Question

(a) State the octet rule. (b) Does the octet rule apply to ionic as well as to covalent compounds? Explain, using examples as appropriate.

Short answer

(a) The octet rule states that atoms of elements tend to combine such that each atom has eight electrons in its valence shell, thus achieving the electron configuration of a noble gas. (b) The octet rule generally applies to both ionic and covalent compounds. In ionic compounds, such as sodium chloride (NaCl), electrons are transferred between atoms to achieve a full octet and form ionic bonds. In covalent compounds, such as water (H2O), electrons are shared between atoms to achieve a full octet and form covalent bonds. Nevertheless, exceptions to the octet rule exist, particularly in transition metals.

Step-by-step solution

(a) Define the Octet Rule

The octet rule is a chemical rule of thumb that states that atoms of elements tend to combine in such a way that each atom has eight electrons in its valence shell. This is done to achieve the electron configuration of a noble gas, which is considered stable.

(b) Discuss the Octet Rule in Ionic Compounds

In the case of ionic compounds, the octet rule often applies. Ionic compounds are formed by the transfer of electrons between a metal and a non-metal atom. The metal loses electrons from its valence shell and the non-metal gains electrons to complete its valence shell. This transfer of electrons results in the formation of positively charged cations and negatively charged anions which then attract each other, forming an ionic bond. One example of an ionic compound is sodium chloride (NaCl). In NaCl, sodium (Na) loses one electron to chlorine (Cl), resulting in both atoms obtaining a full octet and forming an ionic bond between them.

(b) Discuss the Octet Rule in Covalent Compounds

Covalent compounds are formed by the sharing of electrons between non-metal atoms in order to achieve the full octet in their valence shells. In most cases, the octet rule applies to covalent compounds. One example of a covalent compound is water (H2O). In water, each hydrogen atom (H) shares one electron with the oxygen atom (O), and oxygen shares two electrons with each hydrogen atom. This electron sharing results in each hydrogen atom having a full (2-electron) shell and the oxygen atom having a full octet, forming covalent bonds between them. In conclusion, the octet rule generally applies to both ionic and covalent compounds. However, it is important to note that there may be exceptions to this rule, especially in the case of transition metals, where achieving a full octet may not always be necessary to achieve stability.