The molecule \(\mathrm{HCl}\) is known, but the molecule \(\mathrm{HeCl}\) is not. Explain why this is so.

In this case, we have three elements involved: hydrogen (H), helium (He), and chlorine (Cl). Hydrogen (H) has 1 electron, and its electron configuration is 1s^1 Helium (He) has 2 electrons, and its electron configuration is 1s^2 Chlorine (Cl) has 17 electrons, and its electron configuration is 1s^2 2s^2 2p^6 3s^23p^5 The electron configuration of an atom determines how it forms bonds with other atoms.

For the formation of an HCl molecule, hydrogen has one electron in its valence shell and needs one more to achieve a stable electron configuration (1s^2). Chlorine, on the other hand, has 7 electrons in its outer shell (3p^5) and needs one more to achieve a stable configuration (3p^6). A covalent bond can form between hydrogen and chlorine because hydrogen has an electron to share with chlorine, and chlorine has an open space to accept that electron. Because both atoms achieve stable electron configurations by sharing electrons, they are more likely to form an HCl molecule.

For the formation of a HeCl molecule, helium has two electrons in its valence shell (1s^2), which means it already has a stable electron configuration and does not need any additional electrons. Chlorine, as mentioned earlier, has 7 electrons in its outer shell (3p^5) and needs one more to achieve a stable configuration (3p^6). Since helium already has a full valence shell, it is not interested in forming a bond with chlorine, as that would disrupt its stable electron configuration. Additionally, chlorine still wants to accept an electron to achieve a stable configuration, but helium does not have an electron to give. Thus, this mismatch in electron requirements makes it highly unlikely for a HeCl molecule to form.

The reason HCl is a known molecule and HeCl is not is due to the properties of the elements involved and their ability to form bonds based on their electron configurations. Hydrogen and chlorine can form a covalent bond because they both achieve stable electron configurations through sharing electrons. In contrast, helium already has a stable electron configuration and does not need any additional electrons.