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Lewis Symbols. To help us to focus on the valence electrons – those that can participate in bonding - we use Lewis Symbols (in honor of scientist G.N. Lewis). Lewis Dot Symbols. Lewis Dot symbol (or Electron dot symbol) Dots placed around an element ’ s symbol represent valence electrons

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Lewis Symbols

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Lewis Symbols

  • To help us to focus on the valence electrons – those that can participate in bonding - we use Lewis Symbols (in honor of scientist G.N. Lewis)


Lewis Dot Symbols

  • Lewis Dot symbol (or Electron dot symbol)

    • Dots placed around an element’s symbol represent valence electrons

    • Pair electrons as needed

    • Ions are placed in brackets with charge outside

    • Easily shows “Octet rule”

      • Tendency of an atom to achieve an electron configuration having 8 valence electrons

        • Same as the electron configuration of a noble gas

        • The 8 electrons shown as 4 pairs


Group Practice:

  • Write out dot diagrams showing the valence electrons of the following atoms. Use principles of electron configuration to predict and explain the ionic compoundseach pair will react to form:

    • Magnesium fluoride

    • Aluminum oxide

    • Nickel(II) chloride


Molecular Compounds

  • Write the orbital diagram for a hydrogen atom.

  • We have seen evidence from gaseous reactions that elemental hydrogen exists as a diatomic molecule. Use your orbital diagram to explain why hydrogen atoms would be more stable (lower chemical energy) as H2 molecules.


Electron Dot Diagrams for theDiatomic Elements

  • How does hydrogen obtain a noble-gas electron configuration?


Covalent Compounds and Bonding

  • When two nonmetals form a compound, the bond between atoms is covalent.

  • Both atoms are close to the noble-gas electron configuration, so sharing electrons will allow both to obtain it.

  • In a covalent bond, each shared electron is attracted simultaneously to two nuclei.


Covalent Bonds

  • Electrons not transferred in this case

  • Electrons typically shared in pairs


Carbon Dioxide Example

  • The atoms of CO2 molecules are held together by strong covalent bonds.

  • No bonds connect the molecules, so CO2 molecules separate easily from each other into the gas state at room temperature.

Figure 8.13

Figure from p. 28


Whiteboard Practice

  • Use valence electron concepts and electron dot diagrams to represent these compounds:

    • HF

    • CF4

    • O2

    • CO2


The Octet Rule

  • Just as in ionic bonding, covalent bonds are formed so that each atom can have the noble-gas electron configuration. Noble gases have 8 valence electrons, an octet.


The Halogens

  • Do the atoms in each of these molecules have an octet?

  • Why do the halogens exist as diatomic molecules?


Multiple Bonds

  • How many valence electrons does an oxygen atom have?

    • How many does it need to obtain an octet?

    • O2 has a double bond, two pairs of shared electrons

  • How many valence electrons does a nitrogen atom have?

    • How many does it need to obtain an octet?

    • N2 has a triple bond, three pairs of shared electrons


The Octet

  • An unreactive or stable compound usually has the maximum number of valence electrons per core (8)

    • Same as the electron configuration of a noble gas

    • Covalently bonded atoms achieve 8 valence electrons by sharing electrons

    • The 8 electrons exist in 4 pairs

  • H atoms bond with other atoms to obtain a total of 2 electrons like He (duet).


Practice: Valence Electrons and Number of Bonds

  • How many bonds do each of the following atoms tend to form?

    • H

    • Cl

    • O

    • N

    • C


Carbon Compounds

Figure 8.21

  • Carbon has:

    • Four valence electrons

    • The ability to form four bonds

    • The ability to bond to itself

    • Very strong bonds when bonded to itself

  • Carbon molecules are ubiquitous in nature.

  • Aside from what I’ve mentioned here, skip “Bonding in Carbon Compounds”, pp. 307-310 (top).


Hydrocarbons

  • Aromatic hydrocarbons

    • A class of hydrocarbons which has carbon atoms arranged in a six-atom ring with alternating single and double bonds

    • Delocalized structures

Figure 8.22

Figure 8.22


Functional Groups in Hydrocarbons


Ionic and Covalent

  • In ionic compounds, ions are held together by electrostatic forces – forces between oppositely charged ions.

  • In molecular compounds, atoms are held together by covalent bonds in which electrons are shared.

Figure 8.2


Activity: Identifying Types of Bonding

  • Identify the type of bonding in each of the following substances:

    • NaF

    • ClO2

    • FeSO4

    • SO2

    • Ca(ClO2)2


Steps for Writing Lewis Structures

  • Write an atomic skeleton.

  • Sum the valence electrons from each atom to get the total number of valence electrons.

  • Place two electrons, a single bond, between each pair of bonded atoms (can also be drawn as a line)

  • Place remaining valence electrons to complete the octet of each outer atom. These are called non-bonding electrons or lone pairs. If there are “extra” electrons, place the pairs around the central atom.

  • If necessary to satisfy the octet rule, shift unshared electrons from non-bonded positions on atoms with completed octets to positions between atoms to make double or triple bonds.


Activity: Lewis Structures

  • Draw Lewis structures to show how electrons are shared in these molecules.

    • C2H6

    • C2H4

    • C2H2

    • HCN

    • CO2

    • NH3


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