THE ATOMIC SPECTRA

Q.1: The Bohr’s theory of hydrogen atom is based upon several assumptions. Do any of these assumptions contradict classical physics?
Ans: The assumption in Bohr’s theory that an electron moving around the nucleus in a certain orbit does not radiate energy is contrary to classical electrodynamics.

Q.2: Why does the hydrogen gas produced in the laboratory not glow and emit radiations?
Ans: A spectrum is given by the light emitted from an incandescent gas or vapour e.g., electric discharge through a gas or hydrogen-filled discharge tube.

Q.3: Why are the energy levels of the hydrogen atom less than zero?
Ans: The energy levels of the hydrogen atom are negative. This shows that the electron is bound (not free). Thus, one must do work (or expend energy) to remove it from the atom.

Q.4: If the hydrogen gas is bombarded by electrons of energy 13.6 eV, would you expect to observe all the lines of the hydrogen spectrum?
Ans: If a hydrogen atom is bombarded by electrons of energy 13.6 eV, it gets ionized; because 13.6 eV is the ground state energy, which is equivalent to the ionization energy. As such, no spectral lines of hydrogen will be observed.

Q.5: Hydrogen gas at room temperature absorbs light of wavelengths equal to the lines in the Lyman series but not those in the Balmer series. Explain?
Ans: Hydrogen gas at room temperature contains electrons in the ground state (p=1). If the energy supplied to the electron is such that the electron is lifted from its ground state to one of the higher allowed orbits, the atom will be excited, and it will absorb energy equal to the difference of the energies of the electron in the two states. Thus, light of wavelength equal to the lines in the Lyman series will be absorbed.

Q.6: How are x-rays different from visible radiations?
Ans: Both x-rays and visible radiations are electromagnetic waves, but x-rays differ from the visible radiations in the following features:
i. X-rays are highly penetrating. They can pass through many opaque solids such as wood or flesh but are stopped by bones and metals. Hence x-ray photographs are used in medicine.
ii. They cause ionization in gases.
iii. They can eject photoelectrons on striking some metals.
iv. They produce fluorescence in many substances like zinc sulphide, barium platinocyanide, etc.
v. They can damage living tissues if exposed to them for a longer duration.

Q.7: What property of x-rays makes them so useful in seeing otherwise invisible internal structures?
Ans: In solids, the atoms are grouped together in a regular manner. The interatomic distance in a crystal is of the order of the wavelength of x-rays. Hence a crystal is used as a 'transmission grating' to produce diffraction of x-rays. This x-ray crystallography has helped to locate the internal structure of crystal systems (called basic unit cells). Recently developed internal imaging devices (for the human body) include CT (computerized tomography) scanning, MRI (Magnetic Resonance Imaging), and PET (Positron Emission Tomography).

Q.8: Explain the difference between laser light and light from an incandescent lamp (or bulb)?
Ans:

Laser Light	Incandescent Light
i. Laser light is highly monochromatic.	i. Light from an incandescent bulb is a mixture of several wavelengths.
ii. It consists of parallel waves in a narrow beam and is highly directional (i.e., moves straight without spreading).	ii. It is emitted in all directions and spreads out.
iii. It is produced due to stimulated emission of radiation.	iii. It is produced due to spontaneous emission of radiation.

Q.9: Does the light emitted by a neon sign constitute a spectrum or only a few colors? Explain?
Ans: The luminous neon in a discharge tube has a reddish color. Its spectrum is composed of a few colors (line spectrum) of wavelength, very close to each other. So the spectral lines are closely spaced to form a band spectrum.

Q.10: Suppose you are given a glass tube having two electrodes sealed on both ends. The inside is either hydrogen or helium. How can you tell which one it is without breaking the tube?
Ans: The electrodes are connected across a voltage source. If the voltage is gradually increased, then the hydrogen-filled tube will become luminous first because its ionization potential is four times less than that of helium.

The gases can also be differentiated by taking the spectrum of each other.

Q.11: The hydrogen atom contains only a single electron and yet the hydrogen spectrum contains many lines. Why is this so?
Ans: The atoms of hydrogen can be excited to different energy levels. The excited electrons will not stay there. These will jump to the inner orbits. On de-excitation, an electron does not necessarily return to the ground state in a single jump. Rather, it may return by several jumps. Thus, several spectral lines of different frequencies are emitted, depending upon the differences of energies between the levels for the transitions. So, the spectrum of hydrogen contains many lines.

Q.12: The electron in a hydrogen atom requires energy of 10.2eV for the excitation to a higher energy level. A photon and an electron, each of energy 10.5eV, are incident on the atom. Which of these can excite the atom? Give an explanation in support of your answer.
Ans: To excite an electron, energy can be supplied to the electron by direct collision with accelerated particles as well as by the photons of energy hv. The energy of a photon must be exactly equal to the excitation energy (10.2 eV) for the bound electron; otherwise, it will not be absorbed (since it cannot transfer its energy in parts).

On the other hand, accelerated particles can give energy to the bound electron in full as well as in part. Hence an electron 10.5eV (a little higher than the excitation energy of 10.2 eV) can excite the hydrogen atom.

Q.13: Describe the atomic processes in the target of an x-ray tube whereby x-ray continuous spectra and characteristic spectra are produced?
Ans: The x-rays produced by an x-ray tube consist of two parts:
i. A series of un-interrupted wavelengths having a short cut-off wavelength (Xm) are produced when high-velocity electrons are decelerated by a heavy nucleus. This constitutes a continuous spectrum of photons, including x-rays. This process is called 'bremsstrahlung' (German for breaking radiation).
ii. A number of distinct and discreet wavelengths which constitute line (or discontinuous) spectrum of the x-rays are produced when electrons are dislodged from the inner most orbits, followed by electron jumps from the outer orbits. So characteristic spectra result from transitions to a 'hole' in an inner energy level.

Q.14: Explain clearly why x-ray emission lines in the range of 0.1nm are not observed from an x-ray tube when a low atomic number metal is used as the target in the tube?
Ans: For the production of most energetic x-rays, the electrons must be raised from deploying energy levels of the target atoms and certain electrons of innermost shell must be knocked out. The target metal with low atomic number will have x-rays of larger wavelengths. Hence, emission lines of x-rays in the range of 0.001 - 1 nm are not observed.

Q.15: Why do the frequencies of characteristic x-rays depend on the type of the material used for the target?
Ans: The transitions for the emission of characteristic x-rays depend upon the nature of the target material atoms, because frequency of x-rays (v) depends upon the atomic number (z) of the target material [v proportional z-according to Moseley’s law: v: (2.48 x 10^15 Hz) (Z - 1)^2]. Due to Moseley’s work, the characteristic x-ray spectrum became the universally accepted signature of an element.

Q.16: Does the maximum frequency in the ‘bremsstrahlung process’ depend on the nature of the target material?
Ans: No. the maximum frequency and minimum wavelength in the ‘bremsstrahlung process’ do not depend on the material.

Q.17: In laser operation, what process is required to be produced before ‘stimulated emission’?
Ans: Laser operation requires the creation of a non-equilibrium condition, called “population inversion” in which the number of atoms in a high energy state is greater than the number in a lower energy state.

Q.18: Why does laser usually emit only one particular color of light rather than several colors?
Ans: A laser beam is highly coherent and monochromatic, i.e., the emitted photons have the same frequency and wavelength. As each and every color has its own wavelength, so a laser, being monochromatic, emits only one particular color of light.