1. Probability density: - The probability density of finding the electron is maximum at the nucleus.
2. Distance from the nucleus: - The electron can be found at a distance $2 \mathrm{a}_{0}$ from the nucleus.
3. Spherical symmetry: - The 1s orbital is spherically symmetrical.
4. Total energy: - The total energy of the electron is maximum when it is at a distance $\mathrm{a}_{0}$ from the nucleus.
This statement is incorrect. Therefore, the correct answer is (4).
An element has two isotopes having atomic masses 10 and 15 u, respectively. If the percent abundance of lighter isotopes is 80%, then the average atomic mass of the element is: