Question

Identify the group \(4 A\) element that best fits each of the following descriptions: (a) Prefers the \(+2\) oxidation state (b) Forms the strongest \(\pi\) bonds (c) Is the second most abundant element in the Earth's crust (d) Forms the most acidic oxide

Short answer

(a) Lead (Pb), (b) Carbon (C), (c) Silicon (Si), (d) Carbon (C)

Step-by-step solution

Identify the group 4A elements

The group 4A elements, also known as the carbon group, include the elements carbon (C), silicon (Si), germanium (Ge), tin (Sn), and lead (Pb). These are the elements we will consider for each description.

Determine which element prefers the +2 oxidation state

Elements with higher atomic numbers in group 4A (like tin and lead) are more likely to exhibit the +2 oxidation state due to the inert pair effect. Lead (Pb) is especially common in the +2 oxidation state.

Identify the element that forms the strongest π bonds

The ability to form strong π bonds is associated with small size and high electronegativity. Carbon (C), being small and highly electronegative for a metal, forms very strong π bonds like those in double bonds (e.g., CO2, C=C).

Find the second most abundant element in the Earth's crust

Silicon (Si) is the second most abundant element in the Earth's crust, primarily found in the form of silicon dioxide and silicates.

Determine which element forms the most acidic oxide

The acidity of oxides usually decreases down the group. Carbon dioxide (CO2) is the most acidic oxide formed by the group 4A elements.

Key concepts

Oxidation States

An oxidation state, often known as an oxidation number, refers to the degree of oxidation of an atom within a compound. It reflects the total number of electrons that an atom either gains or loses to form a chemical bond.
Group 4A elements, due to their varying electron arrangements, can have different oxidation states. As we move down the group, the oxidation state typically decreases.

• Lead (Pb) frequently displays a +2 oxidation state. This is due in part to the inert pair effect, where the s-electrons are not as available for bonding as the p-electrons.
• Tin (Sn) can also show a +2 state, but it commonly prefers a +4 state due to its position higher up in the group compared to lead.

The inert pair effect becomes more pronounced in heavier group 4A elements, thus making them more stable in lower oxidation states like +2.

Pi Bonds

Pi bonds are a type of covalent bond that forms when the atomic orbitals overlap sideways, allowing for the sharing of electrons in a region above and below the internuclear axis. These bonds are crucial in multiple bond formations, such as double or triple bonds.
Carbon (C), in group 4A, is notorious for forming robust pi bonds due to its small atomic size and high electronegativity. These characteristics allow for effective overlapping of p-orbitals, thus forming strong pi bonds.

• In carbon dioxide (CO extsubscript{2}), the carbon-oxygen double bond includes a pi bond that adds strength to the molecule.
• Similarly, in organic compounds like alkenes, the carbon-carbon double bond is stabilized by the presence of a pi bond.

As you descend the group, elements like silicon are less effective in forming strong pi bonds due to increased atomic size.

Earth's Crust Abundance

The abundance of elements in the Earth's crust varies significantly, and this can affect not just their availability but also their applications in various geological and technological processes. Group 4A elements, while not all equally abundant, have significant roles.
Silicon (Si) stands out as the second most abundant element in the Earth's crust. It primarily exists as silicon dioxide (quartz) or silicate minerals.
This high abundance is due to silicon's role in forming a range of silicate minerals, which are crucial components of most rocks:

• Silicates are foundational to the Earth's crust, making up over 90% of its composition.
• Silicon dioxide is not only an integral mineral but is also vital in sand and clay manufacture.

This abundance makes silicon indispensable for the glass, electronics, and construction industries.

Acidic Oxides

Acidic oxides are oxides that react with water to form acids or that react with bases and alkalis to form salts. The acidity of such oxides usually declines down a group in the periodic table.
In the context of group 4A elements, carbon dioxide (CO extsubscript{2}) is noted for its acidic oxide nature. When dissolved in water, it forms carbonic acid (H extsubscript{2}CO extsubscript{3}).

• Carbon dioxide's small size and ability to readily form covalent bonds aid in its acidic properties.
• As you move to heavier group 4A elements, the acidic nature decreases; for instance, silicon dioxide (SiO extsubscript{2}) does not exhibit the same acidic characteristics, as it is mostly stable and forms no acids with water.

This change is because as elements get larger and more metallic, their oxides tend to be less acidic and more amphoteric or basic.