Define Ionisation enthalpy. Discuss the factor affecting ionisation enthalpy and its trends in the periodic table?

Ionization enthalpy, also known as ionization energy or ionization potential, is the amount of energy required to remove one mole of electrons from one mole of atoms in their gaseous state, resulting in the formation of positively charged ions. The process is represented by the equation:

X(g) → X⁺(g) + e⁻

where X represents an atom, g denotes the gaseous state, X⁺ represents the resulting positively charged ion (cation), and e⁻ represents the removed electron.

Factors affecting ionization enthalpy:

Nuclear Charge: Ionization enthalpy generally increases with the increase in nuclear charge, i.e., as you move across a period in the periodic table from left to right. This is because the protons in the nucleus have a stronger hold on the electrons in the outermost energy shell, making it harder to remove an electron.

Atomic Size: Ionization enthalpy tends to decrease with increasing atomic size, i.e., as you move down a group in the periodic table. This is because larger atoms have more electron shells, and the outermost electrons are farther away from the nucleus. The increased distance reduces the attraction between the nucleus and the outermost electrons, making it easier to remove them.

Electron Shielding: Electron shielding or screening effect occurs when inner electrons repel the outer electrons from the full force of the positive charge in the nucleus. This effect is more pronounced in larger atoms with multiple electron shells, reducing the effective nuclear charge experienced by the outermost electrons and making them easier to remove.

Subshell and Orbital Stability: Half-filled and fully-filled subshells or orbitals are more stable due to electron pairing effects. As a result, removing an electron from these stable configurations requires extra energy, leading to higher ionization enthalpy for elements with these electron configurations.

Trends in ionization enthalpy in the periodic table:

Across a period (left to right): Ionization enthalpy generally increases. This is because the nuclear charge increases, and the atomic size decreases, resulting in stronger attraction between the nucleus and the outermost electrons.

Down a group (top to bottom): Ionization enthalpy generally decreases. This is due to the increase in atomic size, increased electron shielding effect, and the outermost electrons being farther from the nucleus, making them easier to remove.

Exceptions to the trends: There are some notable exceptions to the general trends in ionization enthalpy. For example, the ionization enthalpy of Group 3A (Group 13) elements is higher than that of Group 2A (Group 2) elements due to the presence of stable half-filled p-orbitals in Group 3A elements. Additionally, there are irregularities in ionization enthalpy between s-block and p-block elements, as well as between transition metals due to varying electron configurations and electron repulsion effects.

In summary, ionization enthalpy is a measure of the energy required to remove an electron from an atom. It increases across a period and decreases down a group in the periodic table, with several factors influencing these trends.