Nuclear Physics Concept Page - 3

Definition

Trajectory of α$\alpha$ particles in an electric field

The trajectory traced by an α$\alpha$-particle depends on the impact parameter,b of collision. The impact parameter is the perpendicular distance of the initial velocity vector of the α$\alpha$-particle from the centre of the nucleus. A given beam of α$\alpha$-particles has a distribution of impact parameters b, so that the beam is scattered in various directions with different probabilities . (In a beam, all particles have nearly same kinetic energy.) It is seen that an α$\alpha$-particle close to the nucleus (small impact parameter) suffers large scattering. In case of head-on collision, the impact parameter is minimum and the α$\alpha$-particle rebounds back (θ≅π$\theta \cong \pi$). For a large impact parameter, the α$\alpha$-particle goes nearly undeviated and has a small deflection (θ≅0$\theta \cong 0$). The fact that only a small fraction of the number of incident particles rebound back indicates that the number of α$\alpha$-particles undergoing head on collision is small.

Example

Distance of closest approach to the nucleus of an alpha paticle

An α$\alpha$ particle of energy 5 MeV is scattered through 1800${180}^0$ by a fixed uranium nucleus. The closest distance is :
The distance of closest approach is
d=Ze24πϵ0Ek$d={\displaystyle \frac{Z{e}^2}{4\pi {ϵ}_0{E}_k}}$
Here, Z = 92,
Ek=5MeV=5×106×1.6×10−19J$E_k=5MeV=5\times {10}^6\times 1.6\times {10}^{-19}J$
Hence,
d=9×109×2×92(1.6×10−19)25×106×1.6×10−19$d={\displaystyle \frac{9\times {10}^9\times 2\times 92(1.6\times {10}^{-19}{)}^2}{5\times {10}^6\times 1.6\times {10}^{-19}}}$
d=529.9×10−16=0.5299×10−13$d=529.9\times {10}^{-16}=0.5299\times {10}^{-13}$
d≈10−13cm$d\approx {10}^{-13}cm$

Definition

Atom, Element and Molecules

Atom: An atom is the smallest constituent unit of matter that has the properties of a chemical element.
Element:An element is a substance which cannot be subdivided into two or more similar substances by any chemical or physical means.
Molecule:
A molecule is the smallest amount of a substance that can exist independently. It consists of atoms that are put together in a particular shape or form.

Definition

Atoms, Element and Molecules

Atom : 
An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element. 

Element : Elements are chemically the simplest substances and hence cannot be broken down using chemical reactions. Each element is distinguished by its atomic number, i.e. the number of protons in the nuclei of its atoms. They consists of only one type of atoms.
Examples of elements are Hydrogen, Carbon, Oxygen, Nitrogen etc. 

Molecules : A molecule is an electrically neutral group of two or more atoms held together by chemical bonds.
For example, HCl(g) is a molecule made of one hydrogen atom bonded to one chlorine atom. It is made of two atoms and is called a diatomic molecule.

Definition

Absorption spectrum

When white lightpasses through a gas and weanalyse the transmitted lightusing a spectrometer we findsome dark lines in thespectrum. These dark linescorrespond precisely to those wavelengths which were found in theemission line spectrum of the gas. This is called the absorption spectrumof the material of the gas.

Definition

Emission spectrum

When we passs electricity through an atomic gas or vapour, the gas gets excited. Excited means its energy level increases. Such gas in excited state emits radiations. When these emissions are incident on a spectrometer, we get bright lines on a dark background. This spectrum is called emission spectrum.  

Diagram

Emission spectrum for hydrogen

When an atomic gas or vapour is excited atlow pressure, usually by passing an electric current through it, the emitted radiation has a spectrum which contains certain specific wavelengths only.
The given figure represents the emission spectrum for hydrogen.

Definition

Absorption Spectrum

Absorption Spectrum : Absorption Spectrum consists of dark lines or bands on top of a continuous spectrum. Absorption spectra are produced when light from a hot object travels through a cooler, dilute gas.
When a photon with exactly the right wavelength encounters an atom of the cool gas, it is absorbed and its energy used to kick an electron into a higher orbit; if enough atoms of gas are present, all the photons of that wavelengths are absorbed, while photons with other wavelengths get through.

Definition

Emission Spectrum

2. An emission line spectrum :An emission line spectrum consists of bright lines or bands on a dark background. This spectrum is the opposite of continuous spectrum.
Emission spectra are produced when atoms of a dilute gas are excited' by an electrical current, ultraviolet radiation, or some other source of energy.Excited atoms have electrons in high orbits, and these emit photons with specific wavelengths when they jump back down to lower orbits (as explained above). 

Definition

Various Kinds of Spectrum

There are basically three types of spectra namely 
1. A continuous spectrum
2. An emission line spectrum
3. An absorption line spectrum

BookMarks

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