Question

The ionisation energy of alkali metals decreases from \(L\) i to \(C s\) because (a) the atomic size increases from Li to Cs (b) the distance between nucleus and outermost orbital decreases from Li to Cs (c) dectropositive character decreases down the group (d) melting point decreases from Li to Cs.

Short answer

The ionisation energy of alkali metals decreases from Li to Cs because the atomic size increases from Li to Cs, making it easier to remove an outer electron due to weaker nuclear attraction.

Step-by-step solution

Understanding Ionisation Energy

Ionisation energy refers to the energy required to remove an electron from the outermost shell of an atom in its gaseous state. The trend in ionisation energy within a group of the periodic table generally decreases as one moves down the group.

Recognizing the Trend in Atomic Size

As we move down the group from lithium (Li) to cesium (Cs), the atomic radius increases. This is because each successive element has an additional electron shell, which increases the overall size of the atom.

Connecting Atomic Size to Ionisation Energy

The increase in atomic size from lithium to cesium means that the valence electron is further from the nucleus and is less tightly held by the nuclear charge. Thus, it requires less energy to remove this valence electron, showing that ionisation energy decreases.

Analyzing the Given Options

Among the given options, (a) the atomic size increases from Li to Cs is consistent with the explanation provided for why ionisation energy decreases. The rest of the options do not correctly explain the trend in ionisation energy for alkali metals.

Key concepts

Atomic Size Trend

Understanding the trend in atomic size among elements is crucial when exploring properties like ionisation energy. As one moves down a group in the periodic table, atoms gain additional electron shells. These added layers incrementally increase the distance between the nucleus and the outermost electrons. This effect is particularly noticeable in alkali metals, a group characterized by having one electron in their outermost shell.

For example, lithium (Li), being at the top of the alkali metals group, has the smallest atomic radius, whereas cesium (Cs), located further down, has more electron shells, leading to a larger radius. The significance of this trend? Well, it directly affects how tightly the nucleus holds onto its valence electrons, impacting their ease of removal - a key factor in determining ionisation energy.

Periodic Table Group Trends

The periodic table is structured in such a way that allows us to predict the behavior of elements based on their position. This structure is particularly valuable when examining group trends. In each group, or column, of the periodic table, elements share a common number of valence electrons but differ in the number of occupied electron shells.

Within a group, such as the alkali metals, one sees a progressive increase in atomic size, a decrease in electronegativity, and a reduction in ionisation energy as you proceed from the top to the bottom of the group. This is because the effect of adding an electron shell outweighs the increase in nuclear charge when considering the attraction on the valence electron. Consequently, as you look further down a group, elements tend to lose electrons more readily, reflecting in lower ionisation energies.

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom and they play a pivotal role in chemical reactions and bonding. An atom's chemical properties are largely defined by its valence electrons because they are the most distant from the nucleus and the most loosely bound. In the context of alkali metals, each element has a single valence electron.

These electrons are responsible for the chemical similarities within the group, such as their reactivity and tendency to form +1 ions. Since valence electrons in alkali metals are further from the nucleus in atoms further down the group, the ionisation energy required to remove these electrons decreases. Thus, understanding the behavior and properties of valence electrons is fundamental to explaining trends in the periodic table, such as ionisation energy among alkali metals.