Question

For Group 13-17 elements in the third row of the periodic table and beyond, the octet rule is often not obeyed. A friend of yours says this is because these heavier elements are more likely to make double or triple bonds. Another friend of yours says that this is because the heavier elements are larger and can make bonds to more than four atoms at a time. Which friend is more correct?

Short answer

The friend who stated that heavier elements in Group 13-17 can make bonds with more than four atoms at a time due to their increased atomic radius and availability of d-orbitals is more correct in explaining why these elements often do not obey the octet rule compared to the friend who said it's because they are more likely to make double or triple bonds.

Step-by-step solution

Understanding the Octet Rule

First, let's start by understanding the octet rule. The octet rule states that atoms tend to seek stable electron configurations by attaining eight valence electrons. This corresponds to the electron configuration of a noble gas. This rule works fairly well for lighter elements (those in the first and second periods of the periodic table), but as we move to heavier elements, we'll see that this rule doesn't quite apply.

Overview of the Periodic Table

The periodic table arranges elements in a way that helps us understand their properties and behavior. Elements are organized in groups (vertical columns) and periods (horizontal rows). The number of valence electrons determines the group number (e.g., Group 13 elements have 3 valence electrons; Group 17 elements have 7 valence electrons). Elements in the same group tend to have similar chemical properties. The rows, or periods, signify the number of electron shells, and as we move down to heavier elements, the atomic radius increases.

The Role of Atomic Radius

The size of an atom is a critical factor in determining its bonding properties. As we move down the periodic table, atoms typically get larger, meaning their atomic radii increase. This increase in atomic radius results in valence electrons being further away from the nucleus and weaker attraction to the nucleus. This often allows heavier elements to accommodate more than eight electrons in their outermost shell because the increased distance from the nucleus decreases the repulsion between the electrons. This leads these elements to be more likely to form multiple bonds with other atoms.

Bonding Tendencies of Heavier Elements

The bonding tendencies of heavier elements (i.e., those beyond the first and second row of the periodic table) are also influenced by the availability of d-orbitals in their valence shell. This allows them to accommodate more electrons in their outer shell and form more covalent bonds than lighter elements. For example, elements like phosphorus (P, in the third row, Group 15) and sulfur (S, in the third row, Group 16) can form more than four bonds at a time.

Evaluating the Hypotheses

Now that we have some background knowledge, let's evaluate the two friends' hypotheses. The first friend's claim that heavier elements are more likely to make double or triple bonds is not entirely accurate; while heavier elements can create multiple bonds, this is not the primary reason they don't obey the octet rule. The second friend's claim that heavier elements are larger and can make bonds with more than four atoms at a time is better supported by the evidence discussed, as the increased atomic radius and availability of d-orbitals allow these elements to form more covalent bonds than lighter elements.

Conclusion

In conclusion, the friend who stated that heavier elements in Group 13-17 can make bonds with more than four atoms at a time due to their increased atomic radius and availability of d-orbitals is more correct in explaining why these elements often do not obey the octet rule compared to the friend who said it's because they are more likely to make double or triple bonds.

Key concepts

Periodic Table

The periodic table is a fantastic tool used by chemists because it organizes all known elements according to their properties. It is arranged in groups (vertical columns) and periods (horizontal rows). This arrangement helps us predict how elements will behave, especially during chemical reactions.

Elements within the same group share similar chemical properties because they have the same number of valence electrons. For instance, Group 13 elements have three valence electrons, while Group 17 elements have seven. These valence electrons are crucial because they determine how an element will bond with others.

As you move from top to bottom in a group, the number of electron shells increases, which affects other properties like atomic radius. This arrangement helps students and professionals easily identify and understand how different elements are related chemically and predict their interactions.

Atomic Radius

Atomic radius refers to the size of an atom, which typically increases as you move down the periodic table. This is because additional electron shells are added with each period, making the atoms larger. The greater the atomic radius, the further away the valence electrons are from the nucleus.

This distance impacts how strongly the electrons are attracted to the nucleus. In heavier elements, the valence electrons experience less attraction due to their increased distance. This weakened attraction allows these elements to accommodate more than eight electrons in their outer shell, defying the octet rule. Additionally, the reduced repulsion among electrons in larger atoms makes it possible for these elements to form multiple bonds with other atoms.

The concept of atomic radius is crucial for understanding why some elements in Groups 13-17, especially those beyond the second period, are able to make more than four bonds at a time.

d-Orbitals

d-Orbitals play a significant role when it comes to the chemistry of heavy elements. These orbitals become relevant in the third period of the periodic table and beyond, enabling elements to have more complex electron configurations.

Unlike s and p orbitals, which are common in lighter elements, d-orbitals provide additional space for electrons. This means heavier elements have more room for electrons in their valence shell, allowing them to hold more than the "typical" eight electrons predicted by the octet rule. As a result, they can form more than four covalent bonds, making their chemistry much richer and diverse.

For example, elements such as phosphorus and sulfur can utilize their d-orbitals to form a greater number of bonds compared to lighter elements like nitrogen and oxygen. This ability to expand the octet is a vital concept when considering why heavier elements often do not conform to the octet rule.