Question

$$ \begin{aligned} &\text { Explain which of the following anions is larger, and }\\\ &\text { why: } \mathrm{Se}^{2-} \text { or } \mathrm{Te}^{2-} \text { . } \end{aligned} $$

Short answer

\( \text{Te}^{2-} \) is larger because it is lower in the group.

Step-by-step solution

Determine the Position in the Periodic Table

Selenium (Se) and tellurium (Te) are elements belonging to group 16 in the periodic table and are found in the p-block. They are in the same group but different periods, with selenium in period 4 and tellurium in period 5.

Understand Atomic Size Trends

In the periodic table, atomic size increases as we move down a group. This is because each subsequent element down a group has an additional electron shell that increases the size of the atom.

Compare Ionic Sizes of \( ext{Se}^{2-} \) and \( ext{Te}^{2-} \)

When atoms gain electrons to become anions, they increase in size compared to their neutral atoms. Given that \( ext{Se}^{2-} \) and \( ext{Te}^{2-} \) are both 2- anions of elements from the same group, their relative size largely follows the same trend as their neutral atoms because they both gain the same charge.

Apply the Charge Effect on Anion Size

Both \( ext{Se}^{2-} \) and \( ext{Te}^{2-} \) have gained two electrons, resulting in increased electron-electron repulsion within their outer shells. This repulsion affects both ions similarly due to the same charge, but the initial larger size of tellurium (Te), because it is lower in the group, results in \( ext{Te}^{2-} \) being larger.

Conclusion

Considering the position in the periodic table and the general trend of increasing size down a group, \( ext{Te}^{2-} \), being farther down the group than \( ext{Se}^{2-} \), is larger.

Key concepts

Periodic Table Trends

The periodic table is a treasure trove of information about chemical elements and their properties. One of the most significant trends is the change in atomic size as you move around the table. As you progress from left to right across a period, atomic size typically decreases. This happens because electrons are added to the same electron shell, increasing the nuclear charge without increasing the shielding effect significantly, pulling electrons closer to the nucleus.

Conversely, as you move down a group in the table, the atomic size increases. Each element has a new electron shell compared to the element above it, making atoms larger in size. This trend is vital in comparing elements and ions, especially within the same group, like selenium (Se) and tellurium (Te). It explains why tellurium, located below selenium, naturally has a larger atomic size and affects the size of their respective anions, Se\(^{2-}\) and Te\(^{2-}\). This periodic trend is key to understanding the behavior of elements as they gain or lose electrons.

Atomic and Ionic Size

Atomic and ionic sizes are crucial concepts in understanding element behavior, especially for group 16 anions like Se\(^{2-}\) and Te\(^{2-}\). Atomic size refers to the distance from the nucleus to the outermost electron shell of an atom. This size is influenced by the element's position in the periodic table and the number of electron shells.

When atoms gain electrons and become negative ions or anions, they generally increase in size. The extra electrons induce more substantial electron-electron repulsion in the outer shell, making the ion larger than its neutral atom counterpart. Therefore, both Se\(^{2-}\) and Te\(^{2-}\) are larger than their neutral atoms.

• Se\(^{2-}\) is larger than Se
• Te\(^{2-}\) is larger than Te

However, when comparing these anions directly, it's crucial to remember periodic trends: with tellurium further down the table, Te\(^{2-}\) is larger than Se\(^{2-}\), following the group trend.

Electron Shell Impact

The impact of electron shells on atomic and ionic size is profound. Each new period in the periodic table represents an additional electron shell. For elements like selenium (Se) and tellurium (Te), which belong to the same group, this means that while they share a similar outer electron configuration, tellurium with more electron shells is naturally larger.

This increase in size occurs because each new shell further away from the nucleus makes the atom larger, regardless of how tightly the inner shells are held. When these atoms become ions by gaining extra electrons, as with Se\(^{2-}\) and Te\(^{2-}\), the influence of these additional shells is maintained. They result in larger ionic sizes, especially noticeable as we move down the group. Remember, the more electron shells an element has, the less effectively the nucleus can pull on the outermost electrons. This results in a larger atomic and ionic size, explaining why Te\(^{2-}\) is larger than Se\(^{2-}\).

Group 16 Elements

Group 16 of the periodic table, also known as the chalcogens, includes elements such as oxygen, sulfur, selenium, tellurium, and polonium. These elements share similar properties due to their group affiliation, including their tendency to form -2 anions.

The atomic structure of group 16 elements dictates that they need to gain two electrons to achieve a stable electronic configuration, similar to the nearest noble gas. This gain leads to the formation of anions like Se\(^{2-}\) and Te\(^{2-}\). The s and p outer orbitals are crucial here: elements in this group often have six valence electrons in their outer shell before forming anions.

As you move from selenium to tellurium in group 16, there is an increase in both atomic and ionic size due to an additional electron shell, which increases the electron shielding effect. This makes tellurium-based anions generally larger in size, highlighting why Te\(^{2-}\) is bigger than Se\(^{2-}\). This knowledge about group 16 is essential for understanding their chemical behavior and properties.