Problem 34 - University Physics with Modern Physics

Question

(a) What is the lowest possible energy (in electron volts) of an electron in hydrogen if its orbital angular momentum is \(\sqrt{12} \mathrm{k}\) ? (b) What are the largest and smallest values of the \(z\) -component of the orbital angular momentum (in terms of \(\hbar\) ) for the electron in part (a)? (c) What are the largest and smallest values of the spin angular momentum (in terms of \(\hbar\) ) for the electron in part (a)? (d) What are the largest and smallest values of the orbital angular momentum (in terms of \(\hbar\) ) for an electron in the \(M\) shell of hydrogen?

Step-by-Step Solution

Answer

(a) -0.85 eV; (b) \(3\hbar, -3\hbar\); (c) \(\frac{\sqrt{3}}{2}\hbar, \frac{\sqrt{3}}{2}\hbar\); (d) \(\sqrt{6}\hbar, 0\).

Step 1: Calculate Principal Quantum Number (n)

The orbital angular momentum is given by the equation \( L = \sqrt{l(l+1)} \hbar \). The problem states \( L = \sqrt{12} \hbar \). So we equate: \( \sqrt{l(l+1)} = \sqrt{12} \). Squaring both sides gives \( l(l+1) = 12 \). Solving this, we find \( l = 3 \). Since \( n > l \), the lowest possible \( n \) is 4.

Step 2: Calculate the Lowest Energy (in eV)

The energy level for an electron in a hydrogen atom is given by \( E_n = -13.6 \text{ eV} / n^2 \). For \( n = 4 \), the energy is \( E_4 = -13.6 / 4^2 = -0.85 \text{ eV} \).

Step 3: Identify the Largest and Smallest Lz (z-component of L)

The largest and smallest values of the z-component of the angular momentum \( L_z \) are given by \( L_z = m_l \hbar \) where \( m_l \) can be any integer between \( -l \) and \( l \). For \( l = 3 \), \( m_l \) ranges from -3 to 3. Thus, the largest value of \( L_z \) is \( 3 \hbar \), and the smallest value is \( -3 \hbar \).

Step 4: Determine Spin Angular Momentum Values

The spin angular momentum of an electron is given by \( S = \sqrt{s(s+1)} \hbar \), where \( s = \frac{1}{2} \) for electrons. Therefore, \( S = \frac{\sqrt{3}}{2} \hbar \). The largest and smallest z-components of spin \( S_z \) are \( \frac{1}{2} \hbar \) and \( -\frac{1}{2} \hbar \), respectively.

Step 5: Orbital Angular Momentum for M Shell

The M shell corresponds to \( n = 3 \). The possible values for \( l \) are 0, 1, and 2. The orbital angular momentum is maximized for \( l = 2 \), yielding \( L = \sqrt{6} \hbar \). The smallest possible \( L \) for \( l = 0 \) is 0.

Key Concepts

Electron Orbital Angular Momentum

Orbital angular momentum in quantum mechanics describes the motion of electrons around the nucleus of an atom. It is designated by the symbol \( L \) and is crucial for defining the properties of electron orbitals. Angular momentum is quantized, meaning electrons can only occupy states with specific, discrete values.

The formula for orbital angular momentum is \( L = \sqrt{l(l+1)} \hbar \), where \( l \) is the azimuthal quantum number, and \( \hbar \) is the reduced Planck's constant. For example, if \( l = 3 \), the value of \( L \) becomes \( \sqrt{12} \hbar \).

The \( l \) quantum number describes the electron's subshell or shape of its orbital and is always a non-negative integer. It ranges from 0 to \( n-1 \), where \( n \) is the principal quantum number. The values of \( l \) correspond to various subshells: \( l=0 \) for s, \( l=1 \) for p, \( l=2 \) for d, and \( l=3 \) for f subshells.

Principal Quantum Number

The principal quantum number, denoted as \( n \), primarily determines an electron's energy level and distance from the nucleus. It is a positive integer and plays a critical role in the structure of electron orbitals within an atom.

In a hydrogen atom, the energy levels are marked by the formula \( E_n = -\frac{13.6\, \, \text{eV}}{n^2} \), demonstrating that energy is inversely proportional to the square of \( n \). Larger values of \( n \) indicate electrons positioned farther from the nucleus and higher energy levels, while smaller \( n \) values imply lower energies and tighter electron binding.

Additionally, \( n \) affects the number of possible values for \( l \). As shown in the given exercise, if \( l = 3 \), \( n \) must be at least 4, ensuring \( n \) is larger than \( l \). This relationship maintains the integrity of electron configurations within atoms.

Spin Angular Momentum

Spin angular momentum is intrinsic to particles such as electrons and represents a form of angular momentum independent of any rotational or translational motion around an external point. In quantum mechanics, it is fundamental and cannot be visualized in the same manner as classical spinning.

Electrons possess a spin value of \( s = \frac{1}{2} \). The formula for spin angular momentum is \( S = \sqrt{s(s+1)} \hbar \), resulting in \( S = \frac{\sqrt{3}}{2} \hbar \) for electrons.

The z-component of spin angular momentum \( S_z \) can take on values of \( \pm \frac{1}{2} \hbar \), reflecting the two possible orientations or spin states of an electron: spin-up \( (\frac{1}{2} \hbar) \) and spin-down \( (-\frac{1}{2} \hbar) \). These properties contribute extensively to the magnetic characteristics and overall behavior of atoms.

Hydrogen Atom Energy Levels

Hydrogen, being the simplest atom with only one electron, serves as an excellent model for studying atomic structures and offers insights into the nature of quantum systems.

The energy levels in a hydrogen atom are well-defined, thanks to the principal quantum number \( n \). These levels are quantized, meaning the electron can occupy only certain allowed energy states. As previously mentioned, the energy is calculated using \( E_n = -\frac{13.6\, \, \text{eV}}{n^2} \). This equation reveals that energy levels become closer together as \( n \) increases, which means an electron in higher energy levels requires less energy to move between adjacent levels.

In this quantum model, the negative sign indicates that the electron is bound to the nucleus, with higher energy states closer to zero being less tightly bound. These quantized energy levels are crucial for understanding spectral lines and transitions in atoms, such as those employed in spectroscopic analysis.