Dhristi JEE 2022-24

Henry Moseley Law

After the experimental confirmation of Rutherford’s scattering theory in about the year 1913, the one-to-one relationship or link of an atom with its atomic number Z was proven by the work of Henry Moseley from the year 1887 to 1915. 


Henry Moseley used the structure of Bohr’s atomic model to determine the energy radiated by an electron when it migrates from low-level orbitals. This energy released during migration has a strong dependence on an atomic number ‘Z’ so that by measuring the energy of the X-rays characteristic of any element, its atomic number Z can be confidently determined. 


Moseley Periodic Law

Here, we will measure the x-ray spectra of a number of elements and also identify several unknown elements by looking at their characteristics, viz: X-ray spectra.


Moseley’s law was discovered and published by an English Physicist named Henry Moseley. This law is an empirical law that concerns the characteristics of X-rays emitted by atoms.


The frequency v of X-ray emitted by an atom is related to its atomic number ‘Z’ by the following formula:


v =(a−b)−−−−−− \[\sqrt{(a-b)}\]        .....(1)


Here,


a and b = are constants. We also call these constants proportionality and screening or shielding constants.


Equation (1) is Moseley’s X-ray Characteristic formula and here the two physical constants ‘a’ and ‘b’ are independent constants of an element; however, these two depend on the X-ray series.


For a ‘k’ series, the value of a and b is:

a = \[\frac{3RC}{4}\]−−−−−−\[\sqrt{\frac{3RC}{4}}\]

and

b = 1 


Where,

R = Rydberg’s constant

c = speed of light


For the L series, the value of a and b is as follows:

a = \[\frac{5RC}{36}\]—-------- \[\sqrt{\frac{5RC}{36}}\],

and

b  = 7.4


The relation between a and b is determined by experiments using Henry Moseley’s law and the graph for this relationship is as follows:


The line intersecting in the graph at the Z-axis shows that Z = b, where b is 1 for K series elements and 7.4 for elements in L series.


Moseley Law Statement

A simple idea is that the effective charge of the nucleus decreases by 1 when it is being screened by an unpaired electron that persists behind in the K-shell.


Moseley X-Ray Experiment

X-ray spectrometers are the fundamental foundation-stones of the process of X-ray crystallography. 


The working by Moseley by employing X-ray spectrometers is as follows:


A glass-bulb electron tube was used, inside this evacuated tube, electrons were fired at a metallic substance, which was a sample of the pure element in his work.


The firing of electrons on a metallic substance caused the ionization of electrons from the inner electron shells of the element. The rebound of electrons into the holes in the inner shells then caused the emission of X-ray photons leaving out the tube in a semi-beam, through an opening in the external X-ray shielding. 


Now, these radiated X-rays were then diffracted by a standardized salt crystal, with angular results emitting in the form of photographic lines by the exposure of an X-ray film fixed at the outside the vacuum tube at a known distance. 


Next, Moseley employed the application of Bragg's law after initial guesswork of the mean distances between atoms in the metallic crystal, based on its density next leading to calculate the wavelength of the emitted X-rays.


Analysis of Moseley’s Experiment

To Determine the following things:

  • Firstly, we must confirm Moseley’s law with six known samples of elements. Since the energy is the characteristic X-ray (according to Moseley), which is proportional to (Z - n and channel number N is directly proportional to E, then N is proportional to (Z - n). Therefore, N kZ = − bg n. 

  • Draw a graph plotting N vs. Z for the six known samples. Obtaining the best values of k and n can be observed from this graph. Now, look at your spectra carefully and think about what the uncertainties in your data are. Devise a reasonable method for determining the uncertainties in n and k.

  • Determine Z for the unknowns by comparing the peak position for each with your results from the six known samples and also determine the uncertainty associated with your findings.


So, this is how we can determine the atomic number of a material; by observing the X-ray characteristic of an element. 


Moseley’s Law and a Basic Introduction

Moseley’s law is used to understand the emitted x-rays by the atoms. This law was derived and published by Henry Moseley. He used this law to determine the energy exerted by an atom. Atoms are the smallest particle that exists. And to find the energy that is exerted by the atom, Moseley’s law is used. It is an empirical law that determines the atomic number. 


Students can find more information about Moseley’s law on the Vedantu website. It has all free downloadable content that students can use and study. It is important to practice all the formulas with example questions to get a better understanding of the law. This law is very important as it created the basis of the periodic table and also helped in discovering new elements that were previously unknown to the scientists. 


Statement of the Moseley’s Law

The statement of moseley’s law is: “The square root of the frequency of the x-ray emitted by an atom is proportional to its atomic number”. New elements were also found because of this law. This law came to existence because when Henry Moseley was studying graphs, he found a strange relationship between the lines and the atomic number. This law also helped with organizing the elements on the periodic table based on atomic numbers rather than atomic mass. 


The formula for Moseley’s law is ν=a(Z–b)…(1) 


Importance of Moseley’s law 

Moseley’s law is very important because it proved that atomic numbers are more necessary than atomic mass and it is because of this reason that the entire periodic table was changed based on the element's atomic number. This law also helped with discovering new elements and explained the property of elements way better. 


In 1914, Moseley also published a paper where he spoke about three unknown elements between two others and because of all his experiments and data, we now have more information about how to study elements. He also found that the K lines were related to the atomic number and later found the formula by which the approximated relationship between them could be calculated. 


The formula which is called Moseley’s Law is: 

V = A . (Z – b)²


In this case, 

  • V is the frequency of the x-ray emitted line

  • A and b are constants that depend on the type of notation

FAQs on Moseley Law

1. What is the Basic Idea of Moseley’s Law?

Photographic recording of Kα and Kβ x-ray emission lines for a range of varying elements can be determined by using Moseley’s law; we must note that for the dispersive element, the line position is proportional to the wavelength. Here, Moseley’s law concerns the characteristic X-rays emitted by various atoms.

2. What is Periodic Law?

This law states that all the elements in a periodic table are arranged in increasing order where each element has its own physical and chemical property.

3. How Moseley’s Law Influences Physics?

The development of Moseley's law in X-ray spectra has led to the introduction of advanced atomic physics, nuclear physics, and quantum physics by providing the first experimental evidence in favor of Niels Bohr's theory, apart from the hydrogen atom spectrum that the Bohr theory was designed through.

4. What is Moseley’s Empirical Formula?

Moseley's empirical formula for Kα X-rays was adapted to the Bohr model. The implication of the relationship between these two models is that the single electron in the K-shell before the emission is about 100% effective in shielding the nucleus so that the electron from the L-shell obtains an effective nuclear charge of Z-1.

5. How to study Moseley’s law?

Mosleye law is the empirical law that studies the characteristic emitted x=ray lines from the atom. These lines have various properties that can be calculated using the formula/ Students can learn more about this law in the Vedantu website. They have to login and they will be able to access all the available resources. Moseley’s law, experiments, analysis and solved examples are all available on the website for free and students can make use of these resources to understand the subject well. 

6. How did Moseley’s law help arrange the periodic table?

Mosleye’s law studies the characteristics of the x-ray lines emitted by the atoms. He found out that these elements also have an atomic number. Previously, the periodic table was arranged based on the atomic mass which caused a lot of confusion, but because of this law, the periodic table is now rearranged based on the atomic number and many other elements were also discovered. This law also proved that atomic numbers are more significant in understanding the properties of an element rather than the atomic mass. 

7. What is periodic law in simple terms?

In simple terms, periodic law states that all the elements that are present in the periodic table are arranged in an increasing order and every element has unique properties of its own which can be studied. It also states that these elements are arranged based on their atomic number rather than the atomic mass. It also explains that the physical and chemical properties of the elements occur periodically, systematically and they are predictable. Periodic law is also known as Mendeleev's law.

8. Why is Mosleye’s law important? 

Mosleye’s law is important in physics because it changed the way that scientists looked at elements and atoms. Without Moseley's law, we still wouldn't have discovered atomic numbers and the periodic table would've been arranged based on the atomic mass. The development of mosleye’s law has also helped researchers in advanced physics, nuclear and quantum physics by providing a lot of empirical experiments. 

9. How is the periodic table arranged? 

The periodic table was arranged based on the element's atomic mass, however, after Mosleye’s law came into existence, it was found out that the elements can be arranged based on just their atomic numbers. Atomic numbers represent the positive charge of the atom’s nucleus and it is increased by one number in every atom. So based on this, the entire periodic table was rearranged based on atomic numbers in an increasing order which also helped the researchers to study the properties of the atom.

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