4.1 Introduction to Chemical Bonding: The Octet Rule

The electron configurations of atoms can help us understand how atoms combine to form compounds. In this module, we will focus only on compounds being formed from main group elements. (The main group elements are those from the s and p blocks. The octet rule is a simple, yet very useful rule.)

Atoms tend to acquire the electron configuration of the closest noble gas through sharing or transferring electrons. Transferring or sharing electrons leads to the formation of bonds between atoms.

Recall, all the noble gases (except for He) have the ns2np6 arrangement as part of their valence configuration. These eight electrons form an “octet” of electrons.

Ionic bonds form when one or more electrons are transferred from one atom to another, leading to the creation of oppositely charged monatomic ions. For example, if an electron is transferred from atom X to atom Y, X becomes a positively charged cation, X+ and Y becomes a negatively charged anion, Y−. The electrostatic attraction between X+ and Y− ions results in an ionic bond.

Covalent bonds form when two atoms share one or more pairs of electrons. Each shared pair of electrons creates one covalent bond, so atoms can be held together by single, double, or triple bonds depending on the number of electron pairs.

Recall:

• metals from groups 1 and 2 have low IE’s and low EA’s

• atoms from groups 16, 17 very large IE’s and very large EA’s

We anticipate that group 1 and 2 metals prefer to lose electrons while atoms from groups 14-17 prefer to gain electrons. The following guidelines are useful.

(1) When metals from the s block combine with atoms from the p block, particularly atoms from

Groups 16 and 17, they typically form ionic compounds with oppositely charged ions held

together by ionic bonds.

(2) When p-block atoms combine with each other to form a compound, they typically form molecular compounds with covalent bonds holding the atoms together.

On Your Own: Consider the ground state electron configuration of the atoms indicated below. Use this information, together with the octet rule to answer the following questions. Keep in mind that the atoms can gain noble gas configuration by transferring electrons and becoming ions or sharing electrons to form a molecular compound. (Review ionic versus Molecular Compounds in Module 1).

(i) What are the ions most commonly formed by Na, O, and Cl in their compounds?

(ii) What type of bond (ionic or covalent) is formed between Na and O? What is the formula of the binary compound formed?

(iii) What type of bond (ionic or covalent) is formed between Na and Cl? What is the formula of the binary compound formed?

CHEM 120/121 Module 4: Chemical Bonding and Structure page 2 of 22

On Your Own: Consider the ground state electron configuration of the atoms indicated below. Use this information, together with the octet rule to answer the following questions. Keep in mind that the atoms can gain noble gas configuration by transferring electrons and becoming ions or sharing electrons to form a molecular compound. (Review ionic versus Molecular Compounds in Module 1).