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 tend to combine in such a way that they each have eight electrons in their outermost electron shell, resulting in a more stable electron configuration. (b) Yes, the octet rule generally applies to both ionic and covalent compounds, though there are some exceptions. In ionic compounds, such as sodium chloride (NaCl), metal atoms transfer electrons to non-metal atoms, forming cations and anions that have complete octets. In covalent compounds, like water (H2O), atoms share electrons to achieve stable electron configurations with eight valence electrons (octet rule) or two valence electrons (duet rule, as in the case of hydrogen). However, elements in the third period and beyond may exceed the octet due to the presence of d-orbitals, and some elements like hydrogen follow the duet rule.

Step-by-step solution

(a) State the octet rule.

(In chemistry, the octet rule states that atoms tend to combine in such a way that they each have eight electrons in their outermost electron shell, also known as the valence shell, resulting in a more stable electron configuration. This rule is based on the fact that a complete set of eight electrons provides the most stable electron configuration for an atom.)

(b) Does the octet rule apply to ionic as well as covalent compounds?

(Yes, the octet rule generally applies to both ionic and covalent compounds, though there are some exceptions. An ionic compound forms when a metal atom transfers one or more of its electrons to a non-metal atom, whereas a covalent compound forms when two non-metal atoms share one or more pairs of electrons. In both cases, the atoms involved form bonds to reach a stable electron configuration with eight electrons in their outermost shell. However, some elements, particularly those in the third period and onward in the periodic table, may exceed the octet due to the presence of d-orbitals. Additionally, some elements like hydrogen obey the "duet rule," forming stable compounds with only two electrons in their outermost shell.)

Examples of the octet rule in ionic compounds

(In ionic compounds, the transfer of electrons between atoms results in the formation of positively charged cations and negatively charged anions. For example, consider the formation of sodium chloride (NaCl). Sodium (Na) has one electron in its outermost shell and needs to lose one to achieve stable configuration akin to a noble gas (like Neon), while chlorine (Cl) has seven electrons in its valence shell and requires one additional electron to achieve an octet. Sodium transfers its single valence electron to chlorine, becoming a positively charged Na^+ ion, and chlorine becomes a negatively charged Cl^- ion with a filled octet. The positively charged sodium ion and negatively charged chloride ion form an ionic bond, completing their octets.)

Examples of the octet rule in covalent compounds

(In covalent compounds, atoms share electrons to achieve a stable electron configuration with eight electrons in their valence shell. A common example is water (H2O). Oxygen has six valence electrons, requiring two more to achieve an octet. Hydrogen, following the duet rule, has one electron and needs one more to complete its outermost shell. Each hydrogen atom shares its electron with oxygen, forming two covalent bonds in a water molecule. This electron sharing allows every atom in the molecule to achieve stable electron configurations, with oxygen following the octet rule and both hydrogen atoms following the duet rule.)

Key concepts

ionic compounds

Ionic compounds are formed when one or more electrons are transferred from one atom to another, usually occurring between a metal and a non-metal. In these reactions, metals tend to lose electrons, forming positively charged ions known as cations. Non-metals gain these electrons, forming negatively charged ions known as anions.

For instance, in sodium chloride (NaCl), sodium (Na) donates an electron to chlorine (Cl). Sodium becomes a Na\(^+\) ion, while chlorine becomes a Cl\(^-\) ion. This electron transfer allows both atoms to achieve a stable electron configuration similar to the nearest noble gas.

Once these ions are formed, they attract each other due to their opposite charges, creating ionic bonds. These strong bonds result in the formation of a crystalline lattice structure, typical of many ionic compounds. Ionic bonds are generally strong due to the significant attraction between the positive and negative ions.

covalent compounds

Covalent compounds form when two non-metal atoms share pairs of electrons. This sharing allows each atom involved to reach a stable electron configuration by filling its outer shell. Covalent bonds result in the formation of molecules that can exist in various states, such as gases, liquids, or solids.

Water (H\(_2\)O) is a prime example of a covalent compound. Oxygen shares electrons with two hydrogen atoms to fill its valence shell and satisfy the octet rule, while hydrogen, which follows the duet rule, shares electrons with oxygen to achieve stability. Each shared electron pair constitutes a covalent bond.

This mutual sharing of electrons in covalent compounds generally leads to molecules with specific shapes and properties. Unlike ionic compounds, covalent compounds often have lower melting and boiling points and can be less soluble in water.

electron configuration

Electron configuration refers to the distribution of electrons in an atom's orbitals. This arrangement plays a crucial role in determining how atoms interact and bond with each other. An atom's electron configuration is written in terms of energy levels, sublevels, and orbitals, following a specific order based on increasing energy.

For example, a neutral sodium (Na) atom has an electron configuration of \(1s^2 2s^2 2p^6 3s^1\). When sodium forms compounds like NaCl, it loses its single 3s electron, resulting in a stable configuration resembling that of a noble gas (Neon).

The electron configuration is essential for understanding the chemical properties and reactivity of elements. Electrons in the outermost energy level, known as valence electrons, are particularly significant as they participate in chemical bonding.

valence electrons

Valence electrons are the electrons found in the outermost shell of an atom. They play a pivotal role in determining an element’s chemical properties and its ability to form bonds. Each group in the periodic table shares the same number of valence electrons, which is a major factor in their similar chemical behavior.

In the case of covalent bonds, atoms share valence electrons to achieve stability. For ionic bonds, one atom donates and another accepts these electrons, allowing both to attain a stable electron configuration. For example, chlorine (Cl) has seven valence electrons, needing one more to achieve a complete octet.

Valence electrons are also critical in understanding the octet rule, where atoms aim to reach an electron configuration with eight electrons in their outermost shell, emulating the stable configuration of noble gases.

chemical bonds

Chemical bonds are the forces that hold atoms together in compounds. They are formed by interactions between valence electrons of the atoms involved. Atoms bond to achieve greater stability, often by filling their outer electron shells according to the octet rule.

The two main types of chemical bonds are ionic and covalent bonds. Ionic bonds, as seen in sodium chloride (NaCl), occur when electrons are transferred from one atom to another, creating ions that are held together by electrostatic forces. On the other hand, covalent bonds involve the sharing of electron pairs between atoms, like in the formation of water (H\(_2\)O).

Understanding chemical bonds is essential for grasping the structures and properties of different substances. These bonds determine many physical and chemical properties, such as melting points, boiling points, solubility, and electrical conductivity.