Question

Which group of elements shows lowest ionisation enthalpy? (a) Alkali metals (b) Alkaline earth metals (c) Halogens (d) Noble gases

Short answer

Alkali metals (a) show the lowest ionisation enthalpy.

Step-by-step solution

Understanding Ionisation Enthalpy

Ionisation enthalpy, also known as ionization energy, is the energy required to remove the most loosely bound electron from an isolated gaseous atom to form a cation. It tends to decrease down a group in the periodic table as the atomic size increases, which causes the outermost electron to be further away from the nucleus and less tightly held due to the increased shielding effect.

Comparing Groups on the Periodic Table

Based on the periodic trends, alkali metals, which are in Group 1, have the largest atomic radii within their respective periods compared to the other groups given in the options. Alkali metals have only one electron in their outermost shell, which experiences the maximum shielding effect and is furthest from the nucleus. This makes the electron easier to remove, resulting in a lower ionisation enthalpy.

Selecting the Group with Lowest Ionisation Enthalpy

Since the alkali metals have the weakest hold on their outermost electron, they have the lowest ionisation enthalpy compared to alkaline earth metals, halogens, and noble gases. Therefore, the group of elements that shows the lowest ionisation enthalpy is the alkali metals.

Key concepts

Alkali Metals

Alkali metals, located in Group 1 of the periodic table, are well known for their distinctive properties, owing much to their position in the periodic trends. They are characterized by their single valence electron, which resides in the outermost energy level of their atomic structure.

This aspect of their electron configuration is crucial because it contributes directly to the physical and chemical behaviors observed in these elements, such as their high reactivity and relatively low ionization enthalpy. Ionisation enthalpy is pivotal because it influences how an element reacts and forms compounds, particularly ionic ones.

Examples and Reactivity

Notable examples of alkali metals include lithium (Li), sodium (Na), and potassium (K), to name a few. They react vigorously with water to produce hydrogen gas and a corresponding hydroxide, a testament to their low ionisation enthalpy, which makes the removal of their sole valence electron less energy-intensive compared to other elements.

Periodic Trends

Understanding periodic trends is integral to grasping why certain elements exhibit particular properties, such as ionisation enthalpy. Ionisation enthalpy typically decreases down a group as we move from top to bottom on the periodic table.

This phenomenon is due to the increasing atomic size which stems from adding more electron shells. Consequently, the outermost electrons are further from the attractive force of the nucleus. In contrast, across a period from left to right, ionisation enthalpy tends to increase because of the addition of protons in the nucleus, which enhances the electrostatic pull on the electrons without significantly increasing atomic size.

Atomic Radii

The atomic radius is a measure of the size of an atom, typically the distance from the nucleus to the boundary of the surrounding cloud of electrons. Understanding atomic radii is crucial for explaining trends in ionisation enthalpy.

As you move down a group on the periodic table, the atomic radii increase due to the additional electron shells, which places the valence electron further from the nucleus and weakens its binding energy. It also explains why a larger atom, like those of alkali metals, will typically have a lower ionisation enthalpy, as the increased distance reduces the nuclear attraction on the valence electron.

Shielding Effect

The shielding effect is the phenomenon by which the core electrons block the attraction between the nucleus and the valence electrons. This effect becomes more significant in atoms with a larger number of electron shells.

As the number of shells increases, the inner electrons provide a 'shield', reducing the full positive charge effect felt by the valence electrons from the nucleus. It's a key factor in why elements like the alkali metals, which have more electron shells, experience lower ionisation enthalpies. The larger the shielding effect, the easier it is for an atom to lose its outermost electrons and thus, the lower the ionisation enthalpy will be.