How gases behave

Noble gases, located in Group 18, are characterized by their minimal reactivity due to a filled outer electron shell, making them highly stable. However, labeling the entire group as "inert" is no longer entirely accurate.

It was once thought that noble gases couldn't form bonds, but discoveries in the 1960s showed that heavier noble gases can engage in reactions. Thus, stating they are wholly inert is false.

Noble gases are solitary atoms due to complete outer electron shells, meaning they don't need to share electrons for stability. They are monatomic like $He$, $Ne$, $Ar$, etc., and not diatomic like $O_2$ or $N_2$.

Moving down Group 18, valence electrons become more distant from the nucleus with increased shielding. This results in a lower first ionization energy, making electron loss or sharing easier. Thus, reactivity actually rises as atomic number increases.

Larger noble gases like Xenon ($Xe$) have a low enough ionization energy to react with very electronegative elements, like Fluorine ($F$) and Oxygen ($O$), forming compounds like $Xe{F}_4$ and $Xe{O}_3$. This is consistent with known periodic trends and history.

Let's evaluate Option A. While noble gases are notably stable because of their full valence shells, the term "completely" is too strong a statement. We know from research that noble gases like Xenon can react under specified conditions, so they are not entirely inert.

Consider Option D. A diatomic molecule comprises two identical atoms bonded together, such as $O_2$ or $C{l}_2$. With a stable electronic setup, noble gases do not need to bond with another atom to achieve stability, existing as single atoms or monatomic gases at room temperature.

Checking the trend in Option C, as atomic numbers rise down Group 18, atoms enlarge and outer electrons are less tightly bound due to a decrease in first ionization energy. This makes larger noble gases more reactive than smaller ones, meaning reactivity does not decrease.

After discarding other options, Option B remains. It is accurate: heavier noble gases like Xenon ($Xe$) can react with highly electronegative elements like Fluorine ($F$) and Oxygen ($O$) to form compounds like $Xe{F}_4$ and $Xe{O}_3$.

To address this query, we must consider the Periodic trends within Group 18, the noble gases. As per the IB curriculum, descending a group results in increasing atomic radius and decreasing first ionization energy due to the distance and shielding of valence electrons.

Ionization energy refers to the energy needed to detach one mole of electrons from one mole of gaseous atoms. For noble gases, the pattern is that Helium ($He$) has very high ionization energy due to tightly held electrons, whereas Xenon ($Xe$) has comparatively lower ionization energy.

Despite their general stability from full valence shells, noble gases are not entirely inert. The creation of compounds like $Xe{F}_4$ proves that under specific circumstances, heavier noble gases do undergo reactions.

Our study of ionization energy shows that removing electrons becomes easier with increasing atomic number down the group. Hence, reactivity actually ascends as atomic number rises, not the opposite.

Due to their complete valence shells, noble gases do not need to share electrons with another atom of the same element for stability. Unlike molecules like $O_2$ or $N_2$, noble gases are monatomic at room temperature.

Xenon's ionization energy is sufficiently low to react with the most electronegative elements like fluorine and oxygen, leading to stable compounds. This verifies Option B as the only scientifically correct choice.

On the periodic table, the number of the Period indicates the main energy level ($n$) for the outermost shell. Argon, being in Period 3, has its outer electrons in the third energy level ($n=3$).

As a Noble Gas in Group 18, Argon has a complete outer shell, which includes an "octet" of 8 electrons (note that Helium is an exception with 2). For elements in Period 3, these 8 electrons occupy the $3s$ and $3p$ subshells.

Options that do not align with the third energy level ($n=3$) can be excluded immediately:

• Option A involves the $4s$ and $4p$ subshells, corresponding to Period 4.
• Option C involves the $2s$ and $2p$ subshells, corresponding to Period 2.

Next, examine the electron count in the remaining choices.

Option B presents $3s^{2}3p^5$. The sum is: $2+5=7$ This indicates 7 valence electrons (Group 17), not the 8 needed for a Noble Gas.

Option D is $3s^{2}3p^6$. The total is: $2+6=8$ This matches the third energy level ($n=3$) and the full octet characteristic of Argon, a Noble Gas.

Locate Argon ($Ar$) on the periodic table. As a noble gas in Period 3, its atomic number is $18$. Thus, a neutral Argon atom possesses $18$ electrons distributed among its energy levels.

According to the Aufbau principle, fill the subshells in order of increasing energy (1s, 2s, 2p, 3s, 3p, 0dots) until all $18$ electrons are allocated:

$\begin{array}{rl}& 1s^2\text{ (2 electrons)}\\ & 2s^2\text{ (2 electrons)}\\ & 2p^6\text{ (6 electrons)}\\ & 3s^2\text{ (2 electrons)}\\ & 3p^6\text{ (6 electrons)}\end{array}$

The sum is $2+2+6+2+6=18$.

The valence shell is the outermost energy level with electrons. For Argon, $n=3$ is the highest value, so the third shell includes $3s$ and $3p$ subshells:

$\text{Valence configuration: }3s^{2}3p^6$

Evaluate why other options are unsuitable:

• Option A pertains to Period 4 ($n=4$), characteristic of Krypton.
• Option B depicts 7 valence electrons ($3s^{2}3p^5$), fitting for Chlorine.
• Option C is for Period 2 ($n=2$), matching Neon.

The correct electron configuration for Argon's valence shell is $3s^{2}3p^6$, representing a complete octet at the third energy level.

The question states Argon is in Period 3 of the Periodic Table. This period number signifies the principal quantum number ($n$) for the outer shell, meaning Argon's valence shell should begin with 3 ($3s$ and $3p$).

Using the period number, discard options indicating shells other than $n=3$:

• Option A refers to Period 4 ($n=4$).
• Option C refers to Period 2 ($n=2$, matching Neon).

Being a noble gas, Argon has a full valence shell (an octet). A filled $s$ and $p$ subshell in the third level has 8 electrons (2 in $s$ and 6 in $p$).

For the remaining choices:

• Option B is $3s^{2}3p^5$. This sums to $2+5=7$ valence electrons, typical for a halogen (Chlorine).
• Option D is $3s^{2}3p^6$. This totals $2+6=8$, a complete octet.

After excluding options with incorrect period numbers and electron counts, the correct electron configuration for Argon's full valence shell is identified.

The question requires you to identify a non-noble gas that is used to maintain food freshness by removing oxygen. This is a common technique in Modified Atmosphere Packaging (MAP) to prevent spoilage. The gas must be stable and non-reactive with food.

Examine the options and discard gases unsuitable for food preservation:

• Sulfur (Option C): Sulfur is typically a solid and can be reactive. It isn't used in food packaging due to its chemical activity.
• Oxygen (Option D): Oxygen is reactive and causes oxidation, which is precisely what we're trying to avoid in food preservation.

Helium (Option B) is a non-reactive gas, but it's a noble gas. Although non-reactive, the focus is on non-noble gases specifically for this purpose.

Through recall, we know that Nitrogen ($N_2$) is preferred for food preservation:

1. Inactivity: It has a robust triple bond ($N\equiv N$), making it stable and unreactive.
2. Plentiful: It constitutes about $78\%$ of our atmosphere, making it cost-effective.
3. Non-toxicity: It safely displaces oxygen, preventing oxidation and aerobic bacterial growth.

Given Nitrogen's effectiveness and commonality in the food industry to displace oxygen, the correct choice is Option A.

The question asks for a non-noble gas that helps in preserving food by removing oxygen. The gas must be gaseous at room temperature, non-toxic, and chemically inert to avoid altering food quality.

Sulfur is usually found as a solid and can be reactive. Since we need a gas to displace oxygen, a solid is not appropriate for this function.

Oxygen ($O_2$) is a reactive gas and the very element that leads to spoilage. It's not suitable for preservation as it promotes oxidation.

Helium ($He$) is a noble gas and although it is inert, the focus is specifically on non-noble gases for this application.

Nitrogen ($N_2$) constitutes about $78\%$ of the atmosphere. It is diatomic with a strong triple bond ($N\equiv N$), rendering it inert. It is non-toxic, readily available, and ideal for Modified Atmosphere Packaging (MAP) to avert oxidation.

Begin by identifying the elements in question. Noble gases are positioned in Group 18 of the periodic table, including elements like Helium, Neon, and Argon. This is discussed in the section on Periodic Table Structure.

A key feature of noble gases is their electron arrangement. Atoms strive for a full outer shell for enhanced stability. Noble gases naturally have complete valence shells: Helium ($He$) has 2 electrons, fulfilling its first shell, while others like $Ne$ and $Ar$ have 8 electrons ($s^2{p}^6$ configuration) in their outer shell. This completion makes them nonreactive.

Other elements bond by sharing or transferring electrons to complete their outer shells, for example, Oxygen ($O$) forms $O_2$ to achieve stability. Noble gases, however, are intrinsically stable and don't need to share electrons, so they don't form molecules or large structures.

Options B and C, involving diatomic and triatomic molecules, can be dismissed. Such structures require atoms to bond, but noble gases are stable as solitary atoms, so they don't form these in the atmosphere.

Option D suggests a giant covalent network, similar to diamond. This requires extensive covalent bonding, but noble gases do not form such bonds under normal circumstances.

Since noble gases are stable as lone atoms, they exist as monatomic entities (from 'mono' meaning one). This aligns with their valence shell stability.

Using the Elimination of Bonding Types strategy, start with the physical state. The query addresses noble gases. Giant covalent lattices, like diamond, are solids with high melting and boiling points due to strong bonds. Since noble gases are naturally gaseous, we can exclude giant structures.

Consider molecules like $ext{N}_2$, $ext{O}_2$, or triatomic molecules. Forming these requires covalent bonds to stabilize outer shells. In the Periodic Table, noble gases in Group 18 already have full valence shells (8 electrons, 2 for Helium), so they don't need to share electrons and don't form multi-atomic molecules.

After ruling out giant lattices, diatomic, and triatomic molecules, what remains is that noble gases are single, unbonded atoms. The term for this is monatomic ('mono' means one), reflecting their stable electron setup.

By excluding bond types that involve electron sharing or solid lattices, Option A is confirmed as correct for noble gases. Knowing Group 18 stability is key in Periodic Table topics.