Question

How can you explain higher stability of \(\mathrm{BCl}_{3}\) as compared to \(\mathrm{TICl}_{3}\) ?

Short answer

BCl3 is more stable due to the absence of thallium's inert pair effect, which makes +3 state less stable.

Step-by-step solution

Understand the Elements Involved

First, recognize that \(\mathrm{BCl}_3\) consists of boron (B) and \(\mathrm{Cl}\) atoms, while \(\mathrm{TICl}_3\) consists of thallium (Tl) and \(\mathrm{Cl}\) atoms. Boron is in Group 13 of the periodic table, just like thallium, but it is a smaller and lighter element.

Examine Electronic Configurations

Boron has an electronic configuration of \([He] \ 2s^2 2p^1\), while thallium's configuration is \([Xe] \ 4f^{14} 5d^{10} 6s^2 6p^1\). The presence of fully filled \(4f\) and \(5d\) orbitals causes poor shielding and higher effective nuclear charge for thallium.

Consider Oxidation States

Boron typically forms \(\mathrm{BCl}_3\) with a +3 oxidation state. Thallium, however, prefers a +1 oxidation state due to the inert pair effect, making the +3 state in \(\mathrm{TICl}_3\) less stable.

Apply the Inert Pair Effect

The inert pair effect is prominent in heavier elements, making the +1 state more stable for thallium. This effect is due to the hesitation of the \(6s\) electron pair to participate in bonding, causing \(\mathrm{TICl}_3\) to be less stable.

Compare Molecular Stability

With boron's small size and lack of inert pair effect, \(\mathrm{BCl}_3\) is stable with a +3 oxidation state. In contrast, \(\mathrm{TICl}_3\) is less stable with thallium in the same oxidation state due to the inert pair effect.

Key concepts

Inert Pair Effect

The inert pair effect is a trend seen especially in heavier elements of groups 13 to 17, like thallium. It refers to the tendency of the outermost s-electron pair to remain non-ionized or unshared in chemical bonding. This effect becomes more pronounced down the group as the atomic number increases.

The reason behind this effect is primarily the poor shielding ability of inner electrons found in f and d orbitals. These orbitals do not shield the nuclear charge well, leading to a stronger attraction of the electrons to the nucleus. This makes it energetically unfavorable for the s-electrons to participate in bonding, rendering them "inert."

In the context of thallium in \({TICl}_3\), the inert pair effect leads to the electron configuration where the 6s electrons are less likely to be involved in bonding, favoring a +1 oxidation state rather than +3.

• Heavy elements exhibit a noticeable inert pair effect.
• Results in decreased participation of s-electrons in bonding.
• More pronounced in thallium compared to lighter elements like boron.

Oxidation States

An oxidation state indicates the degree of oxidation of an atom in a chemical compound. Commonly known as oxidation numbers, these states signify how many electrons an atom has gained, lost, or shared when forming a chemical bond.

Boron, despite being in Group 13 along with thallium, exhibits a +3 oxidation state in compounds like \(\mathrm{BCl}_3\). This is because boron has empty p-orbitals available and readily uses its valence shell electrons in bonding. Consequently, it forms stable covalent bonds.

Thallium, on the other hand, displays different behavior mostly due to the inert pair effect. While it can display a +3 oxidation state as in \(\mathrm{TICl}_3\), it is less stable compared to its +1 state. This lesser stability arises because the 6s electrons are less inclined to participate in bonding.

• Boron stabilizes in a +3 oxidation state due to efficient bonding.
• Thallium prefers a +1 oxidation state due to the reluctance of the 6s electron pair.
• Oxidation state stability influences molecular stability.

Effective Nuclear Charge

Effective nuclear charge, often denoted as \(Z_{\text{eff}}\), is the net positive charge experienced by an electron in a multi-electron atom. It represents the actual nuclear charge reduced by the shielding effect of inner-shell electrons.

In heavier elements like thallium, electrons in the f and d orbitals exhibit poor shielding efficiency, resulting in a higher effective nuclear charge. Consequently, outer electrons experience a stronger attraction to the nucleus. This increased attraction limits their availability for bonding, further accentuating effects like the inert pair effect.

For boron, with fewer inner electrons and an absence of f and d orbitals, the effective nuclear charge is relatively simpler, leading to a more stable bonding environment in compounds like \(\mathrm{BCl}_3\).

• Higher effective nuclear charge in thallium's influences electron involvement.
• Poor inner electron shielding in heavy atoms affects bonding.
• Boron benefits from lesser electron-electron repulsion in its configuration.