^Characteristic X – rays

If the fast incident electrons collide with the electrons of the inner most level of the heavy target, knock them out. To fill the vacancy created electrons from the higher energy state jump & an X-ray photon of the corresponding energy gap is emitted in accordance with the Bohr’s hypothesis. Such X-rays are called characteristic X-rays.

The frequency (f) of the characteristic X-rays is given by Moseley law:

f  = k ( Z – b )²

Here Z is atomic no. & k, b are constants, their values depends on the series transition.

Moseley made an extensive study of the characteristic X-ray spectra of a number of heavy elements. Moseley’s work provided for the first time a way to determine experimentally the atomic number Z of an element. He concluded that the atomic number is more fundamental than atomic weight in the emission of characteristic X-rays & thus the elements in the periodic table must be arranged in the order of increasing atomic number.

^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)