In an orbital electrons are filled according to Aufbau principle, Pauli's exclusion principle and Hund's rule of maximum muliplicity .
According to Aufbau principal the orbital are filled in order to their increasing energies . In the order of increase of energy of orbitals cna be calculated from `( n + l )` rule. Lower the value of `( n + l )` for an orbital the lowest energy hence orbital are filled in order of increasing `( n + l ) ` value. If two orbitals have same `( n + l ) ` value, the orbital with lower value of 'n' has lower energy hence it is filled first.
According to Pauli's exclusion principle, an orbital can have maximum two electrons with opposite spin.
According to Hunds rule pairing of electron in degenerate orbitals of the same sub shell does not take place until each orbital belonging to that sub shell has got one electron each i.e., singly occupied.
Assertion `:` The energy of an electorn is largely determined by principal quantum number.
Reason `:` The principle quantum number (n) is a measure of the most probable position finding the electron around the nucleus

Options

A. If both (A) and ( R ) are correct and ( R ) is a correct explanation of ( A )
B. If both ( A ) and ( R ) are correct and ( R ) is not the correct explanation of ( A ).
C. If ( A ) is correct but ( R ) is incorrect
D. If both ( A ) and ( R ) are incorrect.

Answer

To solve the question, we need to analyze the assertion and reason provided, and determine their validity based on the principles of atomic structure.

### Step 1: Understanding the Assertion
The assertion states: "The energy of an electron is largely determined by principal quantum number."

- The principal quantum number (n) indicates the shell in which an electron resides. As n increases, the energy of the electron generally increases because electrons in higher shells are further away from the nucleus and experience less electrostatic attraction. Therefore, the assertion is **true**.

### Step 2: Understanding the Reason
The reason states: "The principal quantum number (n) is a measure of the most probable position of finding the electron around the nucleus."

- The principal quantum number does indeed indicate the shell and thus the average distance of the electron from the nucleus. Higher values of n correspond to higher energy levels and larger average distances from the nucleus. Therefore, the reason is also **true**.

### Step 3: Analyzing the Relationship
Now we need to determine if the reason correctly explains the assertion.

- The assertion relates to the energy of the electron being influenced by n, while the reason explains that n indicates the probable position of the electron. Since the energy of an electron is related to its distance from the nucleus (which is indicated by n), the reason does provide a correct explanation for the assertion.

### Conclusion
Both the assertion and the reason are true, and the reason is a correct explanation for the assertion. Therefore, the correct answer is that both the assertion and reason are correct, and the reason is the correct explanation for the assertion.

### Final Answer
**Both assertion and reason are true, and the reason is the correct explanation for the assertion.**

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