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Covalent Bonding & Molecular Compounds. Depicting Molecular Compounds. Key Terms. A molecule is a neutral group of atoms that are held together by covalent bonds. A molecular compound is a chemical compound whose simplest units are molecules.

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Covalent bonding molecular compounds

Covalent Bonding & Molecular Compounds

Depicting Molecular Compounds


Key terms
Key Terms

  • A moleculeis a neutral group of atoms that are held together by covalent bonds.

  • A molecular compoundis a chemical compound whose simplest units are molecules.

  • A chemical formulaindicates the relative numbers of atoms of each kind in a chemical compound by using atomic symbols and numerical subscripts.

  • A molecular formulashows the types and numbers of atoms combined in a single molecule of a molecular compound.

  • A diatomic moleculeis a molecule containing only two atoms.


Formation of a covalent bond
Formation of a Covalent Bond

  • Describe what happens to potential energy as a bond forms.

  • As a covalent bond forms, the attractive forces between the atoms overpower the repulsive forces between the atoms.

  • When the bond

    has formed,

    potential energy

    is at a minimum.



Characteristics of covalent bonds
Characteristics of Covalent Bonds

  • Bond lengthis the average distance between two bonded atoms.

  • By sharing electrons,

    each H atom has

    a full 1s orbital.

  • Bond energyis the energy required to break a chemical bond and form neutral, isolated atoms.


Bond length vs bond energy
Bond Length vs. Bond Energy

  • As bond length decreases, bond energy increases.

  • As bond length increases, bond energy decreases.

  • Therefore, the relationship between bond length and bond energy is an inverse relationship.


The octet rule
The Octet Rule

  • Chemical compounds tend to form so that each atom has an octet of electrons in its highest occupied energy level.

    • Octet = 8 electrons.

    • An octet results

      in fully filled

      s + p orbitals.


Exceptions to the octet rule
Exceptions to the Octet Rule

  • 1. Some elements are satisfied with fewer than 8 valence electrons:

    • Hydrogen only needs 2 valence electrons (1 bond).

    • Beryllium only needs 4 valence electrons (2 bonds).

    • Boron only needs 6 valence electrons (3 bonds).

  • 2. Some elements can be surrounded by more than eight electrons when they bind to highly electronegative atoms (F, O, Cl). Called an expanded octet.


Lewis structures
Lewis Structures

  • Electron-dot notation used to represent molecules.

    • Dotsrepresent unshared electrons.

      :

    • Dashesrepresent shared electron pairs.

      -

      • A shared pair of dots (electrons) is replaced with a dash.

      • A single dash represents a single bond.


Steps for drawing lewis structures
Steps for Drawing Lewis Structures

  • 1. Determine the type and number of atoms in the molecule.

    • For CH3I the atoms are:

      • 1 carbon, 3 hydrogen and 1 iodine.

    • Obtain the correct element cards from the bag.


Steps for drawing lewis structures1
Steps for Drawing Lewis Structures

  • 2. Write the electron-dot notation for each type of atom and then determine the total number of valence electrons.

    • Use different colored beads to represent the valence electrons around each type of atom.

    • C = 1 atom x 4 valence electrons = 4 val e-

    • H = 3 atoms x 1 valence electron = 3 val e-

    • I = 1 atom x 7 valence electrons = 7 val e-

    • TOTAL = 14 valence electrons


Steps for drawing lewis structures2
Steps for Drawing Lewis Structures

  • 3. Place atom with most open spaces (single electrons) in the middle.

    • Carbon will always be in the center if it is present.

    • Hydrogen and halogens will never be centrally located.

    • Arrange cards around the central atom.


Steps for drawing lewis structures3
Steps for Drawing Lewis Structures

  • 4a. Make sure each element (except hydrogen) is surrounded by eight valence electrons.

    • If not all atoms are surrounded by 8 valence electrons, move electron pairs to form double or triple bonds.

      • Hydrogen and halogens will never form double or triple bonds.


Steps for drawing lewis structures4
Steps for Drawing Lewis Structures

  • 4b. Replace bonding electron pairs with dashes.

    • Replace bonding electron pairs (beads) with straw sticks

  • 5. Check work by counting number of valence electrons.

    • Should have same number of electrons as in step 2.


Double bond example
Double Bond Example

CH2O

  • Step 1:

    • 1carbon, 2 hydrogen, 1 oxygen

  • Step 2:

    • 12 total valence electrons.

  • Step 3:


Double bond example1
Double Bond Example

  • Step 4a:

    • Carbon and oxygen are both one electron short. Move an electron pair between oxygen and carbon.

  • Step 4b:

  • Step 5:

    • Final structure has 12 valence electrons (same as in step 2).


Triple bond example
Triple Bond Example

CO

  • Step 1:

    • 1carbon & 1 oxygen

  • Step 2:

    • 10 total valence electrons.

  • Step 3:


Triple bond example1
Triple Bond Example

  • Step 4a:

    • Move two electron pairs between oxygen and carbon to satisfy octet rule for both.

  • Step 4b:

  • Step 5:

    • Final structure has 10 valence electrons (same as in step 2).


Single vs multiple bonds
Single vs. Multiple Bonds

  • Single bondsconsist of one shared electron pair.

  • Multiple bondsconsist of two of more shared electron pairs.

    • Double bonds have two shared electron pairs.

      • Have shorter bond length than single bonds.

      • Have greater bond energy than single bonds.

    • Triple bonds have three shared electron pairs.

      • Have shorter bond length than double bonds.

      • Have greater bond energy than double bonds.


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