Q.Which of the following have no unit? (Note: more than one of the given options may be correct.)

The key idea here is Standard State Conditions, which are specific conditions under which thermodynamic properties like enthalpy changes are typically measured and reported. However, the question specifically asks about the presence or absence of units for various chemical properties.

Let's analyze each option:

1. (A) Electronegativity: This is a relative measure of an atom's ability to attract electrons in a chemical bond. It is typically expressed on a dimensionless scale (e.g., Pauling scale, Mulliken scale), making it a quantity without units.
2. (B) Electron gain enthalpy: This is the enthalpy change that occurs when an electron is added to a neutral gaseous atom to form a negative ion. As an enthalpy change, it is a form of energy and thus has units, commonly expressed in $\text{kJ/mol}$ or $\text{eV/atom}$. …

In chemistry, many properties are quantified and expressed with specific units, like mass in kilograms or energy in joules. However, some properties are either relative scales, ratios, or qualitative descriptions, and thus do not have any associated units. Understanding the nature of each property helps determine if it is dimensionless.

Let's examine each option:

1. Electronegativity (A)

   • Concept: Electronegativity is a measure of the tendency of an atom to attract a shared pair of electrons towards itself in a covalent bond. It is a relative concept, comparing the electron-attracting power of different atoms.
   • Scales: Several scales exist for measuring electronegativity, such as the Pauling scale, Mulliken scale, and Allred-Rochow scale.
   • Units: The most widely used Pauling scale is derived from bond energies and is a dimensionless quantity. It assigns a value to each element relative to a reference element (often fluorine or hydrogen). While the Mulliken scale, derived from ionization enthalpy and electron gain enthalpy, could theoretically have units of energy, it is typically used as a relative, dimensionless value in practice, especially when comparing trends. For the purpose of general chemical understanding, electronegativity is considered a dimensionless quantity.

2. Electron gain enthalpy (B)

   • Concept: Electron gain enthalpy ($\Delta_{eg}H$) is the enthalpy change that occurs when an isolated gaseous atom in its ground state gains an electron to form a gaseous anion.
   • Units: Since it represents an energy change per mole of atoms, it has units of energy per mole. Common units are kilojoules per mole ($\text{kJ/mol}$) or electron volts per atom ($\text{eV/atom}$).
   • Example: The electron gain enthalpy of chlorine is $-349 \text{ kJ/mol}$.

3. Ionisation enthalpy (C)

   • Concept: Ionisation enthalpy ($\Delta_iH$) is the minimum amount of energy required to remove an electron from an isolated gaseous atom in its ground state. …