^Production of X – rays

If the fast electrons incident on entering a heavy target (a metal piece of high atomic weight & high melting point) X-rays are produced.

Coolidge tube is used in the production of X-rays. It is a hard glass evacuated tube (EGT). One end of the tube has a tungsten filament emitting fast moving electrons (also called cathode rays), while the other end contains the heavy target (HT).

A p.d. » 20 kV is maintained between the cathode and a metallic target accelerates the electrons toward the latter.  In the Coolidge tube it is necessary to maintain vacuum of high order (P = 10^–6 cm of Hg) so that the electrons may not lose energy due to the collisions with gas atoms. Majority part of the KE of the incident electrons is wasted in heating the anode & hence the X-ray production requires targets with high melting such as tungsten. Also cooling of target is required. The efficiency of production of X-rays less than 1%.

X-rays are produced by following two mechanisms

^Isotones

Are the atoms:

having same: n_n

But different: A, Z, n_e, n_p N_N

Examples:  1. ₁H² & ₂He³              2. ₁H³ & ₂He⁴

3. ₃Li⁷ & ₄Be⁸            4. ₁₇Cl³⁷ & ₁₉K³⁹

Symbols used: n_e = Number of electrons

n_p = Number of protons

n_n = Number of neutrons

N_N = Number of nucleons

A = Mass Number

Z = Atomic Number

^Isotopes

Are the atoms:

having same: elements, Z, n_e , n_p Chemical properties

But different: A, n_n, N_N, Physical properties

Examples:  1. ₁H¹, ₁H², ₁H³               2. ₂He³, ₂He⁴, ₂He⁶

                  3. ₈O¹⁶, ₈O¹⁷, ₈O¹⁸         4. ₁₀Ne²⁰, ₁₀Ne²¹, ₁₀Ne²²

^Atomic mass unit (u)

^Limitations of Bohr’s theory

1. It is valid only for single electron system.
2. Nucleus was taken as stationary but it also rotates about its own axis.
3. Couldn’t explain fine structure of spectral line.
4. Provides no information about the relative intensities of spectral lines.
5. Provides no distribution of electrons in an atom.
6. Fails to explain that why do the electrons move only in circular orbits.
7. Bohr’s theory doesn’t explain the Zeeman effect (splitting up of spectral lines in magnetic field) & Stark effect (splitting up of spectral lines in electric field).
8. Bohr’s theory doesn’t explain the doublets in the spectrum of the some atoms e.g. in sodium (5890 A⁰ & 5896 A⁰).
9. Silent about the selection rules which governs the transitions.
10. Use two theories

(i) Quantum (to explain the existence of stationary orbits) &

(ii) Classical (for motion of electrons in the orbits). These two theories essentially oppose each other.