Problem 42 - Chemistry The Molecular Nature of Matter

Question

What is the minimum number of electrons that would be expected to be in the valence shell of \(\mathrm{As}\), in \(\mathrm{As} \mathrm{Cl}_{5}\) ?

Step-by-Step Solution

Answer

The minimum number of valence electrons expected in the valence shell of As in \(\mathrm{AsCl}_5\) is 5.

Step 1: Determine the Group Number

Identify the group number from the periodic table for the element arsenic (As). Arsenic is in Group 15 of the periodic table.

Step 2: Identify Valence Electrons

Elements in Group 15 typically have 5 valence electrons. These are the electrons in the outermost shell that can participate in the formation of chemical bonds.

Step 3: Examine the Compound

The compound given is \( \(\mathrm{AsCl}_5\).\) Since arsenic is the central atom and is capable of expanding its octet using d-orbitals, it will share its valence electrons with each chlorine atom in the form of covalent bonds.

Step 4: Calculate the Minimum Number of Valence Electrons

The minimum number of valence electrons for arsenic in this compound would correspond to the number of covalent bonds formed with chlorine atoms, which is 5. Hence, arsenic must have at least 5 valence electrons available for bonding with the chlorine atoms.

Key Concepts

Valence Electrons and the Periodic Table

Valence electrons are the outermost electrons of an atom and are crucial for forming chemical bonds. These electrons determine the chemical behavior of an element, including its reactivity and bonding capacity. The periodic table, which organizes elements based on atomic number and valence electron configurations, helps us to predict the bonding characteristics of an element. For arsenic (\textbf{As}), located in Group 15, the periodic table indicates it typically has 5 valence electrons. These electrons are available for bonding and define arsenic's ability to engage in chemical compounds like \textbf{AsCl}\(_5\).

Chemical Bonds

Chemical bonds are the connections between atoms that occur when valence electrons are shared, transferred, or pooled. Chemical bonding is driven by atoms reaching a more stable, lower-energy configuration. There are several types of chemical bonds, including ionic, covalent, and metallic bonds. In the case of \textbf{AsCl}\(_5\), arsenic forms covalent bonds with each of the five chlorine atoms, sharing one pair of electrons with each chlorine to achieve bond formation.

The Octet Rule

The octet rule states that atoms tend to form compounds in ways that give them eight valence electrons, resembling the electron configuration of a noble gas. This rule explains the stability associated with noble gases, which do not typically form bonds because they naturally have eight valence electrons. However, some elements, particularly those found in the d-block of the periodic table, can have more than eight electrons in their valence shells due to the involvement of d-orbitals. This is known as the expansion of the octet, which we see with arsenic in the compound \textbf{AsCl}\(_5\).

Covalent Bonding

Covalent bonding is a type of chemical bond where two atoms share pairs of electrons. This sharing allows each atom to attain the electronic configuration of a noble gas, satisfying the octet rule. Compounds like \textbf{AsCl}\(_5\) demonstrate covalent bonding with arsenic sharing its valence electrons with chlorine atoms. Such sharing results in the formation of a molecule where atoms are held together by shared electron pairs.

d-Orbitals and Bonding

d-Orbitals play a significant role in the chemical bonding of transition metals and elements capablle of expanding their octets. These orbitals are available for elements in the d-block of the periodic table, starting from the fourth period onwards. Arsenic, while not a transition metal, can use its d-orbitals to accommodate more electrons than the eight allowed by the octet rule. This allows arsenic to form more than four covalent bonds, as seen in the compound \textbf{AsCl}\(_5\), where a total of five shared electron pairs are present.