1. Chemical Bonding
Chapter 6

2. In nature, most atoms are joined to other atoms by chemical bonds.
Chemical bond: a mutual attraction between the nuclei & valence electrons of different atoms that binds the atoms together

3. Why bond?
Most atoms are less stable (or at a higher potential energy) by themselves than when they are combined with other atoms.

4. Types of chemical bonds
Ionic bonding
Atoms completely give up electrons to other atoms
Metal with a nonmetal
Covalent bonding
Electron pairs are shared between atoms
Nonmetal with a nonmetal
Bonding occurs as a result of redistribution of valance electrons

5. Covalent Bonding
Molecule: a neutral group of atoms that are held together by covalent bonds
Molecular compound: a chemical compound whose simplest units are molecules
Chemical formula: indicates the relative number of atoms of each kind in a chemical compound by using atomic symbols & numerical subscripts
H2SO4
Molecular formula: shows the types & numbers of atoms combined in a single molecule of a molecular compound
H2O
Diatomic molecule: a molecule containing only 2 atoms O2

6. Characteristics of a Covalent Bond
Bond length: the average distance between 2 bonded atoms
Bond energy: energy required to break a chemical bond & form neutral isolated atoms
Units are kJ/mol
Bond lengths & energies vary with the types of atoms that have combined.
See Table 6-1 on page 168

7. Octet Rule
Chemical compounds tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its outermost ring.

8. Electron-Dot Notation
Electron-dot notation is an electron configuration notation in which only the valence electrons of an atom of an element are shown
Indicated by dots placed around the elements symbol

9. Cont’d…
Electron-dot structures can also be used to represent molecules
Dots between atoms represent shared electrons
Unshared, or lone pair, electrons: pair of electrons not involved in bonding

10. Cont’d…
Lewis structures: formulas in which atomic symbols represent nuclei & inner shell electrons, dot-pairs or dashes between 2 atomic symbol represent electron pairs in covalent bonds, & dots adjacent to only one atomic symbol represent unshared electrons
Structural formula: lewis structures written to show only the electrons that are shared, using dashes to represent the bonds

11. Draw the Lewis structure for iodomethane, CH3I.
Problem
Draw the Lewis structure for iodomethane, CH3I.

12. Covalent Bond Types
Single bond: produced by the sharing of one pair of electrons

13. Covalent Bond Types
Double bond: sharing of 2 pairs of electrons between atoms
Triple bond: sharing of 3 pairs of electrons
Double & triple bonds are known as multiple bonds.

14. Draw the Lewis structure for formaldehyde, CH2O.
In writing Lewis structures for molecules that contain C, N, & O, multiple bonds between pairs are possible.
Draw the Lewis structure for formaldehyde, CH2O.

15. Resonance
Some molecules cannot be represented by a single Lewis structure
Ozone, O3
Resonance: refers to bonding in molecules or ions that cannot be correctly represented by a single Lewis structure
Actually has an average of 2 structures

16. Ionic Bonds
Ionic compound: composed of positive & negative ions that are combined so that the numbers of positive & negative charges are equal
Most exist as crystalline solids
Held together more strongly than covalent compounds

17. Cont’d…
Formula unit: the simplest collection of atoms from which an ionic compounds formula can be established
The ratio of ions in a formula unit depends on the charges of the ions combined
Calcium fluoride
Ca+2 F-1 CaF2

18. Polyatomic Ions A charged group of covalently bonded atoms NH4+
Combine with oppositely charged ions to form ionic compounds
Charge results from an excess of electrons (negative) or a shortage of electrons (positive)

19. Properties of Ionic and Covalent Compounds
1. Ionic compounds have high melting and boiling points, while covalent compounds have low melting and boiling points.

20. Properties cont’d…
2. Ionic compounds conduct electricity when dissolved in water or molten. Covalent compounds never conduct electricity.

21. Properties cont’d…
3. Ionic compounds rarely burn, while covalent compounds burn more frequently.

22. Properties cont’d…
4. Ionic compounds are hard and brittle, while covalent compounds have a wide variety of textures.

23. Properties cont’d…
5. Ionic compounds form crystals, while covalent compounds have many different appearances.