Question

In the periodic table, the maximum chemical reactivity is at the extreme left (alkali metals) and extreme right (halogens). Which properties of these two groups are responsible for this? (a) Least ionisation enthalpy on the left and highest negative electron gain enthalpy on the right. (b) Non-metallic character on the left and metallic character on the right. (c) High atomic radii on the left and small atomic radii on the right. (d) Highest electronegativity on the left and least electronegativity on the right.

Short answer

The maximum chemical reactivity of alkali metals and halogens is due to the least ionization enthalpy on the left (alkali metals) and highest negative electron gain enthalpy on the right (halogens), as stated in option (a).

Step-by-step solution

Understand the Trends in Reactivity for Alkali Metals and Halogens

Alkali metals are highly reactive because they have one valence electron that they can easily lose to achieve a stable noble gas electron configuration. This is facilitated by having low ionization enthalpy. Halogens are highly reactive because they require one additional electron to achieve a stable configuration, which is facilitated by a high negative electron gain enthalpy, attracting the needed electron.

Evaluate the Given Options

Option (a) correlates with the known chemical properties: alkali metals have the lowest ionization enthalpy because they can easily lose an electron, while halogens have the highest negative electron gain enthalpy since they readily accept an electron. The other options do not directly explain the reactivity of these groups. Non-metallic character and metallic character (option b), atomic radii (option c), and electronegativity (option d) are related properties but not the primary reason for reactivity in the context of the question.

Select the Correct Option

Considering the previous analysis, option (a) provides the correct explanation for the high reactivity of alkali metals and halogens. Alkali metals have the least ionization enthalpy which aids in the loss of an electron, and halogens have the highest negative electron gain enthalpy which aids in the gain of an electron.

Key concepts

Ionization Enthalpy

Understanding ionization enthalpy is essential to grasp why some elements tend to lose electrons more easily than others. Ionization enthalpy, also referred to as ionization energy, is the energy required to remove the most loosely bound electron from an isolated gaseous atom to form a positively charged ion. In the context of alkali metals, which reside on the extreme left of the periodic table, they exhibit the lowest ionization enthalpies of all elements.

This low energy requirement is due to their electronic configuration, with a single electron in the outermost shell far from the nucleus. Because these electrons are less tightly held by the nuclear charge, it takes less energy to remove them, facilitating the formation of cations (positively charged ions). This inherent property of alkali metals explains their eagerness to participate in chemical reactions, often resulting in the metals giving up their lone valence electron to achieve stability, mimicking the noble gas electron configuration.

Electron Gain Enthalpy

On the other hand, electron gain enthalpy is a concept tied to an atom's proclivity to accept extra electrons. It indicates the amount of energy released when an electron is added to an isolated gaseous atom to form an anion. Halogens, found on the extreme right side of the periodic table, signify the group with the highest negative electron gain enthalpies. With one electron short of a complete valence shell, halogens readily attract extra electrons to fill this gap, releasing a significant amount of energy in the process.

Since attaining a full valence shell is a fortuitous state for an atom, the high negative electron gain enthalpy demonstrates the halogens' reactivity, and their tendency to form anions. This high affinity for an additional electron is why halogens are so reactive and form salts when combined with alkali metals that are willing to donate electrons.

Alkali Metals

Alkali metals hold a unique position in the periodic table due to their distinctive characteristics. They include elements like lithium, sodium, and potassium, among others, all of which share similarities such as softness, high conductivity of heat and electricity, and vigorous reactions with water.

The reactivity of alkali metals can be mainly attributed to their electronic structure, consisting of one electron in the outermost shell. This single valence electron can be easily shed, which is a key factor in how alkali metals form ionic compounds, especially with nonmetals. For example, when alkali metals come into contact with halogens, they readily give up their electron, forming a stable ionic bond in the process. It's this characteristic that largely governs their position in the reactivity series, as well as their extensive use in a wide array of applications from batteries to biochemistry.

Halogens

Contrasting the alkali metals are the halogens, encompassing fluorine, chlorine, bromine, iodine, and astatine. Halogens are non-metallic elements incredibly reactive due to their electron configurations, which lack a single electron from being complete. They are notorious for their roles in forming a wide range of compounds through various types of chemical bonds.

Due to their high negative electron gain enthalpy, halogens have an innate capacity to attract electrons from other atoms, often resulting in the formation of ionic or covalent bonds. In nature, they typically exist as diatomic molecules (e.g., F2, Cl2) and exhibit a gradation of physical properties across the group. Fluorine, the most reactive halogen, is highly electronegative and can attack nearly all other elements to form compounds, illustrating the extreme reactivity that can be found at this end of the periodic table.