : F: ⋅⋅ ⋅⋅ These two electrons are not shared equally The shared electrons spend more Bme with fluorine Fluorine is electron rich = δ- Hydrogen is electron deficient = δ+ A truly covalent bond can only occur when both atoms are idenBcal unequal sharing of electrons in the pairs = polar covalent bonds Electronega>vity EN increases EN: a measure of the ability of an atom to aKract electrons in a chemical bond. Elements with high EN aKract electrons more than do elements with low EN. The difference in electronegaBvity determines the extent of bond polarity. EN increases The most electronegaBve elements, the upper right corner of the periodic table The least electronegaBve elements, the lower lem corner of the periodic table ΔEN determines the extent of polarity. H2 : H—H ΔEN = ENH − ENH = 2.1 − 2.1 = 0 Cl2 : Cl—Cl ΔEN = EN Cl − EN Cl = 3.0 − 3.0 = 0 HCl : H—Cl ΔEN = EN Cl − EN H = 3.0 − 2.1 = 0.9 nonpolar covlent nonpolar covlent polar covlent The greater the difference in electronegaBvity between two atoms, the greater the polarity of their bond Practice Which would be more polar, a H-F bond or HCl bond? H- F 4.0 - 2.1 = 1.9 H- Cl 3.0 - 2.1 = 0.9 The HF bond is more polar than the HCl bond Nomenclature a correct and unambiguous name to each chemical compound, either ionic compounds or covalent compounds Formulas of Compounds A formula denotes compound using chemical symbols and numerical subscripts, idenBfies the number and type of the atoms that make up the compound unit The number of like atoms in the unit is shown by a subscript. Ionic Compounds •  Metals and nonmetals usually react to form ionic compounds •  The metals are caBons and the nonmetals are anions •  The caBons and anions arrange themselves in a regular three- dimensional repeaBng array called a crystal ladce •  Formula of an ionic compound is the smallest whole- number raBo of ions in the substance Formulas of Ionic Compounds •  Determine the charge of each ion –  Metals have a charge equal to group number –  Nonmetals have a charge equal to the group number minus eight •  CaBons and anions must combine to give a formula with a net charge of zero •  It must have the same number of posiBve charges as negaBve charges Predict Formulas Predict the formula of the ionic compounds formed from combining ions of the following pairs of elements: 1. sodium and oxygen 2. lithium and bromine 3. aluminum and oxygen 4. barium and fluorine Wri>ng Names of Ionic Compounds from the Formula of the Compound •  Name the caBon followed by the name of the anion •  A posiBve ion retains the name of the element; change the anion suffix to - ide If the caBon of an element has several ions of different charges (as with transiBon metals) use a Roman numeral following the metal name •  •  Roman numerals give the charge of the metal Examples: •  FeCl3 is iron(III) chloride •  FeCl2 is iron(II) chloride •  CuO is copper(II) oxide Common Nomenclature System •  Use - ic to indicate the higher of the charges that ion might have •  Use - ous to indicate the lower of the charges that ion might have •  Examples: •  FeCl2 is ferrous chloride •  FeCl3 is ferric chloride Common Monatomic CaBons and Anions •  Polyatomic ions - ions composed of 2 or more atoms bonded together with an overall posiBve or negaBve charge –  Within the ion itself, the atoms are bonded using covalent bonds –  The posiBve and negaBve ions will be bonded to each other with ionic bonds •  Examples:NH4+ (ammonium ion), SO42- (sulfate ion) Common Polyatomic CaBons and Anions Name These Compounds NH4Cl, BaSO4, Fe(NO3)3, CuHCO3, Ca(OH)2 Wri>ng Formulas of Ionic Compounds from the Name of the Compound •  Determine the charge of each ion •  Write the formula so that the resulBng compound is neutral •  Example: Barium chloride: Barium is +2, Chloride is - 1 Formula is BaCl2 Write the formula for the following ionic compounds: 1.  sodium sulfate 2 .  ammonium sulfide 3.  magnesium phosphate 4.  chromium(II) sulfate Covalent Compounds •  Covalent compounds are typically formed from nonmetals. •  Molecules - compounds characterized by covalent bonding. •  Not a part of a massive three- dimensional crystal structure •  Exist as discrete molecules in the solid, liquid, and gas states Naming Covalent Compounds 1)  The names of the elements are wriKen in the order in which they appear in the formula 2)  A prefix indicates the number of each kind of atom 3)  If only one atom of a parBcular element is present in the molecule, the prefix mono- is usually omiKed from the first element. Example: CO is carbon monoxide 4)  The stem of the name of the last element is used with the suffix –ide 5)  The final vowel in a prefix is omen dropped before a vowel in the stem name Name These Covalent Compounds 1.  SiO2 2 .  N2O5 3.  CCl4 4.  IF7 Wri>ng Formulas of Covalent Compounds •  Use the prefixes in the names to determine the subscripts for the elements •  Examples: •  nitrogen trichloride NCl3 •  diphosphorus pentoxide P2O5 •  Some common names that are used: –  –  –  –  H2O NH3 C2H5OH C6H12O6 water ammonia ethanol glucose Provide Formulas for These Covalent Compounds 1.  nitrogen monoxide 2 .  dinitrogen tetroxide 3.  diphosphorus pentoxide 4.  nitrogen trifluoride •  Physical State –  Ionic compounds are usually solids at room temperature –  Covalent compounds can be solids, liquids, and gases •  MelBng and Boiling Points –  Mel>ng point - the temperature at which a solid is converted to a liquid –  Boiling point - the temperature at which a liquid is converted to a gas Physical ProperBes •  MelBng and Boiling Points –  Ionic compounds have much higher melBng points and boiling points than covalent compounds –  A large amount of energy is required to break the electrostaBc aKracBons between ions –  Ionic compounds typically melt at several hundred degrees Celsius •  Structure of Compounds in the Solid State –  Ionic compounds are crystalline –  Covalent compounds are crystalline or amorphous – having no regular structure Physical ProperBes •  SoluBons of Ionic and Covalent Compounds –  Ionic compounds omen dissolve in water, where they dissociate - form posiBve and negaBve ions in soluBon –  Electrolytes - ions present in soluBon allowing the soluBon to conduct electricity –  Covalent solids usually do not dissociate and do not conduct electricity - nonelectrolytes Comparison of Ionic vs. Covalent Compounds Ionic Covalent Composed of metal + nonmetal nonmetals Electrons transferred shared Physical state solid / crystal any / crystal or amorphous DissociaBon Yes, electrolytes No, nonelectrolytes Boiling/MelBng High Low ...
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