Question

Beryllium is divalent white carbon is tetravalent in their compounds. Justify the statement on the basis of hybridisation.

Short answer

Beryllium is divalent and carbon is tetravalent due to their respective hybridisations. Beryllium's two 2s electrons hybridise to form two sp orbitals, allowing it to form two bonds. Carbon, however, promotes one 2s electron to 2p, allowing the four available electrons to hybridise into four sp3 orbitals and form four bonds.

Step-by-step solution

- Understanding Valency

Valency is the property of an element that determines the number of other atoms with which an atom of the element can combine. Beryllium is a divalent because its valency is two i.e., it can combine with two atoms of other elements. Carbon, on the other hand, is tetravalent because it combines with four atoms of other elements.

- Beryllium Hybridisation

To better understand beryllium's divalency, it's important to look at its electronic configuration, which is \( \[He] 2s^2 \]\). During formation of compound, the two 2s electrons of beryllium can hybridise to form two sp hybridised orbitals which can form two sigma bonds, thus explaining beryllium's divalency.

- Carbon Hybridisation

Carbon's tetravalency can similarly be explained by its electronic configuration \( \[He] 2s^2 2p^2 \]\). It can promote one of the 2s electrons to the 2p orbital and the four available electrons can then hybridize to form four sp3 hybrid orbitals, capable of forming four sigma bonds with other atoms. This explains carbon's tetravalency.

Key concepts

Understanding Valency

Valency is a fundamental concept in chemistry that refers to the combining power of an element, indicating the number of chemical bonds an atom can form with other atoms. It reflects the ability of an atom to gain, lose, or share electrons in order to achieve a stable electronic configuration, often matching the nearest noble gas structure.

For example, in the given exercise, beryllium with an atomic number of 4 has two electrons in its outer shell and hence it has a valency of two, making it divalent. This means it can form bonds with two atoms by sharing its two valence electrons. On the other hand, carbon, with an atomic number of 6, has four electrons in its outer shell and thus has a valency of four, allowing it to be tetravalent.

To make the concept more comprehensible for students, one could discuss the Octet Rule, which states that atoms tend to form bonds to have eight electrons in their valence shell, thereby achieving a stable electron arrangement similar to the noble gases. This foundational rule helps explain why atoms like oxygen typically form two bonds (since it has six valence electrons and needs two more for an octet), or why nitrogen forms three bonds (with five valence electrons, it requires three more to complete its octet).

Electronic Configuration

The electronic configuration of an atom is a description of the distribution of its electrons in atomic orbitals. It is crucial for understanding how atoms will interact and bond with each other. For elements like beryllium and carbon, their electronic configurations can help explain their valency and bonding behavior.

Beryllium has the electronic configuration of \[He] 2s^2\], which means it has two electrons in the 2s subshell beyond the helium core. In the case of carbon, its configuration is \[He] 2s^2 2p^2\], indicating four electrons are available to occupy the second principal energy level. The configuration hints at how these electrons can be shared or exchanged during the formation of chemical bonds.

To delve deeper, one can introduce the concept of orbitals—where electrons are located around the nucleus—and how electrons fill these orbitals according to the Aufbau principle, Pauli exclusion principle, and Hund's rule. This framework provides students with a roadmap of electron arrangement and supports their understanding of why atoms exhibit certain valencies.

Sigma Bonds

Sigma (\(\text{\sigma}\)) bonds represent the strongest type of covalent chemical bond and are formed by the head-on overlapping of atomic orbitals. They are key to molecular structure and define the way atoms are connected in a molecule. In both beryllium and carbon compounds, sigma bonds are vital in maintaining structural integrity.

In our exercise context, beryllium's two sp hybrid orbitals overlap with orbitals from other atoms to form two sigma bonds. This overlapping allows beryllium to achieve a stable molecular configuration. Carbon, with its four sp3 hybrid orbitals, can form four sigma bonds, one with each of the four orbitals, leading to its tetravalency in compounds such as methane (\(CH_4\)).

Breaking it down further, a single sigma bond is often visualized as a single 'line' between two atoms in molecular diagrams. For students, understanding that a single line represents a stable electron pair shared between two atoms can be a springboard to exploring more complex molecular geometries and types of chemical bonding.