Identify Correct Statements on Atomic Structure and Quantum Mechanics

To determine the correct statements, let us evaluate each statement individually:

1. Statement A: Heisenberg uncertainty principle is applicable to electrons.

   • Heisenberg's Uncertainty Principle is given by the relation: $\Delta x \cdot \Delta p \ge \frac{h}{4\pi}$
   • Since the electron is a microscopic subatomic particle with an extremely small mass, the uncertainty in position and momentum is significant and measurable. Thus, the principle is applicable to electrons.
   • Statement A is correct.

2. Statement B: The size of $2\text{p}_x$ orbital is less than the size of $3\text{p}_x$ orbital.

   • The size of an orbital depends primarily on the principal quantum number ($n$). As $n$ increases, the average distance of the electron density from the nucleus increases.
   • For $2\text{p}_x$, $n = 2$, and for $3\text{p}_x$, $n = 3$. Thus, the size of $2\text{p}_x < 3\text{p}_x$.
   • Statement B is correct.

3. Statement C: The energy of $2\text{s}$ orbital of $\text{H}$ atom is equal to the energy of $2\text{s}$ orbital of $\text{Li}$.

   • The energy of an orbital depends on the nuclear charge ($Z$). For single-electron species, the energy of an orbital in state $n$ is given by: $E_n = -13.6 \times \frac{Z^2}{n^2}\text{ eV}$
   • Since hydrogen ($\text{H}$) has $Z = 1$ and lithium ($\text{Li}$) has $Z = 3$, the nuclear charge experienced by electrons is vastly different. Furthermore, in a neutral multi-electron $\text{Li}$ atom, the attractive force of the nucleus is stronger ($Z_{\text{eff}} > 1$), causing the $2\text{s}$ orbital of $\text{Li}$ to have significantly lower energy (more negative/more stable) than the $2\text{s}$ orbital of $\text{H}$.
   • Therefore, their energies are not equal.
   • Statement C is incorrect.

4. Statement D: The electronic configuration of $\text{Cr}$ is $[\text{Ar}]\, 3\text{d}^5\, 4\text{s}^1$.

   • Chromium ($\text{Cr}$) has an atomic number $Z = 24$.
   • Due to the extra stability associated with half-filled $\text{d}$-orbitals ($\text{d}^5$) and smaller inter-electronic repulsion, an electron from the $4\text{s}$ orbital is promoted to the $3\text{d}$ subshell, giving the ground state configuration: $\text{Cr} : [\text{Ar}]\, 3\text{d}^5\, 4\text{s}^1$
   • Statement D is correct.

Hence, statements A, B, and D are correct.

Correct Option: B (A, B and D Only)