Question

Write Lewis structures for \(\mathrm{SeF}_{4}\) and \(\mathrm{SeF}_{6}\). Is the octet rule satisfied for Se?

Short answer

Se in \( \mathrm{SeF}_4 \) and \( \mathrm{SeF}_6 \) does not satisfy the octet rule; it uses an expanded octet.

Step-by-step solution

Count Valence Electrons for Se

Selenium (Se) is in group 16 of the periodic table and has 6 valence electrons. Fluorine (F) is in group 17 and has 7 valence electrons.

Calculate Total Valence Electrons for \( \mathrm{SeF}_4 \)

For \( \mathrm{SeF}_4 \), there is 1 Se atom and 4 F atoms: - Se contributes 6 valence electrons.- 4 F atoms contribute \( 4 \times 7 = 28 \) valence electrons. - Total valence electrons: \( 6 + 28 = 34 \).

Draw Skeleton Structure for \( \mathrm{SeF}_4 \)

Place the Se atom in the center and the 4 F atoms around it, each connected with a single bond (dash) to the Se atom.

Distribute Remaining Electrons for \( \mathrm{SeF}_4 \)

- Each F gets 6 more electrons to complete its octet (2 from the bonds, 6 more = 8 total).- Remaining are \( 34 - 4 \times 2 = 26 \) electrons.- Place the remaining electrons on the Se atom. With 4 bonding pairs, Se initially uses 8 electrons.

Verify Octet Rule for \( \mathrm{SeF}_4 \)

In \( \mathrm{SeF}_4 \), Se uses an expanded octet to accommodate all electrons (Se has 4 pairs + 2 lone pairs = 10 electrons). The octet rule is not satisfied for Se since it exceeds 8 electrons.

Calculate Total Valence Electrons for \( \mathrm{SeF}_6 \)

For \( \mathrm{SeF}_6 \), there is 1 Se atom and 6 F atoms: - Se contributes 6 valence electrons.- 6 F atoms contribute \( 6 \times 7 = 42 \) valence electrons. - Total valence electrons: \( 6 + 42 = 48 \).

Draw Skeleton Structure for \( \mathrm{SeF}_6 \)

Place the Se atom in the center with 6 F atoms surrounding it, each connected to Se with a single bond.

Distribute Remaining Electrons for \( \mathrm{SeF}_6 \)

- Each F atom gets 6 more electrons to complete its octet (2 from the bonds, 6 more = 8 total).- Remaining are \( 48 - 6 \times 2 = 36 \) electrons.- Each of the 6 bonds uses 2 electrons, and all remaining electrons are on the F atoms.

Verify Octet Rule for \( \mathrm{SeF}_6 \)

In \( \mathrm{SeF}_6 \), Se uses an expanded octet, accommodating 12 electrons. The octet rule is not satisfied for Se since it has more than 8 electrons.

Key concepts

Octet Rule

The octet rule is a fundamental concept in chemistry that helps us understand how atoms form stable structures in chemical compounds. According to the octet rule, atoms tend to bond in such a way that each atom has eight electrons in its valence shell, achieving a noble gas electron configuration. This is often observed in many compounds, particularly for main group elements. However, there are exceptions to this rule, especially for elements in period 3 and beyond on the periodic table.

• Atoms form bonds to achieve a stable electron configuration.
• The octet rule describes this tendency for having eight valence electrons.
• Exceptions to the octet rule can be seen in compounds with elements like sulfur and phosphorus.

In molecules like SeF_4 and SeF_6, selenium demonstrates an expanded octet, forming stable compounds without having eight electrons in its valence shell.

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a key role in chemical bonding. They are involved in forming bonds with other atoms. Therefore, understanding the number of valence electrons is crucial for drawing Lewis structures.
Each element's valence electrons can be determined by looking at its group number in the periodic table. For instance:

• Selenium (Se) is in group 16, so it has 6 valence electrons.
• Fluorine (F) is in group 17, with 7 valence electrons.

Combining these valence electrons in different ratios allows us to determine the number of electrons involved in bonding and to draw accurate Lewis structures for molecules.

Expanded Octet

An expanded octet occurs when an element can have more than eight electrons in its valence shell. This is possible because elements starting from period 3, like selenium, have d-orbitals available that can accommodate extra electrons.
In SeF_4 and SeF_6, selenium uses an expanded octet to achieve stability. This means selenium can accommodate more than the usual eight electrons:

• In SeF_4, selenium has 10 electrons around it.
• In SeF_6, selenium accommodates 12 electrons.

Expanded octets allow for the formation of compounds that wouldn't be possible under the strict constraints of the octet rule, increasingly common in compounds with heavier central atoms.

Selenium

Selenium (Se) is a chemical element that is particularly interesting due to its ability to form compounds with an expanded octet, such as SeF_4 and SeF_6. Selenium is located in group 16 and period 4 of the periodic table, which positions it as a heavier congener of sulfur.
Selenium's ability to exceed the octet rule stems from its electronic structure:

• It has 6 valence electrons, allowing it some flexibility in bonding.
• Its position in period 4 grants it access to d-orbitals, crucial for accommodating extra electrons.

This property of selenium enables it to form stable compounds with elements like fluorine, despite having more than the traditional eight valence electrons.

Fluorine

Fluorine (F) is the most electronegative element on the periodic table, meaning it strongly attracts electrons in bonding scenarios. It resides in group 17 and has 7 valence electrons, making it one electron short of a full octet.
In compounds, fluorine consistently seeks to achieve a stable octet by gaining one additional electron:

• This need makes fluorine a consistent bonding partner in many compounds, always forming a single bond.
• In SeF_4 and SeF_6, fluorine always completes its octet, ensuring stability for the compound.

The bonding behavior of fluorine is a key element in determining the structure and properties of molecules like SeF_4 and SeF_6.