Problem 72 - Introductory Chemistry Atoms First

Question

Chlorine (Cl), neon (Ne), and helium (He) all exist as gases, but only one of them is diatomic. Which is it, and why is it diatomic while the others are monatomic?

Step-by-Step Solution

Answer

Chlorine (Cl) is the diatomic element among the three given elements (Cl, Ne, and He) and exists as a diatomic molecule (Cl2). The reason for chlorine being diatomic is that it requires one more electron to achieve a stable electron configuration with a full valence shell, which it achieves by sharing electrons with another chlorine atom. In contrast, neon (Ne) and helium (He) exist as monatomic gases due to their already stable electron configurations with full valence electron shells, eliminating the need for bonding with other atoms.

Step 1: Identify the electronic configurations of Chlorine, Neon, and Helium

To determine the electronic configurations of chlorine (Cl), neon (Ne), and helium (He), we need to refer to the periodic table. The atomic numbers for these elements are Chlorine: 17, Neon: 10, and Helium: 2. Now, let's write their electronic configurations: - Chlorine (Cl): \(1s^2 2s^2 2p^6 3s^2 3p^5\) - Neon (Ne): \(1s^2 2s^2 2p^6\) - Helium (He): \(1s^2\)

Step 2: Determine the bonding behavior of Chlorine, Neon, and Helium

We can now discuss the bonding behavior of the three elements based on their electronic configurations. The most stable state for any element is to have a full valence electron shell (either by sharing, gaining, or losing electrons). For Noble gases (Group 18 elements), they are already stable due to having full valence shells. - Chlorine (Cl): With 7 valence electrons (in the 3p subshell), chlorine needs 1 more electron to achieve a full valence electron shell (3p6). Chlorine can achieve this by sharing 1 electron with another chlorine atom, forming a diatomic molecule, Cl2. - Neon (Ne): Neon has a full valence electron shell (2s^2 2p^6). Since it already has a stable electron configuration, it does not need to bond with other atoms. As a result, Neon exists as a monatomic gas. - Helium (He): Helium also has a full valence shell (1s^2), making it stable and not requiring any bonding with other atoms. Therefore, it exists as a monatomic gas.

Step 3: Identify the diatomic element and explain the reason

Based on our analysis in Steps 1 and 2, we can now determine that chlorine (Cl) is the diatomic element among the three given elements (Cl2). The reason for chlorine being diatomic is that it requires one more electron to achieve a stable electron configuration with a full valence shell. By sharing its electrons with another chlorine atom, it forms a diatomic molecule (Cl2). In contrast, neon (Ne) and helium (He) exist as monatomic gases due to their already stable electron configuration with full valence electron shells. Since they are already stable, there is no need for them to bond with other atoms, resulting in their existence as monatomic gases.

Key Concepts

Diatomic Molecules

When we talk about diatomic molecules, we are referring to molecules that consist of two atoms. These can be the same element or different elements. Chlorine is a great example of a diatomic molecule because it naturally exists as Cl extsubscript{2}. Why does this happen? Chlorine has 7 valence electrons, and it prefers to have a full shell of 8, like the noble gases. By sharing one electron with another chlorine atom, each atom fills its valence shell, forming a stable Cl extsubscript{2} molecule.

• Diatomic molecules consist of two atoms.
• In Cl extsubscript{2}, the two chlorine atoms share electrons.
• This shared configuration allows Cl to have a full valence shell.

Electron Configuration

Electron configuration is like the address of electrons in an atom. It shows us where each electron is located within atom shells and subshells. Let's take a closer look:

• Chlorine (Cl): Its electron configuration is \(1s^2 2s^2 2p^6 3s^2 3p^5\). It has 7 electrons in its outer shell.
• Neon (Ne): With the configuration \(1s^2 2s^2 2p^6\), it has a full outer shell, making it very stable.
• Helium (He): It is \(1s^2\), with a full first shell, also very stable.

This configuration is key to understanding why elements form molecules. Elements strive to achieve similar stability through bonds by adopting the noble gas configuration.

Noble Gases

Noble gases are unique due to their naturally full valence electron shells. This full outer shell makes them incredibly stable and not reactive under normal circumstances. The noble gases include helium, neon, and others, sitting in Group 18 of the periodic table.

• Helium (He): Only 2 electrons but full in the 1s subshell, thus very stable.
• Neon (Ne): Completed 2s and 2p shells lead to remarkable stability.

Their complete electron configurations mean they don't easily react or form bonds with other elements, unlike chlorine, which is not a noble gas. Instead, chlorine actively seeks additional electrons to reach stable electron configuration, driving it to form molecules like Cl extsubscript{2}.

Valence Electrons

Valence electrons are the electrons found in an atom's outermost shell. These are the electrons involved in bonding between atoms. How many valence electrons an element has determines how it interacts with others:

• Chlorine (Cl) has 7 valence electrons, just one short of a stable structure, encouraging the formation of bonds.
• Neon (Ne) possesses a complete outer shell with 8 valence electrons (in this case, the second shell), resulting in its stability.
• Helium (He) owns 2 valence electrons completing its shell, making it also stable.

Elements with fewer than the ideal number will seek to share, gain, or lose electrons to achieve a stable configuration, which is why only chlorine forms diatomic molecules naturally, unlike neon and helium.