Question

For elements in the third row of the periodic table and beyond, the octet rule is often not obeyed. What factors are usually cited to explain this fact?

Short answer

Elements in the third row of the periodic table and beyond often do not obey the octet rule due to factors like the availability of d-orbitals, higher nuclear charge, larger atomic size, and increased polarizability. These factors allow the central atom to accommodate more electrons and form more than four bonds, resulting in a deviation from the octet rule. Examples include phosphorus (P) in PCl5 and sulfur (S) in SF6.

Step-by-step solution

1. Understanding 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. This rule applies to most elements in the first and second periods of the periodic table, as achieving a full valence shell provides stability.

2. Factors Causing Deviation from Octet Rule

There are several factors that cause elements in the third row of the periodic table and beyond to not follow the octet rule. These factors include: a) Availability of d-orbitals: Starting from the third period, elements have access to d-orbitals in addition to their s and p orbitals. These d-orbitals can accommodate more electrons, which allows the central atom to form more than four bonds and thus have more than an octet of electrons around it. b) Higher nuclear charge: Elements in the third period and beyond have a higher number of protons in their nucleus. This leads to a stronger attraction between the nucleus and the valence electrons, allowing the atom to hold more than eight electrons. c) Size of the atom: As we move from the second to the third period, the atomic size increases. This results in a longer bond distance between the central atom and the surrounding ligands, which allows the central atom to accommodate more electrons without significant repulsion. d) Polarizability: Elements in the third period and beyond are more polarizable due to their larger atomic size. This means they have a greater ability to distort their electron cloud and accommodate more electrons.

3. Examples of elements disobeying the Octet Rule

Some common examples of elements in the third row of the periodic table and beyond that disobey the octet rule include: a) Phosphorus (P): Phosphorus can have up to ten electrons around it, as shown in the compound PCl5 (phosphorus pentachloride), where phosphorus forms five bonds with chlorine atoms. b) Sulfur (S): Sulfur can have up to 12 electrons around it, as seen in the compound SF6 (sulfur hexafluoride), where sulfur forms six bonds with fluorine atoms. To summarize, several factors, including the availability of d-orbitals, higher nuclear charge, larger atomic size, and increased polarizability, contribute to the deviation from the octet rule for elements in the third row of the periodic table and beyond.

Key concepts

d-orbitals

In the context of the octet rule, d-orbitals play a crucial role for elements starting from the third period of the periodic table. Unlike the elements in the first and second periods which only have s and p orbitals, elements like phosphorus and sulfur have access to additional d-orbitals. This availability of d-orbitals allows these elements to expand their valence shell beyond the traditional eight-electron configuration.
This means that these elements can accommodate more electrons when forming bonds. For example:

• Phosphorus in phosphorus pentachloride (PCl5) uses its d-orbitals to hold a total of ten electrons.
• Sulfur in sulfur hexafluoride (SF6) expands its electron count to twelve.

Thus, the flexibility and additional capacity offered by d-orbitals explain why these elements do not need to stick to the octet rule. The ability to fill these extra orbitals is significant for the formation of multiple bonds, making them quite peculiar in terms of chemical bonding.

nuclear charge

Nuclear charge significantly influences how elements in the third period and beyond can potentially break away from the confines of the octet rule. The nuclear charge refers to the number of protons present in the nucleus of an atom, which directly affects the attraction between the nucleus and the electrons.
As the atomic number increases throughout the periodic table, so does the nuclear charge, resulting in:

• A stronger pull on electrons, allowing these atoms to hold onto more electrons than the typical eight.
• The possibility for these atoms to stabilize additional electrons within their expanded valence shell, making them form more complex molecules.

This stronger pull from the nucleus effectively allows more electron crowding around the central atom, breaking away from the restraining nature of the octet rule, while maintaining the atom's stability.

atomic size

The size of the atom plays a major role in why elements of the third row and beyond deviate from the octet rule. As you move down the periodic table from one period to the next, the atomic size increases. With more space within an atom, there's a greater ability to localize more electrons around the nucleus.
The increased atomic size results in:

• Longer bond lengths, which minimize the repulsive interactions between electrons, allowing more electrons to fit around the central atom comfortably.
• The ability to form more than four bonds, such as in phosphorus pentachloride (PCl5) and sulfur hexafluoride (SF6).

Thus, the increase in atomic size offers the space needed to surpass the conventional eight electrons around a central atom, rendering the octet rule less applicable for these atoms.

polarizability

Polarizability is another critical factor contributing to the deviation from the octet rule, especially in larger atoms like those found in the third period and beyond. Polarizability refers to the ease with which an atom's electron cloud can be distorted.
Larger atoms tend to be more polarizable because:

• Their electron clouds are more spread out, making it easier to distort these clouds to accommodate extra electrons.
• The ease of distortion allows these atoms to engage in bonding situations that would be highly strained for smaller, less polarizable atoms.

This flexibility in how the electron cloud can be shared or distributed while forming bonds permits the holding of more than the usual eight electrons, thus, circumventing the limitations of the octet rule for these elements.