Octet Rule: Videos & Practice Problems

The octet rule is a fundamental concept in chemistry where main group elements tend to achieve a stable configuration by having eight electrons in their valence shell, akin to noble gases. This stability is usually achieved through chemical bonding, where covalent bonds allow the sharing of valence electrons between atoms. However, there are exceptions to this rule. Elements with an incomplete octet, such as hydrogen, helium, beryllium, and certain Group 3A elements, are stable with fewer than eight electrons. Conversely, elements capable of an expanded octet—found in period 3 and below, like phosphorus and sulfur—can be stable with more than eight valence electrons due to their larger electron shells. These deviations from the octet rule are crucial for understanding the chemical behavior and bonding capabilities of different elements.

Octet Rule

The octet rule is a fundamental concept in chemistry that describes the tendency of main group elements to achieve a stable electron configuration by having eight electrons in their outermost shell, similar to that of noble gases. This stability is often achieved through chemical bonding, where elements share or transfer electrons. In covalent bonding, two elements share a pair of valence electrons, which contributes to each element's total count of octet electrons.

Valence electrons are the outermost electrons of an atom and are determined by the element's group number in the periodic table. For instance, aluminum, which is in group 3A, has three valence electrons. When elements form bonds, they can gain shared electrons, which are the electrons contributed by another atom during the bonding process. The total number of octet electrons for an element is the sum of its valence electrons and the shared electrons it acquires through bonding.

To satisfy the octet rule, most main group elements aim for a total of eight octet electrons. However, hydrogen is an exception; it only requires two electrons to achieve the stable electron configuration of helium, thus it does not conform to the octet rule in the traditional sense. Understanding these concepts of valence, shared, and octet electrons is crucial for grasping the behavior of elements in chemical reactions and bonding scenarios.

Octet Rule Example 1

In the given compound, a nitrogen atom is centrally located and surrounded by three fluorine atoms. Understanding the behavior of nitrogen, which is a main group element, is crucial as it aims to achieve a stable octet configuration, typically consisting of eight electrons in its valence shell. This goal influences the bonding characteristics of nitrogen.

Nitrogen, positioned in group 5A of the periodic table, possesses five valence electrons. These electrons are essential for its participation in chemical bonding. When nitrogen forms bonds with the three fluorine atoms, it shares electrons with them. Each fluorine atom contributes one electron, resulting in a total of three shared electrons. Therefore, the total number of electrons around nitrogen becomes:

Valence electrons from nitrogen: 5

Shared electrons from fluorine: 3

Total octet electrons: 5 + 3 = 8

This configuration allows nitrogen to achieve a complete octet, fulfilling its stability requirement. Consequently, the correct answer to the question regarding the true statement about the compound is option b, as it accurately reflects the electron sharing and bonding dynamics involved in achieving the octet rule.

How many shared electrons are around the oxygen atom?

• A. 4
• B. 2
• C. 6
• D. 10
• E. 8

Answer: (B)

Which of the following atoms has the most valence electrons?

• A. O
• B. Be
• C. He
• D. N
• E. Si

Answer: (A)

Octet Rule

Chemistry often presents exceptions to the octet rule, which states that many main group elements strive to have eight electrons in their valence shell, resembling noble gases. However, certain elements can achieve stability with a non-octet number of electrons, categorized into two main types: incomplete octets and expanded octets.

An incomplete octet refers to elements that can maintain stability with fewer than eight electrons. For instance, elements like hydrogen, helium, beryllium, boron, aluminum, gallium, and indium are known to have incomplete octets. In contrast, an expanded octet occurs when elements possess more than eight electrons in their valence shell. This phenomenon is typically observed in elements found in the third period and below, such as phosphorus, sulfur, chlorine, arsenic, selenium, bromine, krypton, tellurium, iodine, xenon, and radon.

Interestingly, the number of electrons that these elements can stably accommodate often correlates with their group number. Specifically, the non-octet number of electrons is generally twice the group number. For example, a group 2A element can stabilize with four electrons (2 x 2), while a group 3A element can stabilize with six electrons (2 x 3). Continuing this pattern, group 4A elements can stabilize with eight electrons, group 5A with ten, group 6A with twelve, and so forth, up to group 8A, which can stabilize with sixteen electrons.

Understanding these exceptions is crucial for predicting the behavior of elements during chemical bonding. While noble gases are inherently stable and typically do not engage in bonding, elements with expanded octets can form bonds due to their larger electron shells, allowing for greater flexibility in their bonding preferences.

Octet Rule Example 2

To determine the number of octet electrons around a phosphorus atom in a compound, it's essential to understand the concept of octet electrons, which is the sum of valence electrons and shared electrons. Phosphorus, located in group 5A of the periodic table, has 5 valence electrons. This can be visualized as follows: 1, 2, 3, 4, and 5.

In addition to its valence electrons, phosphorus can form chemical bonds with other elements, such as hydrogen. Each bond contributes shared electrons. In this case, if we consider the bonds formed with hydrogen, we can count an additional 5 shared electrons, leading to a total of 10 electrons around the phosphorus atom.

This aligns with the general rule that for elements not adhering to the octet rule, the total number of electrons can often be calculated as twice the group number. Since phosphorus is in group 5A, multiplying by 2 gives us 10, confirming that phosphorus can accommodate 10 electrons in its outer shell. Therefore, the correct answer regarding the number of octet electrons around the phosphorus atom is 10.

Which of the following contains an atom that may have an incomplete octet?

• A. CCl₄
• B. NCl₃
• C. OCl₂
• D. SeCl₆
• E. BCl₃

Answer: (E)

Here’s what students ask on this topic:

What is the octet rule?

The octet rule is a chemical rule of thumb that reflects a pattern observed in the outermost electrons of atoms in stable molecules. It states that atoms tend to combine in such a way that they each have eight electrons in their valence shell, giving them the same electronic configuration as a noble gas. The rule is based on the principle that having a full or complete outer shell with eight electrons is energetically favorable and confers stability.

This rule helps explain why atoms form certain types of ions and molecules. For instance, sodium ($\mathrm{Na}$) has one electron in its outer shell and will typically lose that electron to achieve an octet in the next lower shell, becoming a positively charged ion (${\mathrm{Na}}^{+}$). Chlorine ($\mathrm{Cl}$), on the other hand, has seven electrons in its valence shell and will gain an electron to complete its octet, becoming a negatively charged ion (${\mathrm{Cl}}^{-}$). When these ions combine to form sodium chloride ($\mathrm{NaCl}$), both achieve stable octet configurations. However, it's important to note that the octet rule has exceptions and doesn't apply to all elements, particularly those beyond the second period of the periodic table.

What are the exceptions to the octet rule?

The octet rule states that atoms tend to combine in such a way that they each have eight electrons in their valence shell, giving them the same electronic configuration as a noble gas. However, there are several exceptions to this rule:

1. Incomplete Octet: Some elements are stable with less than eight electrons in their valence shell. Examples include hydrogen (which needs only two electrons to fill its shell), boron, and aluminum which often form stable compounds with six valence electrons.
2. Odd-Electron Molecules: Molecules with an odd number of electrons, like nitric oxide (NO), cannot distribute electrons to give eight to each atom, resulting in an unpaired electron.
3. Expanded Octet: Elements in the third period and beyond can have more than eight electrons because they have d orbitals available for bonding. Examples include phosphorus (which can have 10 electrons in compounds like PF₅) and sulfur (which can have 12 electrons in compounds like SF₆).
4. Ionic Compounds: In ionic compounds, atoms do not share electrons to achieve noble gas configuration; instead, they transfer electrons. As a result, the octet rule does not apply in the same way as it does to covalent compounds.

Which elements do not follow the octet rule?

The octet rule is a chemical guideline that reflects the observation that atoms of main-group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. However, there are several exceptions to this rule:

• Hydrogen and Helium: These elements follow the "duet rule," as they are stable with just two electrons in their outermost shell.
• Boron and Beryllium: These elements can be stable with six and four electrons, respectively, in their valence shell.
• Elements in Period 3 and beyond: These elements can have more than eight electrons due to the availability of d-orbitals. Examples include phosphorus (which can have 10 electrons), sulfur (12 electrons), and chlorine (14 electrons).
• Molecules with an odd number of electrons: For example, nitric oxide (NO) and nitrogen dioxide (NO₂) do not have an even number of electrons to distribute into pairs for an octet.
• Ions or molecules with less than an octet: Some compounds are stable with fewer than eight electrons around an atom, such as the aluminum ion in AlCl₃.

These exceptions occur because the elements either do not have enough electrons to complete an octet or have the capacity to expand their valence.