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Chapter 10

Chapter 10. Molecular Structure: Solids and Liquids. Homework. Assigned Problems ( odd numbers only) “Questions and Problems” 10.1 to 10.53 (begins on page 292) “Additional Questions and Problems” 10.65 to 10.91 (page 325) “Challenge Questions” 10.93 to 10.99, (page 327).

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Chapter 10

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  1. Chapter 10 Molecular Structure: Solids and Liquids

  2. Homework • Assigned Problems (odd numbers only) • “Questions and Problems” 10.1 to 10.53 (begins on page 292) • “Additional Questions and Problems” 10.65 to 10.91 (page 325) • “Challenge Questions” 10.93 to 10.99, (page 327)

  3. Representing Valence Electrons with Dots: Electron-Dot Formulas The valence electrons are the most important (chemically) electrons in the atom These electrons are situated in the highest, occupied principal energy level (n) They are farthest from the nucleus, away from the stable (filled) inner core of electrons Determine the chemical properties of an element

  4. Representing Valence Electrons with Dots: Electron-Dot Formulas In the main-group elements, the valence electrons are the in the outermost s- and p- orbitals The Roman numeral at the top of each column represents the number of valence electrons in each member of that group The valence electrons for the main-group elements can be represented by Lewis Structures

  5. Representing Valence Electrons with Dots: Electron-Dot Formulas The Lewis (Dot) structure consists of an elements symbol with one dot per each valence electron placed about the element’s symbol The representative elements in the same group have the same number of valence electrons The maximum number of valence electrons for any element is eight

  6. Representing Valence Electrons with Dots: Electron-Dot Formulas • Only the noble gases have the maximum number of eight electrons • The exception is helium with only two valence electrons Octet rule Duet rule

  7. •• •• • •• •• Li• Be• •B• •C• •N• •O::F: :Ne: • • • • • • •• Representing Valence Electrons with Dots: Electron-Dot Formulas outer electron configuration ns1 ns2 ns2np1 ns2np2 ns2np3 ns2np4 ns2np5 ns2np6 Example: dot structures for representative elements in period 2 The valence electrons are in the outer s and p orbitals

  8. Representing Valence Electrons with Dots: Electron-Dot Formulas The force of attraction between two types of atoms is a chemical bond The ionic bond is the attraction between positive and negative ions Compounds held together by ionic bonds are called ionic compounds Each element forms an ion with a stable noble gas configuration

  9. Lewis Structures for Ionic Compounds: Electrons Transferred During ionic bond formation, ions form when atoms of two elements are present: An element that can lose electrons and an element that can gain electrons The most stable of all outer electron configurations is based on the chemical properties of the noble gases By chemical reactions and compound formation, elements can attain a stable outer electron configuration

  10. Lewis Structures for Ionic Compounds: Electrons Transferred • Metals: Low ionization energy, form ions (cations) • Nonmetals: High ionization energy, accept electrons (anions) • The representative elements lose or gain electrons to form ions with outer electron configuration of the nearest noble gas

  11. Electron Configuration of Ions • An ion: An atom that is electrically charged from loss or gain of electrons • Atoms are neutral due to the equal number of protons and electrons • Loss or gain of an electron will leave a net charge on the atom

  12. Electron Configuration of Ions • Consider sodium • Can attain the noble gas configuration (neon) if it loses 1 electron to obtain Na+ • Noble gas core remains but it does not have the same chemical properties of neon Na Na+ Loss of 1 e- 1s22s22p6 1s22s22p63s1 Electron configuration of neon

  13. Electron Configuration of Ions • Consider chlorine • Can attain the noble gas configuration (argon) if it gains 1 electron to obtain Cl- • Attains same E configuration of the nearest noble gas but it does not have the same chemical properties of argon Cl Cl- Gain of 1 e- 1s22s22p63s23p5 1s22s22p63s23p6 Electron configuration of argon

  14. Electron Configurations • We know for Representative Elements: • Group 1A metals form ions with +1 charge • Group 2A metals form ions with +2 charge • Group 7A nonmetals form ions with -1 charge • Group 6A nonmetals form ions with -2 charge • Group 8A nonmetals do not form ions, in fact they are extremely unreactive • Transition Elements: All of the elements of the d area of the periodic table • Elements differ in the number of electrons in the d subshell • Octet rule does not apply here • Loss of electrons does not lead to the noble gas structure

  15. Ions of the Metals of Groups IA, IIA, IIIA • Metals form cations by losing enough electrons to get the same electron configuration as the previous noble gas • A stable noble gas core attained in each case

  16. Ions of the Nonmetals of Group VIA, VIIA • Nonmetals form anions by gaining enough electrons to get the same electron configuration as the next noble gas • A stable noble gas core attained in each case

  17. Sodium Chloride Formed from sodium and chlorine atoms An ionic bond forms consisting of a sodium ion (+ charge) and a chloride ion (- charge) Each sodium loses one electron to achieve an octet Each chlorine atom gains one electron to achieve an octet Formula is NaCl •• •• Na• + ClNa+[ F ]- • •• Lewis Structures for Ionic Compounds: Electrons Transferred •• •• •• ••

  18. Magnesium Chloride Formed from magnesium and two chlorines An ionic bond forms consisting of a magnesium ion (2+ charge) and two chloride ions (- charge each) Each magnesium loses two electrons to achieve an octet Each chlorine atom gains one electron to achieve an octet Formula is MgCl2 Lewis Structures for Ionic Compounds: Electrons Transferred

  19. •• •• • •• •• Li• Be• •B• •C• •N• •O::F: :Ne: • • • • • • •• •• •• Li• Li1+:F: [:F:]1- • •• Lewis Structures for Ionic Compounds: Electrons Transferred • Bonding involves ONLY valence electrons • Transfer of the s and p valence electrons achieves stability of the nearest noble gas • Illustrates the sequence of atoms • Shows atoms and their valence electrons • How they are distributed in a molecule • Use a dot or X to represent an electron

  20. Lewis Structures for Ionic Compounds: Electrons Transferred • Give the number of valence electrons for Mg, N, and Br. Draw the Lewis dot symbol for each of these elements • Mg has 2 valence electrons • N has 5 valence electrons • Br has 7 valence electrons Mg N Br

  21. Covalent Lewis Structures:Electrons Shared Chemical bonding also occurs between two nonmetals Since nonmetals do not readily lose electrons, when two nonmetals bind together the electrons are shared A covalent bond is a pair of electrons shared by two atoms It is the binding force that results from two nuclei attracting the same shared electrons

  22. Covalent Lewis Structures:Electrons Shared • A covalent Lewis structure: A 2-D representation of how atoms are covalently bonded together • Each covalent bond is represented by a pair of dots (bonding electrons) • Must also show all unshared pairs of (nonbonding) electrons • All valence e- from every atom in a molecule must be accounted for in the form of bonds or nonbonding pairs

  23. Covalent Lewis Structures:Electrons Shared • The atoms in covalently bonded molecules often have bonding and nonbonding electrons • The number of covalent bonds that an atom forms is equal to the number of electrons needed to form a noble gas configuration (octet) • The exception is hydrogen which needs only two electrons • Shared electrons can be also represented by dashes

  24. Formation of a Hydrogen Molecule • The simplest covalent bonding condition • Hydrogen has one 1s electron • H atom requires one additional electron to obtain the stable noble gas configuration of helium • Each H atom contributes its one electron • The electron pair shared by the two atoms, forming diatomic hydrogen H2

  25. Covalent Lewis Structure: Electrons Shared • Duet Rule • Hydrogen wants two electrons to attain the noble gas configuration of helium • Octet Rule • All other main group elements want eight electrons to achieve the noble gas configuration • Filled valence shell is achieved by gaining/losing electrons or by sharing electrons

  26. Covalent Lewis Structures:Double and Triple Bonds • A single covalent bond is where two atoms share one pair of valence electrons • Many molecules exist that need two or three pairs of electrons to provide a complete octet of electrons per atom • Multiple covalent bonds: Covalent bonds where two pairs or three pairs of valence electrons are shared between the same two atoms

  27. Covalent Lewis Structures:Double and Triple Bonds • Two identical nonmetal atoms (diatomic molecules) • Each atom will share valence electrons with the other • The shared pair of electrons allow each atom to achieve a stable noble gas configuration • This configuration can be achieved by a single, double, or triple shared pair of electrons ●● ●● ●● ●●● ●●●

  28. Covalent Lewis Structures:Double and Triple Bonds • Single Bond is a chemical bond where two atoms share one pair of valence electrons • Double Bond is a chemical bond where two atoms share two pairs of valence electrons • Triple Bond is a chemical bond where two atoms share three pairs of valence electrons

  29. Two nonidentical nonmetal atoms The number of covalent bonds an atom forms will equal the number of electrons needed to form a stable, noble gas configuration Hydrogen follows the duet rule and forms a stable, helium noble gas configuration Sharing Electrons Between Atoms of Different Elements

  30. Oxygen has 6 valence electrons and 2 octet vacancies Can complete its octet by forming two covalent bonds Nitrogen has 5 valence electrons and 3 octet vacancies Can complete its octet by forming three covalent bonds Bonding Behavior of Elements Group 6A Group 5A

  31. Carbon has 4 valence electrons and 4 octet vacancies Can complete its octet by forming four covalent bonds Fluorine has 7 valence electrons and 1 octet vacancy Can complete its octet by forming one covalent bond Bonding Behavior of Elements Group 4A Group 7A

  32. Bonding Behavior of Elements Group 4A Group 5A Group 6A Group 7A

  33. Writing Lewis Structures for Covalent Compounds A dot structure is a two-dimensional representation of a molecule to show how atoms are joined together by covalent bonding To write a dot structure: A bond is shown as a pair of dots or a dash Dot structures also show the location of electron pairs not used in bonds All valence electrons from every atom in the molecule or PA ion must be accounted for

  34. Writing Lewis Structures for Covalent Compounds • Determine the arrangement of atoms within a molecule • If there are three or more atoms, the central atom (usually) appears only once in the formula • Halogens are often terminal atoms (at the ends) unless it is combined with O as in oxyacids • Hydrogen is always a terminal atom

  35. Writing Lewis Structures for Covalent Compounds • Determine the total number of valence electrons • For main-group elements the group numbers equal the number of valence electrons for the element of that group • If it is an anion, add one electron to the total for each negative charge • If it is a cation, subtract one electron from the total for each positive charge

  36. Writing Lewis Structures for Covalent Compounds • Attach the central atom to each bonded atom by a pair of electrons • Subtract two electrons (from total valence) for each single bond drawn in the structure

  37. Writing Lewis Structures for Covalent Compounds • Distribute the remaining electrons • Add electrons to each atom bonded to the central atom until each has eight electrons (octet rule) • Exception: hydrogen (duet rule) • Any extra electrons should go to the central atoms

  38. Writing Lewis Structures for Covalent Compounds • If the central atom does not fulfill the octet rule, share one or more lone pairs between a terminal atom and the central atom (form multiple covalent bonds) • Double or triple bonds are formed ONLY when one or both of the atoms are C, N, O or S

  39. Writing Lewis Structures for Covalent Compounds Example: Create the electron-dot formula for Fluorine gas = F2 • Determine the arrangement of the atoms F F ● ● • Determine the total number of valence electrons for the dot structure (2 × 7 = 14) • Begin with bonding electrons • Distribute the remaining electrons among the two atoms, giving octets to each fluorine atom, then proceed to lone pairs on each atom

  40. Writing Lewis Structures forCovalent Compounds and Polyatomic Ions • Write a Lewis structure for the following: • NH4+ • SO42- • CO • SCN-

  41. Writing Lewis Structures forCovalent Compounds: CO • Determine the arrangement of the atoms • C: 4 electrons • O: 6 electrons • Total valence electrons: 10 • If octets are not complete, form one or more multiple covalent bonds

  42. Writing Lewis Structures forPolyatomic Ions: NH4+ • Determine the arrangement of the atoms • Total number of valence e- • N: 5 valence electrons • H: 4×1 valence electron • Positive Charge: Subtract one electron • Total valence electrons: 8

  43. Writing Lewis Structures forPolyatomic Ions: SO42- • Determine the arrangement of the atoms • Total number of valence electrons • S: 6 valence electrons • O: 4 x 6 valence electrons • Negative Charge: Add two electrons • Total valence electrons: 32

  44. Writing Lewis Structures forPolyatomic Ions: SCN- • Determine the arrangement of the atoms • Total number of valence electrons: • S: 6 electrons • C: 4 electrons • N: 5 electrons • Charge: add one electron • Total electrons: 16 • If octets are not complete, form one or more multiple covalent bonds

  45. Resonance: Equivalent Lewis Structuresfor the Same Molecule • Resonance occurs when no single dot structure adequately describes bonding in molecule • Resonance structures are two or more dot structures for a molecule or ion that has the same arrangement of atoms • Contain the same number of electrons • Differ only in the location of electrons • The true structure is the average of the individual structures

  46. Writing Resonance Structures • Write a Lewis structure for nitrate ion and include resonance structures • Determine the arrangement of the atoms • Total number of valence electrons • Use a double-headed arrow to connect the structures (3 ×6) + 5 + 1 = 24 valence elec 2 2 2 1 1 1 3 3 3

  47. Writing Resonance Structures • Write a Lewis structure for ozone and include resonance structures (O3) • Determine the arrangement of the atoms • Total number of valence electrons • The actual structure is called a “resonance hybrid” of the two contributing structures 3 ×6 = 18 valence elec Resonance structures Hybrid

  48. Writing Resonance Structures

  49. Predicting the Shapes of Molecules • (Lewis) Dot structures describe the distribution of valence electrons among bonding pairs and nonbonding pairs • They do not give info on the 3-D shape of the molecule • The 3-D arrangement of atoms in a molecule is determined by its molecular geometry

  50. Predicting the Shapes of Molecules • A Lewis structure for water • Are the atoms arranged in a straight line or do they form a v-shape? •• •• •• ••

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