Chapter 6 Chemical Bonding

Transcription

1 Chapter 6 Chemical Bonding

2 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?

3 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.

4 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.

5 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.

6 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. 6

7 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!! = Non-polar covalent = Polar covalent = Ionic

8 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.

9 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

10 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?

11 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.

12 Formation of a Covalent Bond

13 Formation of a Covalent Bond (cont d)

14 The Octet Rule

15 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.

16 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.