9 - 1 The Octet Rule Except for hydrogen and helium, atoms are most energetically stable if they have a completely filled valence shell. A completely filled valence shell is called an octet because eight electrons are involved. Atoms will form ionic compounds by gaining or losing valence electrons or covalent compounds by sharing electrons.

9 - 2 Lewis Structures A simple way to show the valence electrons present in an atom. Valence electrons are those electrons found in the highest numbered principal energy level (PEL). Valence electrons are found only in the s and p sublevels and in most cases are the electrons responsible for bonding.

9 - 3 Lewis Structures The chemical symbol represents the kernel of the atom. The kernel of an atom consists of the nucleus and the core electrons. Example X s px px py py pz pz Start with s and proceed cw using Hund’s Rule!

9 - 4 Lewis Structures – Second Period LiBeBC NOFNe

9 - 5 Drawing Lewis Structures Count the number of valence electrons.  Remember, the number of valence electrons can be determined by the group (family) numbers. Draw a “skeleton” structure for the covalent structure assuming single (sigma) bonds. Terminal atoms will usually be hydrogen, oxygen, and halogens.

9 - 6 Determine the number of valence electrons that are left. Distribute the remaining electrons to the atoms surrounding the central atom to satisfy the octet rule. Distribute the remaining electrons as pairs around the central atom.

9 - 7 When all of the valence electrons have been used, ensure that the central atom has an octet.  If the central atom does not have an octet, form one or more double or triple (pi) bonds.  To form multiple bonds, move one or more pairs of electrons from a surrounding atom to the bond connecting the central atom.

9 - 8 Lewis Structure Examples NH 3 ammonia 1 N val e - 3 H N H H H H H H N shared pair unshared pair

9 - 9 OF 2 oxygen difluoride 1 O val e - 2 F O F 16 FF F F O - 12 F FO

HCNhydrogen cyanide 1 H val e - 1 C N H C N A triple bond is needed for both C and N to have a complete octet. 6

NH 4 + ammonium 1 N val e - 4 H NHH H H + A 1 is subtracted because a positively charged polyatomic ion has 1 less electron Square brackets are used for anions and cations.

O 2 oxygen 2 O val e O O O O O O

C 2 H 5 OHethyl alcohol (ethanol) 2 C val e - 6 H O H H HH HH H H H H CCO H H CC O

N 2 H 4 hydrazine 2 N val e - 4 H H H NN H H H H NN H H

Resonance When there is more than one equivalent Lewis structure, all the structures are given to represent the molecule or the polyatomic ion. Resonance can only occur when all the structures:  satisfy the octet rule  have the same type and number of bonds

Resonance NO 2 - nitrite 1 N val e - 2 O O O N N O O O O N O O N

Resonance -- O N O Note that the placement of the atoms in these structures is the same but the electron arrangement is different. O NO

Note that polyatomic ions are placed inside square brackets and the charge is placed after the brackets as a superscript. It is necessary to include all the structures separated by a double-headed arrow.

Exceptions to the Octet Rule Three major exceptions to the octet rule:  Molecules or ions with more than eight electrons around the central atom.  Species with fewer than eight electrons around the central atom.  Species with an odd number of valence electrons.

Expanded Octets Starting with period three, atoms have the capability to accommodate d electrons (3d). AX 4 E molecules such as SF 4 are able to accommodate 4 bonding pairs of electrons and one nonbonding pair of electrons. This results in S being surrounded by 5 electron pairs.

The favored bonding scenario includes large central atoms (starting in the third period) and small terminal atoms such as fluorine, chlorine, and oxygen. As shown below, S also has the ability to accommodate six pairs of valence electrons as found in SF 6.

Less Than an Octet Molecules having either boron or beryllium as their central atom result in the central atom having only 2 or 3 valence pairs of electrons. These molecules are very reactive with a molecule having an unshared pair of electrons. BeCl 2 BF 3

Odd Number of Valence Electrons Most molecules have an even number of valence electrons. In rare cases, molecules such as NO and NO 2, there is one unpaired electron which is very reactive.

Odd Number of Valence Electrons nitrogen(II) oxide nitrogen(IV) oxide N OONO

Electronegativity Electronegativity is a measure of the attraction of an element for a shared pair of electrons. H Cl Comparing the electronegativity values of hydrogen and chlorine, chlorine has a value of 3.2 and that of hydrogen is 2.2. δ-δ- δ+δ+

Range of Ionic Character % Ionic 100% 50% 5% 0% Character Δ EN 0.0 – 0.3Nonpolar Covalent 0.4 – 1.7Polar Covalent 1.8 – 4.0 Ionic

Properties of Ionic Compounds The building blocks of ionic compounds are cations (positive ions) and anions (negative ions). The force holding an ionic compound together is the ionic bond. Properties  have only empirical formulas

 high melting and boiling points  solids at room temperature (25°C, 298K)  conductors in the liquid phase (l) or in an aqueous solution (aq)  some are water soluble, some are not – check solubility chart

Nonpolar Covalent Compounds The building blocks of nonpolar covalent (molecular) compounds are molecules. The force holding a nonpolar molecule together is the covalent bond. Properties  have both empirical and molecular formulas

 low melting and boiling points  often a gas or a liquid at room temperature (25°C, 298K)  nonconductors of heat and electricity  insoluble in water  exists as discrete molecular units

Polar Covalent Compounds The building blocks of polar covalent (molecular) compounds are molecules. The force holding a polar molecule together is the covalent bond. Properties  have both empirical and molecular formulas

 usually higher melting and boiling points  often a gas or a liquid at room temperature (25°C, 298K)  some are conductors of heat and electricity  more likely to be water soluble