Chapter 6 Chemical Bonding  

Section 6.1: Introduction to Chemical Bonding Things That You Should Know  

•  What is a chemical bond? •  Why do atoms form chemical bonds?

•  What is the difference between ionic and covalent bonding?

•  Why are most chemical bonds neither purely ionic nor purely covalent?

•  How can you determine the type of bonding that will result between any 2 elements?

Why Bond?  

•  Atoms rarely exist singly; instead, they exist in combinations with other atoms held together by chemical bonds.

•  Chemical bonds are mutual, electrical attractions between the nuclei and valence electrons of different atoms that bind the atoms together.

•  As independent particles, atoms have relatively high potential energy. However, nature prefers more stable arrangements.

•  Chemical bonds provide the atoms a low energy (more stable) arrangement.

Types of Bonding  

•  There are 3 main types of bonding between atoms.

•  Ionic •  Covalent

•  Metallic •  When atoms bond, their valence electrons are

redistributed. This redistribution determines which type of bond will result.

Types of Bonding (cont’d)  

•  Ionic bonding results from electrical attraction between cations (+ ions) and anions (- ions).

•  Covalent bonding results from the sharing of valence electrons between atoms.

Ionic or Covalent???  

•  Purely ionic or purely covalent bonds are extremely rare due to the attractive forces between atoms and electrons.

•  By using the attractiveness of atoms to electrons (electronegativity), we can determine whether a bond is ionic or covalent.

•  In particular, if we use the two elements’ electronegativities, we can predict bond type.

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Ionic or Covalent??? (cont’d)  

•  In order to predict bond type, we need a table of electronegativities. [Refer to Figure 5.20 on page 151 for such a table!]

•  Guidelines to Know!! •  0.0 – 0.3 = Non-polar covalent

•  0.3 – 1.7 = Polar covalent •  1.7 – 3.3 = Ionic

Non-polar vs. Polar  

•  In order for a bond to be classified as either non-polar or polar, we must look at the sharing of the valence electrons.

•  If there is an unequal sharing of electrons, the bond will be polar. As a result, a partial positive charge and partial negative charge will develop on the molecule.

•  If there is an equal sharing of electrons, the bond will be non-polar. In this instance, the atoms are either the same or are very similar.

What’s the Difference?  •  Classify the type of bonding that will result between

the following pairs of elements. •  Sulfur and hydrogen •  Chlorine and calcium •  Sodium and iodine •  Hydrogen and fluorine •  Cesium and chlorine •  Oxygen and fluorine

Section 6.2: Covalent Bonding and Molecular Compounds

Things You Should Know  •  What is a molecule? •  What is a molecular formula?

•  What are the relationships between potential energy, distance between approaching atoms, bond length, and bond energy?

•  What is the octet rule? •  What are the 6 basic steps used to write Lewis

structures?

Molecules, Molecules, Molecules  

•  Many compounds that surround us and that we encounter exist in molecular form.

•  Molecules are neutral groups of atoms that are held in place by covalent bonds.

•  Molecules can be composed of either only one type of atom or more than one type.

•  We use chemical formulas to tell what types and numbers of atoms are in compounds.

•  Molecular formulas tell how many atoms and what type of atom are in a molecular compound.

Formation of a Covalent Bond  

Formation of a Covalent Bond (cont’d)  

The Octet Rule  

The Octet Rule (cont’d)  •  Noble gas atoms are unreactive because their electron

configurations are especially stable.

– This stability results from the fact that the noble-gas atoms’ outer s and p orbitals are completely filled by a total of eight electrons.

•  Other atoms can fill their outermost s and p orbitals 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.

Are there any exceptions?  •  Exceptions to the octet rule include those for atoms that cannot fit

eight electrons, and for those that can fit more than eight electrons, into their outermost orbital.

– Hydrogen forms bonds in which it is surrounded by only two electrons.

– Boron has just three valence electrons, so it tends to form bonds in which it is surrounded by six electrons.

– Main-group elements in Periods 3 and up can form bonds with expanded valence, involving more than eight electrons.