Question

Which of the following hydrides is electron deficient? (a) \(\mathrm{NaH}\) (b) \(\mathrm{CaH}_{2}\) (c) \(\mathrm{CH}_{4}\) (d) \(\mathrm{B}_{2} \mathrm{H}_{6}\)

Short answer

Dihydridoboron, or \(\mathrm{B}_{2} \mathrm{H}_{6}\), is the electron deficient hydride.

Step-by-step solution

Understand Electron Deficient Hydrides

Electron deficient hydrides are compounds where the central atom has less than an octet of electrons. These are generally formed by elements of group 13 in the Periodic Table such as boron and aluminum.

Analyze the Given Compounds

For each given hydride, check if the central atom (the non-hydrogen atom) has a complete octet or not. Sodium (Na) and calcium (Ca) are group 1 and group 2 elements respectively; they typically form ionic compounds and their hydrides are not electron deficient. Methane (CH4) is a covalent compound with carbon having a complete octet. Boranes, like B2H6, often have boron atoms with incomplete octets.

Identify the Electron Deficient Hydride

Among the options, B2H6 has boron atoms, which typically have 3 valence electrons and can form compounds that are electron deficient. In B2H6, each boron atom has only 6 electrons in its valence shell, making it electron deficient.

Key concepts

Periodic Table Group 13

The elements that belong to Group 13 of the Periodic Table are known for their unique chemical properties, primarily due to their placement in the p-block. This group is comprised of boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Tl). One standout feature of these elements is their tendency to form electron deficient compounds.

Electron deficient compounds are characterized by having central atoms that lack a full octet of electrons in their valence shells. The classical octet rule states that atoms tend to form compounds in ways that give them eight valence electrons, leading to more stable configurations. However, Group 13 elements often buck this trend by forming stable structures with less than eight electrons.

Boron Hydrides

For example, boron, the simplest group 13 element, forms a number of electron deficient hydrides, known as boranes. The electron deficiency arises because boron has only three valence electrons, yet it still forms stable compounds without reaching an octet.

Covalent Compounds

Covalent compounds are chemical species where two or more non-metal atoms share pairs of electrons to attain stability. Differing from ionic bonding where electrons are transferred to achieve full valence shells, covalent bonding involves the mutual sharing of electrons.

In the context of electron deficient hydrides, such as those formed with Group 13 elements, there is an interesting twist to the usual covalent bonding scenario. While typical covalent compounds like water (H₂O) or methane (CH₄) have central atoms with a complete octet, electron deficient hydrides do not. These species utilize multi-center bonding or 'banana bonds' where two or more atoms share a bonding pair of electrons, which allows them to maintain stability despite having fewer electrons in the outermost shell compared to the octet rule.

Valence Electrons

Valence electrons play a pivotal role in the chemical bonding of elements. These are the electrons that are typically found in the outermost shell of an atom and are responsible for forming bonds with other atoms. The number of valence electrons directly influences how an element reacts and what kinds of compounds it can form.

In the Group 13 elements, the number of valence electrons is three, which is key to their ability to form electron deficient compounds. Boron, for instance, has three valence electrons, enabling it to form compounds like diborane (B₂H₆). In diborane, each boron atom is effectively short by three electrons if we apply the concept of the octet rule. This shortfall leads to the formation of three-center two-electron bonds, which are non-conventional types of covalent bonds that contribute to the electron deficiency of such hydrides.

Chemical Bonding

Chemical bonding is the force that holds atoms together in compounds. The three primary types of chemical bonds are ionic, covalent, and metallic. Ionic bonds are formed through the transfer of electrons from metals to non-metals, covalent bonds from the sharing of electrons between non-metals, and metallic bonds are between metal atoms involving a 'sea' of delocalized electrons.

The concept of chemical bonding is especially intriguing when considering electron deficient hydrides. These compounds often exhibit bonding that doesn't fit neatly into conventional categories. For example, diborane includes 'three-center two-electron bonds', which involve two boron atoms and one hydrogen atom sharing a pair of electrons. Such innovative bonding arrangements underscore the adaptive nature of elemental interactions and the depth of diversity within chemical bonding paradigms.