Question

What is not a true statement? (a) Compounds with \(\mathrm{C}=\mathrm{C}\) double bonds and \(\mathrm{Si}=\mathrm{Si}\) double bonds are quite common. (b) \(\mathrm{BF}_{3}\) is a gaseous molecular halide but \(\mathrm{AlF}_{3}\) is a high melting ionic solid. (c) \(\mathrm{BeO}\) is amphoteric but the oxides of the other group \(2 \mathrm{~A}\) elements are basic. (d) B differs from other elements of group \(3 \mathrm{~A}\) by forming mainly covalent molecular compounds.

Short answer

Statement (a) is not true.

Step-by-step solution

Analyze Statement (a)

Examine if compounds with \(\mathrm{C} = \mathrm{C}\) and \(\mathrm{Si} = \mathrm{Si}\) double bonds are common. Known chemical behavior indicates that \(\mathrm{C} = \mathrm{C}\) double bonds are common in organic chemistry, but \(\mathrm{Si} = \mathrm{Si}\) double bonds are rare due to silicon's preference for single bonds and the tendency to form network structures. Thus, statement (a) is likely not true.

Analyze Statement (b)

Review the properties of \(\mathrm{BF}_3\) and \(\mathrm{AlF}_3\). \(\mathrm{BF}_3\) is a well-known gaseous molecule where boron forms covalent bonds, whereas \(\mathrm{AlF}_3\) forms an ionic solid with a high melting point due to aluminum's greater positive charge and ability to attract fluoride ions more strongly. Statement (b) describes these properties accurately; hence, it is true.

Analyze Statement (c)

Consider the amphoteric nature of \(\mathrm{BeO}\) compared to other group 2A oxides. \(\mathrm{BeO}\) is known for its amphoteric behavior, reacting with both acids and bases, whereas the oxides of other group 2A elements (like \(\mathrm{MgO}, \mathrm{CaO}\)) are typically basic, reacting primarily with acids. This statement accurately reflects the property differences within the group and is true.

Analyze Statement (d)

Evaluate the tendency of boron to form covalent compounds as compared to other group 3A elements. Boron commonly forms covalent molecular compounds due to its small size and higher electronegativity, unlike aluminum and other heavier group 3A elements that more readily form ionic compounds. Statement (d) correctly characterizes this distinction, thus being true.

Key concepts

C=C Bonds

In organic chemistry, the presence of carbon-carbon double bonds, denoted as \(\mathrm{C}=\mathrm{C}\), is a defining trait. These bonds are crucial in many organic molecules, making them very common. The double bond consists of a sigma bond and a pi bond. As a result, these bonds create a planar structure that can impact the geometry and reactivity of the molecules. You'll find \(\mathrm{C}=\mathrm{C}\) bonds in alkenes, which are hydrocarbons like ethene.

Each carbon in the \(\mathrm{C}=\mathrm{C}\) bond is typically bonded to other atoms by single sigma bonds, making these compounds unsaturated and prone to addition reactions, where other atoms can add across the double bond. This property makes alkenes reactive and useful in various chemical reactions, like polymerization and halogenation. Unlike carbon, silicon does not commonly form stable double bonds such as \(\mathrm{Si}=\mathrm{Si}\), favoring structures where it creates a series of single bonds, leading to network structures like silicate minerals.

Amphoteric Oxide

An amphoteric oxide can react with both acids and bases, effectively demonstrating dual behavior. A classic example is beryllium oxide (\(\mathrm{BeO}\)). This unique property of \(\mathrm{BeO}\) sets it apart from other group 2A oxides like \(\mathrm{MgO}\) or \(\mathrm{CaO}\), which mainly show basic characteristics. These basic oxides typically react with acids to form water and salts.

The ability of an oxide to exhibit amphoterism often depends on the nature of the element it is formed from and its position in the periodic table. Beryllium's small size and relatively high charge density contribute to \(\mathrm{BeO}\)'s amphoteric nature. In contrast, the larger sizes and lower charge densities of magnesium and calcium favor a straightforward basic reaction profile.

Ionic Compound

Ionic compounds are formed when atoms transfer electrons between each other, creating ions that attract one another due to opposite charges. A typical example is aluminum fluoride (\(\mathrm{AlF}_3\)), which consists of aluminum cations (\(\mathrm{Al}^{3+}\)) and fluoride anions (\(\mathrm{F}^{-}\)). These ions arrange in a repeating pattern called a lattice structure. This structure results in high melting points and the characteristic crystalline form of ionic solids.

In contrast to covalent bonds where atoms share electrons, in ionic compounds, one atom donates one or more electrons while the other atom accepts, leading to electrostatic attraction. This is why \(\mathrm{AlF}_3\) is considered an ionic compound with properties such as high melting points and solubility in water, where they can dissociate into their respective ions.

Covalent Bonding

Covalent bonding is all about sharing electrons between atoms, leading to the formation of a molecule with shared electron pairs. An example of this is found in molecules like boron trifluoride (\(\mathrm{BF}_3\)), where boron covalently bonds with three fluorine atoms. In covalent bonds, the atoms involved typically have similar electronegativities, allowing them to share electrons relatively equally.

In covalent molecular compounds, the bonds form specific geometries, determining the shape of the molecule. For instance, in \(\mathrm{BF}_3\), the molecule is trigonal planar due to the three bond pairs around boron. This type of bonding results in molecules with distinct properties, such as lower melting points compared to ionic compounds, and many exist as gases or liquids at room temperature.

• Covalent bonds provide stability to compounds through electron sharing.
• The strength of covalent bonds depends on the atoms involved and their ability to attract shared electrons.