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

Understanding valence electrons is crucial when learning how to draw Lewis structures for chemical compounds. Valence electrons are the electrons that reside in the outermost shell of an atom and are integral to the formation of chemical bonds.
For instance, in the case of Selenium (Se), which is situated in Group 16 of the periodic table, it possesses six valence electrons. On the other hand, Fluorine (F), found in Group 17, comes with seven. When constructing Lewis structures for compounds like SeF₄ andSeF₆, it's essential to count the total number of valence electrons, which serve as the basis for the bonding framework. Counting these electrons helps in predicting the molecular structure and understanding the molecule's chemical behavior.
To enhance comprehension, remember that:

• The total valence electrons for SeF₄ is calculated as (6 + 4*7) which sums up to 34.
• For SeF₆, the total valence electrons are (6 + 6*7), adding up to 48.

These electrons are then distributed to achieve stable structures for the molecules.

The octet rule is a well-accepted guideline in chemistry which asserts that atoms are most stable when they have eight electrons in their valence shell. This rule is based on the observation that atoms with eight electrons achieve a noble gas electron configuration, which is especially stable.
In Lewis structures, following the octet rule means ensuring that each atom (mainly those from the second period of the periodic table) is surrounded by eight electrons, leading to a stable valence shell. During the construction of Lewis structures for SeF₄ and SeF₆, it's notable that fluorine atoms, which always seek to complete their octet, succeed in achieving this stable configuration. Meanwhile, selenium, which can expand its valence shell beyond eight electrons, does not strictly follow the octet rule in these compounds.
For a clear depiction of how the octet rule operates, consider the structure of SeF₄ where selenium ends up with 10 electrons, an apparent deviation from the rule.

The 'expanded octet' refers to a phenomenon where certain elements, particularly those from the third period of the periodic table and beyond, have the capacity to accommodate more than eight electrons in their valence shell, thereby breaking the conventional octet rule. This is possible thanks to the presence of available d-orbitals that can be used to house the extra electrons.
The concept of an expanded octet comes into play with molecules like SeF₆, where selenium, a third-period element, exhibits an ability to hold up to 12 electrons around it. Exploring the Lewis structure for SeF₆ showcases selenium central to six fluorine atoms, with the resulting structure displaying an expanded octet. This evolution beyond the octet is possible due to selenium's ability to utilize its d-orbitals, offering a deeper insight into the versatile nature of chemical bonding in heavier elements.
In summary, while the octet rule is a fundamental guideline in chemistry, the expanded octet serves as a necessary extension, accommodating the diverse behaviors of larger atoms in complex molecules.