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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.
The Octet Rule When two atoms form a covalent bond, their shared electrons form overlapping orbitals. This achieves a noble-gas configuration. The bonding of two hydrogen atoms allows each atom to have the stable electron configuration of helium, 1s2.
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.