Problem 168 - Chemistry: An Atoms-Focused Approach

Question

Which of the following molecules contains an atom with an expanded valence shell? (a) \(\mathrm{XeF}_{2} ;\) (b) \(\mathrm{GaCl}_{3} ;\) (c) \(\mathrm{ONF}_{3}\);(d) \(\mathrm{SeO}_{2} \mathrm{F}_{2}\)

Step-by-Step Solution

Answer

Answer: (a) \(\mathrm{XeF}_{2}\) and (d) \(\mathrm{SeO}_{2}\mathrm{F}_{2}\)

Step 1: Determine the valence electrons for each central atom

To determine the number of valence electrons in the central atom of each molecule, we need to look at the group number in the periodic table. Keep in mind that elements in Period 3 or later, such as sulfur, phosphorus, and the noble gases, can expand their valence shell because they have access to the d orbitals. (a) \(\mathrm{XeF}_{2}\): Xe is a noble gas, so it has 8 valence electrons. (b) \(\mathrm{GaCl}_{3}\): Ga is in Group 13 and has 3 valence electrons. (c) \(\mathrm{ONF}_{3}\): N is in Group 15 and has 5 valence electrons. (d) \(\mathrm{SeO}_{2}\mathrm{F}_{2}\): Se is in Group 16 and has 6 valence electrons.

Step 2: Calculate total electrons in the valence shell

We need to calculate the total number of electrons in the valence shell of each central atom for the given molecules. (a) \(\mathrm{XeF}_{2}\): Xe has 8 valence electrons + 2 single bonds with F (2x2 = 4) = 12 electrons (b) \(\mathrm{GaCl}_{3}\): Ga has 3 valence electrons + 3 single bonds with Cl (3x2 = 6) = 9 electrons (c) \(\mathrm{ONF}_{3}\): N has 5 valence electrons + 1 double bond with O (4) + 3 single bonds with F (3x2 = 6) = 15 electrons (d) \(\mathrm{SeO}_{2}\mathrm{F}_{2}\): Se has 6 valence electrons + 2 double bonds with O (2x4 = 8) + 2 single bonds with F (2x2 = 4) = 18 electrons

Step 3: Identify molecule with atom having expanded valence shell

Now, let's identify the molecule containing an atom with an expanded valence shell (more than 8 electrons) from the calculated electron counts. (a) \(\mathrm{XeF}_{2}\): 12 electrons in Xe's valence shell (expanded) (b) \(\mathrm{GaCl}_{3}\): 9 electrons in Ga's valence shell (not expanded) (c) \(\mathrm{ONF}_{3}\): 15 electrons in N's valence shell (not expanded because N belongs to Period 2) (d) \(\mathrm{SeO}_{2}\mathrm{F}_{2}\): 18 electrons in Se's valence shell (expanded) The molecules containing an atom with an expanded valence shell are \(\mathrm{XeF}_{2}\) and \(\mathrm{SeO}_{2}\mathrm{F}_{2}\). Hence, both (a) and (d) are the correct answers.

Key Concepts

Valence Electrons

Valence electrons are the electrons present in the outermost shell of an atom. These electrons play a pivotal role in chemical bonding because they are the electrons that atoms share, lose, or gain. Understanding valence electrons is crucial for predicting how an atom will interact with others. Each element's position in the periodic table gives insights into its valence electrons. For example:

• Group 1 elements (like sodium) have 1 valence electron.
• Group 17 elements (like fluorine) have 7 valence electrons.
• Noble gases in Group 18 typically have 8 valence electrons, making them stable.

These electrons determine an atom's chemical properties and help to identify whether an atom can have an expanded valence shell, typically seen in elements beyond Period 2.

Periodic Table Groups

The periodic table is organized into groups and periods to help scientists quickly determine the properties of elements. Each column of the periodic table is called a group. Elements in the same group share similar chemical properties and the same number of valence electrons.
Some important points about periodic table groups include:

• Group 1: Known as alkali metals, very reactive due to having just one valence electron.
• Group 2: Alkaline earth metals, with two valence electrons, are also quite reactive.
• Group 13: Contains elements like gallium, which have three valence electrons.
• Group 17: Known as halogens, very reactive non-metals with seven valence electrons.
• Group 18: Contains noble gases, which have a full valence shell of eight electrons.

This organization helps predict the likelihood of elements forming compounds and their possible chemical reactions.

Noble Gases

Noble gases reside in Group 18 of the periodic table and include elements like helium, neon, and xenon. These elements are characterized by having a complete valence shell, typically with eight valence electrons (except helium, which has two). This full valence shell makes them extremely stable and largely unreactive. However, under certain conditions, heavier noble gases like xenon can form compounds.
Here's how noble gases generally behave in the context of chemical bonding:

• Due to their full valence shell, noble gases rarely form bonds under normal conditions.
• Xenon, unlike its lighter counterparts, can expand its valence shell and form compounds such as \(\mathrm{XeF}_{2}\).
• In such cases, xenon exceeds the octet rule due to available d orbitals beyond the third period.

Understanding noble gases' properties is essential for recognizing when they might form unusual chemical compounds.

Chemical Bonding

Chemical bonding involves the interaction of valence electrons between atoms as they seek stability. There are several types of chemical bonds, with the most common being ionic and covalent bonds. The type of bonding an element undergoes depends greatly on its position within the periodic table, particularly its number of valence electrons.

• Ionic bonds occur when electrons are transferred from one atom to another, usually between a metal and a non-metal.
• Covalent bonds form when electrons are shared between non-metal atoms, allowing them to attain full valence shells.
• Molecules like \(\mathrm{XeF}_{2}\) illustrate how expanded valence shells can participate in chemical bonding, accommodating more than eight electrons.
• Expanded valence shells are feasible because elements from period 3 onward can utilize d orbitals.

Through bonding, atoms achieve a more stable electronic configuration, often resulting in more complex molecular structures.