SECTION 1. INTRODUCTION TO CHEMICAL BONDING CHEMISTRY CHAPTER 6. CHEMICAL BONDING SECTION 1. INTRODUCTION TO CHEMICAL BONDING Chemical bond = electrical attraction between nuclei and valence electrons of different atoms, binding the atoms together.

Three types of bonds: Ionic: some atoms give up electrons to other atoms; anions and cations attract Covalent: sharing of pairs of electrons between atoms Metallic: attraction between positive metal atoms and the surrounding “sea” of electrons

Covalent Bonding - 75482 Ionic Bonding - 75108 Chemical Bond - 75107

Difference in electro-negativity % ionic character type of bond example 0 to 0.3 0 to 5 nonpolar-covalent Cl-Cl 3.0 - 3.0 = 0 0.3 to 1.7 5 to 50 polar-covalent O-H 3.5 – 2.1 = 1.4 more than 1.7 more than 50 ionic NaCl 3.0 (Cl) – 0.9 (Na) = 2.1

Comparing Polar and Nonpolar Covalent Bonds

Using Electronegativity Difference to Classify Bonding 75110

SECTION 2. COVALENT BONDING AND MOLECULAR COMPOUNDS A molecule is a neutral group of atoms that are held together by covalent bonds. A chemical compound whose simplest units are molecules is called a molecular compound.

A molecule is a neutral group of atoms that are held together by covalent bonds. In organic chemistry and biochemistry, “molecule” is also used to refer to such groups even if they are charged.

Molecules

The composition of a compound is given by its chemical formula. A chemical formula indicates the relative numbers of atoms of each kind in a chemical compound by using atomic symbols and numerical subscripts. If the number is “1,” the subscript is omitted. Ex. – H2O, not H2O1

For a molecular compound, the chemical formula (molecular formula) shows the types and numbers of atoms combined in a single molecule. Ex. – glucose C6H12O6 A diatomic molecule has two atoms. Ex. – O2, HCl

Chemical Formula 75113

Review from Chapter 5 notes: The Octet Rule Review from Chapter 5 notes: Main group elements tend to get sets of 8 electrons in their highest occupied energy levels. This set of 8 electrons is called an octet.

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 or transferring electrons in bonding. In this section we will discuss covalent bonding; later we will see how this works with ionic 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.

When two atoms share a pair of electrons in a covalent bond, the electrons in the bond help both atoms achieve octets (or, for H, a filled shell of 2 electrons).

Cl has 7 valence electrons. Ex. – HCl H has one electron. Cl has 7 valence electrons. In forming a covalent bond, each contributes one electron. The H now “has” 2 e- (from the bond) The Cl now “has” 8 valence e- (2 from the bond, plus the remaining 6) avon-chemistry.com

Exceptions to the Octet Rule 1. Hydrogen forms bonds in which it is surrounded by only two electrons, since this fills the 1st main energy level. 2. Boron has just 3 valence electrons, so it tends to form bonds in which it is surrounded by 6 electrons.

3. Some atoms can make 5 or 6 bonds, so they have more than 8 electrons around them. Ex. – PF5 3dchem.com

The Octet Rule 75123

The inner-shell electrons are not shown. Electron-Dot Notation Represents valence electrons, using dots placed around the element’s symbol. The inner-shell electrons are not shown.

Electron-Dot Notation 75124

Sample Problem: Write the electron-dot notations for hydrogen and nitrogen. a. A hydrogen atom has only one occupied energy level, the n = 1 level, which contains a single electron. b. The group notation for nitrogen’s family of elements is ns2np3. Nitrogen has 5 valence electrons.

Lewis Structures Lewis structures use electron-dot notation to represent molecules. Example: H2 The pair of dots between the two symbols represents the shared electron pair of the hydrogen-hydrogen covalent bond.

For a molecule of fluorine, F2, the electron-dot notations of two fluorine atoms are combined. The pair of dots between the two symbols represents the shared pair of a covalent bond. In addition, each fluorine atom is surrounded by three pairs of electrons that are not shared in bonds.

An unshared pair, also called a lone pair, is a pair of electrons that is not involved in bonding and that belongs exclusively to one atom. The pair of dots representing a shared pair of electrons in a covalent bond is often replaced by a long dash. examples:

Lewis Structures 75126

Lone Pair of Electrons

A structural formula indicates the kind, number, and arrangement, and bonds but not the unshared pairs of the atoms in a molecule. example: F–F H–Cl

Structural Formula 75115

Multiple Covalent Bonds A double bond is a covalent bond in which 2 pairs of electrons are shared between two atoms. Double bonds are often found in molecules containing carbon, nitrogen, and oxygen. A double bond is shown either by two side-by-side pairs of dots or by two parallel dashes.

A triple bond is a covalent bond in which 3 pairs of electrons are shared between two atoms. example 1—diatomic nitrogen: example 2—ethyne, C2H2:

Comparing Single, Double, and Triple Bonds 75116

IONIC COMPOUNDS Most of the rocks and minerals that make up Earth’s crust consist of positive and negative ions held together by ionic bonding. example: table salt, NaCl, consists of sodium and chloride ions combined in a one-to-one ratio—Na+Cl–—so that each positive charge is balanced by a negative charge.

SECTION 3. IONIC BONDING AND IONIC COMPOUNDS An ionic compound is composed of positive and negative ions that are combined so that the numbers of positive and negative charges are equal. So the overall charge is zero.

Most ionic compounds exist as crystalline solids. A crystal of any ionic compound is a three-dimensional network of positive and negative ions attracted to each other.

In contrast to a molecular compound, an ionic compound is not composed of independent, neutral units that can be isolated. The chemical formula of an ionic compound represents the simplest ratio of the compound’s ions.

A formula unit is the simplest collection of atoms from which an ionic compound’s formula can be established. Example – MgCl2 A formula unit is 1 Mg2+ ion and 2 Cl- ions.

The ratio of ions in a formula unit is one that gives no overall charge. Example 1: Na+ and Cl- 1:1 formula is NaCl (if no subscript number, it is understood to be “1”)

Example 2: Mg2+ and Cl- 2 Cl- ions are needed to balance one Mg2+, so the formula is MgCl2. Example 3: What would the formula be for Al3+ and S2-?

Example 2: Mg2+ and Cl- 2 Cl- ions are needed to balance one Mg2+, so the formula is MgCl2. Example 3: What would the formula be for Al3+ and S2-? Answer: Al2S3 Al: 2 x (+3) = +6 S: 3 x (-2) = -6

Ionic vs. Covalent Bonding

Formation of Ionic Compounds We can show the formation of an ionic compound using electron-dot diagrams. Ex. 1: NaCl

Note octets in the final ions. Ex. 2: MgI2 I + Mg + I → I- + Mg2+ + I- Note octets in the final ions.

In an ionic crystal, ions minimize their potential energy by combining in an orderly arrangement known as a crystal lattice. Attractive forces exist between oppositely charged ions within the lattice. Repulsive forces exist between like-charged ions within the lattice. The structure of the crystal depends on the sizes and charges of the ions (see Fig. 14 and 16).

Characteristics of Ion Bonding in a Crystal Lattice 75129

NaCl and CsCl Crystal Lattices

A Comparison of Ionic and Molecular Compounds The forces of attraction between ions are much stronger than those between molecules (within a molecule the covalent bonds are strong). So ionic compounds generally have higher melting points and boiling points than molecular compounds.

Melting and Boiling Points of Compounds

Ionic compounds are hard but brittle Ionic compounds are hard but brittle. The atoms cannot shift their positions easily because this would bring similar charges (which repel) closer together.

Ionic compounds are not electrical conductors in the solid state, because the ions cannot move in order to carry electric charges. They are conductors when melted. Some can dissolve in water, and then the solutions are conductors because there are dissolved ions that can move.

Comparing Ionic and Molecular Compounds 75127

Polyatomic Ions Certain atoms bond covalently with each other to form a group of atoms that has both molecular and ionic characteristics. A charged group of covalently bonded atoms is known as a polyatomic ion.

Like other ions, polyatomic ions have a charge that results from either a shortage or excess of electrons. The polyatomic ion behaves as a unit in forming compounds. The charge belongs to the group as a whole.

Lewis structures for polyatomic ions are often written in brackets. Examples: NH4+ NO3- SO42-

In drawing Lewis structures, determine the total valence electrons of the atoms, and correct for the charge. Give all atoms octets (or 2 e- for H) if possible using single, double, and triple bonds.

Ex: NH4+ : N has 5 valence electrons; 4 H’s each have 1; subtract 1 for positive charge NO3- : 5 + (3 x 6) + 1 = 24 SO4-2 : 6 + (4 x 6) + 2 = 32 (see earlier slide for structures)