Question

Dissolving NaF in selenium tetrafluoride (SeF \(_{4}\) ) produces NaSeF\(_{5}\), Draw the Lewis structures of \(\mathrm{SeF}_{4}\) and \(\mathrm{SeF}_{5}\). In which structure does Se have more than eight valence electrons?

Short answer

Answer: Both structures have selenium with more than eight valence electrons (12 electrons in both cases).

Step-by-step solution

Count the total valence electrons available.

To draw the Lewis structures of SeF\(_4\) and SeF\(_5^{-}\), we first need to count the total number of valence electrons available for each molecule. For SeF\(_4\), we have 1 Se atom and 4 F atoms. Se has 6 valence electrons, and F has 7 valence electrons. Total valence electrons = (1 x 6) + (4 x 7) = 6 + 28 = 34 For SeF\(_5^{-}\), we have 1 Se atom, 5 F atoms, and 1 extra electron due to the -1 charge. Total valence electrons = (1 x 6) + (5 x 7) + 1 = 6 + 35 = 41.

Draw the Lewis structures.

Now it's time to draw the Lewis structures for both molecules. For SeF\(_4\): 1. Place the least electronegative atom in the center, which is Se. 2. Arrange the four F atoms around Se and connect them with single bonds. 3. Distribute the remaining electrons (28 left) to fulfill the octet rule around each atom. For SeF\(_5^{-}\): 1. Place the least electronegative atom in the center, which is Se. 2. Arrange the five F atoms around Se and connect them with single bonds. 3. Distribute the remaining electrons (31 left) to fulfill the octet rule around each atom.

Determine which structure has Se with more than eight valence electrons.

Observe the Lewis structures: SeF\(_{4}\) has: - Se atom with 1 single bond to each of the 4 F atoms => 4 electron pairs (8 electrons). - 2 lone pairs (1 pair above and 1 pair below Se). Total electrons surrounding Se = 4 bonds * 2 electrons each + 2 lone pairs * 2 electrons each = 8 + 4 = 12. SeF\(_{5^{-}}\) has: - Se atom with 1 single bond to each of the 5 F atoms => 5 electron pairs (10 electrons). - 1 lone pair. Total electrons surrounding Se = 5 bonds * 2 electrons each + 1 lone pair * 2 electrons each = 10 + 2 = 12. Both Lewis structures have Se with more than eight valence electrons (12 electrons in both cases). The Se atom in both structures is an exception to the octet rule due to its access to d-orbitals.

Key concepts

Valence Electrons

Valence electrons are the electrons located in the outermost shell of an atom. They play a vital role in determining how atoms interact and bond with each other. To understand a molecule's structure, it’s important to count these electrons. This can help in drawing Lewis structures, which depict the bonding between atoms.

In selenium tetrafluoride (SeF\(_{4}\)), selenium (Se) has 6 valence electrons, while each fluorine (F) atom has 7 valence electrons. Hence, for SeF\(_{4}\), the total number of valence electrons is calculated as:

• Se: 1 atom x 6 electrons = 6 electrons
• F: 4 atoms x 7 electrons = 28 electrons

Total valence electrons = 34

Understanding the number of valence electrons helps set the stage for applying the octet rule.

Octet Rule

The octet rule is a guideline in chemistry stating that atoms tend to form bonds until they are surrounded by eight valence electrons, resembling the electron configuration of noble gases.

When drawing Lewis structures, the octet rule helps us ensure atoms form stable arrangements. However, some atoms, like selenium in SeF\(_{4}\) and SeF\(_{5}^{-}\), sometimes exceed this typical number due to their size and additional available orbitals. This is evident in SeF\(_{5}^{-}\), where the selenium atom eventually has 12 valence electrons around it, rather than stopping at 8.

The octet rule is useful for predicting molecular shapes and reactivity, although exceptions do exist. Exploring why such exceptions occur leads us into the realm of d-orbitals.

D-Orbitals

D-orbitals are part of the more complex electron orbitals that some elements have beyond the basic s and p orbitals. In the periodic table, elements starting from the third period can accommodate electrons in these d-orbitals.

Selenium is one such element capable of utilizing its d-orbitals. This ability allows selenium to hold more than eight electrons, which is why selenium in the compounds SeF\(_{4}\) and SeF\(_{5}^{-}\) has 12 electrons around it instead of just 8. This expanded valence shell makes it possible for selenium to form more bonds than one might expect by strictly following the octet rule.

Understanding d-orbitals aids in comprehending how elements can form complex bonds and exhibit such unique chemical properties. These structures are crucial for advanced chemistry studies, revealing more about molecular geometry and chemical reactivity.