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A sample of hydrogen atoms is irradiated with light with wavelength \(85.5 \mathrm{nm},\) and electrons are observed leaving the gas. (a) If each hydrogen atom were initially in its ground level, what would be the maximum kinetic energy in electron volts of these photoelectrons? (b) A few electrons are detected with energies as much as 10.2 eV greater than the maximum kinetic energy calculated in part (a). How can this be?

Short Answer

Expert verified
(a) The maximum KE is 0.94 eV. (b) Electrons could be from excited states, gaining an additional 10.2 eV energy.

Step by step solution

01

Calculate the energy of the incident photons

Use the formula for energy of a photon, \( E = \frac{hc}{\lambda} \), where \( h \) is Planck's constant \( 6.626 \times 10^{-34} \text{ J s} \), \( c \) is the speed of light \( 3 \times 10^8 \text{ m/s} \), and \( \lambda = 85.5 \text{ nm} \) converted to meters (\( 85.5 \times 10^{-9} \text{ m} \)). This gives \( E \approx 2.33 \times 10^{-18} \text{ J} \). Convert this energy to electron volts by dividing by the charge of an electron \( 1.602 \times 10^{-19} \text{ C} \). The energy of the photon \( E \) is approximately \( 14.54 \text{ eV} \).
02

Determine the ionization energy from ground level

Hydrogen atoms have an ionization energy of \( 13.6 \text{ eV} \) when they are in their ground state.
03

Calculate the maximum kinetic energy of the ejected electrons

The maximum kinetic energy of photoelectrons is given by the difference between the energy of the incident photons and the ionization energy: \( \text{Max KE} = E - 13.6 \text{ eV} = 14.54 \text{ eV} - 13.6 \text{ eV} = 0.94 \text{ eV} \).
04

Analyze the appearance of higher energy electrons

The observation of electrons with energies up to 10.2 eV greater than the calculated maximum kinetic energy may occur if the hydrogen atoms were initially excited to a higher energy level, such as the first excited state. The energy difference between the first excited state and the ground state of hydrogen corresponds to \( 10.2 \text{ eV} \) and accounts for the observed higher kinetic energies. These electrons are ejected with energies \( 0.94 \text{ eV} + 10.2 \text{ eV} = 11.14 \text{ eV} \).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Hydrogen Ionization
Hydrogen ionization refers to the process of removing an electron from a hydrogen atom, thereby ionizing it. When a hydrogen atom, which is the simplest atom consisting of just one proton and one electron, is exposed to enough energy, its electron can be freed. This process requires a specific amount of energy, known as the ionization energy. For hydrogen in its ground state, or the lowest energy state where the electron is closest to the nucleus, the ionization energy is approximately 13.6 electron volts (eV).

Ionizing a hydrogen atom involves overcoming the attractive force between the positively charged proton and the negatively charged electron. When energy from a photon is absorbed, it may transfer sufficient energy to the electron to overcome this attraction, resulting in ionization. The calculation of this process is significant in fields like astrophysics and quantum mechanics, as it helps understand the behavior of atoms in different environments, such as stars or atmospheric conditions.
Photon Energy Calculation
Calculating the energy of a photon is essential to understanding phenomena like the photoelectric effect. The energy of a photon is related to its wavelength by the equation: \( E = \frac{hc}{\lambda} \), where:
  • \( E \) is the energy of the photon
  • \( h \) is Planck's constant, approximately \(6.626 \times 10^{-34} \text{ J s} \)
  • \( c \) is the speed of light, roughly \(3 \times 10^8 \text{ m/s}\)
  • \( \lambda \) is the wavelength of the light in meters
To perform a photon energy calculation, you first need to ensure that your wavelength is in the correct unit of meters. For example, a wavelength of 85.5 nm (nanometers) must be converted to meters by multiplying by \(10^{-9}\).

Once the wavelength is correctly converted, substitute these values into the equation to find the energy in joules. For easier interpretation, this energy is often converted to electron volts (eV) by dividing by the charge of an electron, approximately \(1.602 \times 10^{-19} C\). This conversion provides a more intuitive sense of how much energy is involved in effects like the photoelectric effect or ionization processes.
Kinetic Energy of Photoelectrons
When a photon hits a material like hydrogen and causes an electron to be ejected, the photoelectron can have kinetic energy. The maximum kinetic energy of emitted photoelectrons can be calculated through the energy of the incoming photon minus the ionization energy of the atom. This is expressed by the equation: \( \text{Max KE} = E_{ ext{photon}} - E_{ ext{ionization}} \).In the context of a hydrogen atom, if a photon has energy greater than 13.6 eV, the excess energy becomes the kinetic energy of the ejected electron. For instance, if a photon provides 14.54 eV, subtracting the ionization energy (13.6 eV) results in a maximum kinetic energy of 0.94 eV for the ejected electron.

Interestingly, we sometimes observe photoelectrons with kinetic energies greater than the calculated maximum. This discrepancy can happen if the atom was initially in an excited state, which requires less energy to ionize than from the ground state. Thus, these electrons exhibit more kinetic energy post-ejection due to their initial excited energy status. Understanding photoelectron kinetic energy is crucial in fields like spectroscopy and material science, where it provides insights into atomic and electronic structures.

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