Rutherford Scattering Experiment

Rutherford Scattering Experiment

The next step in the development of atomic model was given by Ernest Rutherford in 1911. Rutherford used a "Gold foil experiment" for explaining the atomic model given by J. J. Thomson. A decades earlier, Rutherford only identified one of type of radiation given off by radioactive elements like polonium, uranium etc and named them as alpha particles.

The alpha particles are fast moving and positively charged Helium nuclei with two protons and two neutrons.
Rutherford observed the deflection of alpha particles after passing through metal sheet and purposed his atomic model also called as planetary model because of its resembles with arrangement of plants around the sun.

Rutherford’s Alpha Scattering Experiment

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Rutherford directed a narrow beam of alpha particles (after passing through a slit in a lead screen) obtained from polonium (²¹⁴Po) at an extremely thin sheet (about 5 $\times$10^-7 meter thick) of a metal like silver and gold.

After passing through the metal sheet, the alpha particles strike on fluorescent screen which was coated with zinc sulphide and produce a visible flash of light called scintillation.

Rutherford was expecting results according to the plum pudding model in which electrons and protons were distributed randomly. He was expecting that all alpha beams will move in the same direction of propagation with a little deflection. But results were much more different than his expectations.

Observation of Rutherford's alpha scattering experiment

Rutherford observed that most of alpha particles strike on the fluorescent screen without any deflection. However very few alpha particles deflected slightly and very less number of alpha particle; approx one particle deflect with the angle of 180°, i.e. almost bounce back.

Conclusion of Rutherford's Alpha Scattering Experiment

Based on the observation of alpha scattering experiment combined with the discovery of neutrons by James Chadwick, Rutherford concluded.

• An atom consists of a minute positively charged body at its center called as nucleus. The nucleus though small, contains all the protons and neutrons. Since the total mass of atom is because of protons and neutrons, hence the entire mass located at the center of atom. Hence few alpha particles deflected due to positive charge of nucleus.

• An atom consists of a sufficient number of extremely small negatively charged electrons distributed around the nucleus to balance the positive charge of nucleus. Hence maximum volume of an atom remains empty so many alpha particles moved without any deflection.

• Since very less number of alpha particles deflected, hence the volume of nucleus is very less compare to whole atom. The size of nucleus is less than 2x 10^-14 m while the size of an atom is around 10^-10 m.

Objection of Rutherford's Alpha Scattering Experiment

1. By following the classic electromagnetic science J.C.Maxwell had shown that whenever an electric charge emits radiation and loses energy when it is subjected to acceleration. In an atom, electron is also negatively charged particle; hence it must radiate energy during its movement around the nucleus and should lose energy and reached to zero. As a result, its orbit should become smaller and smaller and finally it should drop in to the nucleus in a helical path instead of circular path. In other words, an atom is not a stable species. On the contrary atom is stable and electrons and their energy in one of these orbits stay same.
2. Another discordance regarded the Rutherford's experiments was the spectrum obtained from the electron. An electron which continually emitted radiation must forms a continuous atomic spectrum. In other words there must be no line for fixed frequency. However the atomic spectrum is not continues spectrum but a line spectrum with many lines of fix frequency. Hence the Rutherford atomic model failed to explain the line spectrum of atoms. To overcome the first objection that why the electrons do not fall into the nucleus on account of mutual electrostatic attraction, Rutherford gave the explanation that the electrons are revolving with extremely high speed and at great distance from center. So centrifugal force arising from this motion balances the force of electrostatic attraction. Hence electrons do not fall in nucleus to form stable atom. But there was no clarification for atomic spectrum pattern.

Rutherford Scattering Formula

 

When an alpha particle strike to a positively charge body that is nucleus, the electrostatic force of repulsion worked strongly and alpha particles get scattered. The scattering process can be considered as cross section for interaction with nucleus which has point charge Z_e.

Let us take $\theta $ is the scattering angle and b is the impact parameter, the number of particles striking the per unit are of detector is given by Rutherford formula.

Rutherford Scattering Derivation

An alpha particle which scatters by the central positive charge of nucleus leads to a hyperbolic trajectory.
By using the scattering angle $\theta$, and momentum (p) of alpha particle, we can calculate the impact parameter (b) and the closest approach to the target nucleus.

According to Rutherford formula the number of alpha particle deflected by the angle $\theta$ is inversely proportional to the fourth power of the sine function of one half the angle of deflection

CHEMISTRY: Representative Elements

Representative Elements

Here in this page we are going to discuss about concept called representative elements. The metallic elements which are found on the left side and in the center of the periodic table are known as representative elements. The metals of Groups 1 and 2 are known as the representative metals and those which are present in the center of the periodic table are called the transition representative metals. The lanthanides and actinides are special classes of transition metals.

1. The elements of "s" and "p" blocks except "d" group elements are called as representative elements.
2. Their outer shells are not completely filled with electrons.
3. The elements get the nearest inert gas configuration by losing or gaining or sharing of electrons.
4. They are chemically active. A few metals, all the non-metals and metalloids are representative elements.
5. "s" block elements are placed at left side and "p" block elements are placed at the right side of the periodic table i.e., representative elements are placed at left and right side of the periodic table.

"S" Block Elements:

As an "s" orbital can have a maximum of two electrons, "s" block has two groups.They are

• Group I : (H, alkali metals) Electronic configuration of outer shell is ns¹
• Group II : (alkaline earth metals) Electronic configuration of outer shell is ns²

The electronic configuration of "s" block elements is ns¹ or ns².

"P" Block Elements:

As a "p" orbital can have a maximum of six electrons, .p' block has six groups.
They are

• Group III : (Boron family) Electronic configuration of outer shell is ns²np¹
• Group IV : (Carbon family) Electronic configuration of outer shell is ns²np²
• Group V : (Nitrogen family) Electronic configuration of outer shell is ns²np³
• Group VI : (Oxygen family) Electronic configuration of outer shell is ns²np⁴
• Group VII : (Halogens) Electronic configuration of outer shell is ns²np⁵

The electronic configuration of "p" block representative elements varies from ns²np¹ to ns²np⁵.

Properties of Representative Elements

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Recollect from the former analysis of electron configurations that Hydrogen, Lithium and sodium all ended with s1 configuration. Whenever we look at other representative chemical element therein grouping we will have that the relation proceeds on down the periodic table. All of a common elements in group IA end in s1 configuration for their electron configuration.

H 1s¹

Li 2s¹

Na 3s¹

K 4s¹

Rb 5s¹

Cs 6s¹

Fr 7s¹

Classification of Representative Elements

 

The representative elements are classified into 3 main groups. These groups are as follows:

1) The alkali metals,

2) The alkaline earth metals, and

3) The post-transitional metals.

1) The Alkali Metals

The given alkali metals in table are the group IA (1) chemicals element. They make strongly basic hydroxides, thus the term "alkaline" being utilized in basic substances. They have got a high metallic behavior and are good reducing factors in the table.

2) The Alkaline Earth Metals

The given alkaline earth metal in the table are precisely the group IIA (2) chemicals element. They are named alkaline earth metal because "earths" of this grouping, lime (CaO), and magnesium oxide (MgO), yield alkaline chemicals reaction in the column. They have very good metallic properties, that includes conduction, reduction power, luster, softness, malleability, and ductility etc.

3) The Post-Transition Metals

The given post-transition metallic element combine the lower chemical element of group IIIA (13), IVA (14), and VA (15), formatted in a helical fashion in the table. There properties having the like analogy to the alkaline earth metal as the alkaline earth metal have to the alkaline metal in the table.