Define ionization enthalpy. Discuss the factors affecting ionisation enthalpy of the elements and its trends in the periodic table.

Ionization enthalpy, also known as ionization energy or ionization potential, is the energy required to remove an electron from a gaseous atom or ion in its ground state. It is typically measured in units of kilojoules per mole (kJ/mol) or electron volts (eV).

Mathematically, ionization enthalpy can be expressed as:

A(g)→A+(g)+e−A(g)→A+(g)+e−

Where:

• A(g) represents the gaseous atom.
• A^+ represents the resultant cation after losing one electron.
• e^- represents the removed electron.

Factors affecting ionization enthalpy of elements:

1. Nuclear Charge: The greater the nuclear charge (number of protons) in the nucleus of the atom, the stronger the attraction between the nucleus and the outermost electron. Consequently, it becomes more difficult to remove the outermost electron, resulting in higher ionization enthalpy. As you move across a period from left to right in the periodic table, the nuclear charge increases, leading to higher ionization enthalpy.

2. Atomic Radius: The distance between the outermost electron and the nucleus affects ionization enthalpy. A larger atomic radius implies that the outermost electron is farther away from the nucleus, experiencing weaker attraction. Therefore, atoms with larger atomic radii tend to have lower ionization enthalpies. As you move down a group in the periodic table, atomic radius increases due to the addition of electron shells, leading to lower ionization enthalpy.

3. Shielding Effect: The presence of inner electron shells shields the outermost electron from the full effect of the nuclear charge. Therefore, atoms with more inner electron shells experience less effective nuclear charge felt by the outermost electron, resulting in lower ionization enthalpy.

4. Subshell Stability: Fully filled or half-filled electron subshells tend to have greater stability. Elements with electron configurations that result in fully filled or half-filled subshells exhibit higher ionization enthalpy compared to elements with partially filled subshells.

Trends in ionization enthalpy in the periodic table:

1. Across a Period (Left to Right): Ionization enthalpy generally increases across a period due to the increasing nuclear charge. As you move from left to right across a period, the nuclear charge increases, resulting in stronger attraction between the nucleus and the outermost electron, making it more difficult to remove the electron.

2. Down a Group (Top to Bottom): Ionization enthalpy generally decreases down a group in the periodic table. This is because, as you move down a group, the outermost electron is farther away from the nucleus due to the addition of electron shells. As a result, the outermost electron experiences weaker attraction to the nucleus, making it easier to remove and leading to lower ionization enthalpy.

In summary, ionization enthalpy is the energy required to remove an electron from an atom, and it is influenced by factors such as nuclear charge, atomic radius, shielding effect, and subshell stability. These factors lead to predictable trends in ionization enthalpy across periods and down groups in the periodic table.