Chemical Bonding Chapter 6

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

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

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

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

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

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.

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

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

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

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

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

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.

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.

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

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

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

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)

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

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

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

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

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