Question

Give reasons for the following: (a) Tripositive or trinegative charged ions are rare. (b) Some elements show variable convalency. (c) \(\mathrm{CCl}_{4}\) is non-polar but \(\mathrm{CHCl}_{3}\) is polar (d) \(\mathrm{PCl}_{5}\) exists but \(\mathrm{NCl}_{5}\) does not exist.

Short answer

Tripositive or trinegative ions require high energy for three electrons transfer hence are rare. Elements show variable convalency due to the availability of different electron shells for bonding. \(\mathrm{CCl}_{4}\) is non-polar due to symmetry of chlorines around the carbon, while \(\mathrm{CHCl}_{3}\) is polar due to overall dipole. \(\mathrm{PCl}_{5}\) exists because phosphorus can increase its covalency by using vacant d-orbitals, which is not possible for nitrogen, hence \(\mathrm{NCl}_{5}\) doesn't exist.

Step-by-step solution

Explain tripositive or trinegative charged ions rarity

Tripositive or trinegative ions are rare due to the energy required to remove or add three electrons. Removing three electrons from an atom requires a substantial amount of energy, which is not typically available in natural reactions. Likewise, adding three electrons to an atom is difficult because it necessitates overcoming the electric repulsion from the other negatively charged electrons.

Discuss variable convalency of some elements

Some elements, especially transition metals, exhibit variable convalency because they have more than one electron shell available for bonding. This is due to the presence of d-orbitals, which can accommodate additional electrons and thus cause variation in the convalency.

Explain the polarity of \(\mathrm{CCl}_{4}\) and \(\mathrm{CHCl}_{3}\)

The difference in polarity in \(\mathrm{CCl}_{4}\) and \(\mathrm{CHCl}_{3}\) is due to their molecular geometry. In \(\mathrm{CCl}_{4}\), the chlorines are symmetrically distributed around the carbon, making it non-polar as the dipole moments cancel out. In contrast, in \(\mathrm{CHCl}_{3}\), one of the bonds is a Carbon-Hydrogen bond which generates an overall dipole, making the molecule polar.

Discuss the existence of \(\mathrm{PCl}_{5}\) and \(\mathrm{NCl}_{5}\)

\(\mathrm{PCl}_{5}\) exists and \(\mathrm{NCl}_{5}\) does not exist due to the number of valence electrons in the outer shell and the expansion of the octet. Nitrogen has 5 valence electrons, meaning it can form three covalent bonds and doesn’t have d-orbital to expand its octet. Phosphorus can expand its octet as it has vacant d-orbitals to accommodate more than 8 electrons.

Key concepts

Tripositive and Trinegative Ions

Understanding the rarity of tripositive and trinegative ions requires an insight into the energy changes accompanying electron gain or loss. Atoms seek stability through the gain or loss of electrons to achieve a noble gas electron configuration. However, acquiring a charge of \(3+\) or \(3-\) involves significant energy. For tripositive ions, this means removing three outer electrons, where each successive electron removal encounters greater nuclear attraction, thus requiring more energy. Meanwhile, trinegative ions must add three electrons against the repulsive force of existing electrons. Natural processes rarely supply the energy necessary for such transformations, making these ions uncommon.

An example of a rare tripositive ion is the aluminum ion, \(Al^{3+}\), which occurs because aluminum can lose three electrons more readily compared to elements with a larger atomic radius.

• Energy and stability considerations limit the formation of highly charged ions
• Tripositive ions like \(Al^{3+}\) are more common in metals with fewer electron shells

Variable Convalency

The concept of variable convalency can be intriguing as it showcases an element's ability to form different numbers of covalent bonds. Elements with access to d-orbitals, such as transition metals, have the fascinating characteristic of exhibiting variable convalency because these orbitals can house additional electrons. This allows the element to share not only the s or p electrons but also the d electrons in bonding, leading to multiple valencies. For instance, iron can exist as \(Fe^{2+}\) or \(Fe^{3+}\), displaying valencies of 2 and 3, respectively.

An essential point in this context is that variable convalency is predominantly observed in elements that have incomplete d-orbitals, and this property is vital in the formation of complex compounds.

• Transition metals are prime examples of variable convalency
• The presence of accessible d-orbitals allows for a variety of bonding configurations

Molecular Polarity

Molecular polarity arises from the distribution of electrons within a molecule and the shape of the molecule itself. If the electrons are symmetrically arranged and the molecular shape is such that it allows for an even distribution of charge, the molecule will be non-polar since the dipole moments cancel each other out. In contrast, \(CHCl_{3}\) is polar because the hydrogen atom creates an asymmetry in charge distribution, resulting in a net dipole moment.

The polar nature of molecules affects properties such as solubility and boiling points, influencing how substances interact with one another.

• Electronegativity differences and molecular geometry dictate polarity
• Dipole moments in symmetrical molecules like \(CCl_{4}\) cancel out, making them non-polar

Octet Rule and D-orbitals

The octet rule is a chemical guideline that reflects the observation that atoms tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The rule is based on the stability associated with the full s and p orbitals of the noble gases. However, for elements in the third period and below, such as phosphorus in \(PCl_{5}\), the presence of d-orbitals allows the element to accommodate more than eight electrons, thus expanding the octet.

This expansion is not possible for elements like nitrogen in \(NCl_{5}\), which do not have available d-orbitals in their valence shell. Consequently, while \(PCl_{5}\) can form five bonds, \(NCl_{5}\) cannot exist due to the unavailability of d-orbitals to expand beyond the octet.

• D-orbitals in period three and below allow expansion beyond the octet
• \(PCl_{5}\) exemplifies the octet rule's exceptions, whereas \(NCl_{5}\) demonstrates its limitations