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Hi guys, today we will be studying about the atomic structure. So let&#39;s start learning by no whether delaying.

<h2>Atomic Structure</h2>

John Dalton (1808) regarded the atom as a hard, dense and smallest indivisible particle of matter. However, the emission of negatively and positively charged particles from radioactive elements as well as from gases on the passageof electricity at very low pressures, convinced the scientists that the atom is not indivisible but consists of smaller fundamental particles. It is now fully established that an atom consists of three major fundamental particles called electrons, protons, and neutrons.

<h2>Atom</h2>

John Dalton proposed (in 1808) that the atom is the smallest indivisible particle of matter. Atomic radii are of the order of 10-8 cm or 10-10 m. It contains three subatomic particles namely electrons, protons, and neutrons.

(i) Electron - Electron was discovered as a result of the study of cathode rays by JJ Thomson. It was named by Stony. It carries a unit negative charge (1.6 × 10-19 C). Mass of the electron is 9.11 × 10-31kg and the mass of one mole of electrons is 0.55 mg.

(ii) Proton - Rutherford discovered protons on the basis of an anode ray experiment. It carries a unit positive charge (+1.6 × 10-19 C). The mass of a proton is 1.007276 u.

(iii) Neutron - Neutrons are neutral particles. It was discovered by Chadwick (1932). The mass of a neutron is 1.675 × 10- 24g or 1.008665 amu or u.

Information about some known subatomic particles:

<ul>
	<li>Positron: Discovered by Dirac (1930) and Anderson (1932).</li>
	<li>Neutrino and antineutrino: Particles having no mass and charge. Discovered by Fermi (1934)</li>
	<li>Meson: Discovered by Yukawa (1935)</li>
	<li>Antiproton: Discovered by Segre and Weigh land (1955).</li>
</ul>

<h2>J.J Thomson Model</h2>

In 1897, British physicist JJ Thomson measured the ratio of electrical charge (e) to the mass of the electron (me ) by using cathode ray tube and applying electrical and magnetic field perpendicular to each other as well as to the path of electrons. Thomson argued that the amount of deviation of the particles from their path in the presence of electrical or magnetic field may vary as follows: The e/me ratio of the proton is 9.58 × 10-4 C/g. (e/me ratio is maximum for H2 gas)

Atom is a positive sphere with a number of electrons distributed within the sphere. It is also known as the plum pudding model. It explains the neutrality of an atom. This model could not explain the results of the Rutherford scattering experiment.

<h2>Rutherford&#39;s Nuclear model of atom </h2>

It is based upon the α-particle scattering experiment. Rutherford presented that:

<ol>
	<li>Most of the atoms are empty. Most of the α- particles passed through gold foil, without undergoing any deflection from their original path.</li>
	<li>an atom possesses a highly dense, positively charged center, called nucleus of the order 10-13 cm.</li>
	<li>The entire mass of the atom is concentrated inside the nucleus.</li>
	<li>electrons revolve around the nucleus in circular orbits.</li>
	<li>electrons and the nucleus are held together by electrostatic forces of attraction.</li>
</ol>

Atomic Number (Z): Atomic number of an element corresponds to the total number of protons present in the nucleus or total number of electrons present in the neutral atom.

Mass Number (A): The mass of the nucleus is due to protons and neutrons; thus they are collectively called nucleons. The total number of nucleons is termed as the mass number of the atom. Mass number of an element = Number of Protons + Number of Neutrons

<h2>Formulation of Bohr’s model of atom</h2>

Two developments played a major role in the formulation of Bohr’s model:

<ol>
	<li>Dua character of the electromagnetic radiation which means that radiation possesses wave-like (interference, diffraction, refraction, polarization, etc) and particle-like properties (black body radiations and photoelectric effect).</li>
	<li>Atomic spectra explained by the electronic energy level in atoms.</li>
</ol>

Maxwell’s Electromagnetic Wave Theory:

The energy is emitted from source continuously in the form of radiations and magnetic fields. All electromagnetic waves travel with the velocity of their propagation. An electromagnetic wave has the following characteristics:

(i) Wavelength: It is the distance between two successive crests or troughs of a wave. It is denoted by the Greek letter λ(lambda).

(ii) Frequency: It represents the number of waves which pass through a given point in one second. It is denoted by µ (nu).

(iii) Velocity (v): It is defined as the distance covered in one second by the waves. Velocity of light is 3 × 1010 cms-1 .

(iv) Wave number: It is the reciprocal of wavelength and has units cm-1. It is denoted by ṽ (nu bar).

(v) Amplitude (a): It is the height of the crest or depth of the trough of a wave.

Wavelength, frequency and velocity of any electromagnetic radiations are related to each other as: f= vλ

Electromagnetic wave theory was successful in explaining the properties of light such as interference, diffraction etc., but it could not explain: 1. Black body radiation 2. Photoelectric effect

These phenomena could be explained only if electromagnetic waves are supposed to have particle nature. Max Planck provided an explanation for the behaviour of black body and photoelectric effect.

Planck’ Quantum theory of radiation:

Planck explains the distribution of intensity of the radiation from black body as a function of frequency or wavelength at different temperatures. E= h= hc/λ where, h = Planck’s Constant = 6.63 × 10-34 Js-1  E = energy of photon or quantum = frequency of emitted radiation n is the number of quanta of a particular frequency and ET is total energy.

Photoelectric Effect: Einstein received the Nobel Prize for his work on the photoelectric effect. It is the phenomenon in which a beam of light of a certain frequency falls on the surface of metal and electrons are ejected from it. This phenomenon is known as photoelectric effect.

<h2>Bohr’s model of atom</h2>

Neils Bohr proposed his model in 1931. Bohr’s model is applicable only for one electron system like H, He+ , Li2+ etc.

Emission Spectrum of Hydrogen:

According to Bohr’s theory, when an electron jumps from ground state to excited state, it emits a radiation of definite frequency (or wavelength) corresponding to the wavelength of each photon of light emitted, a bright line appears in the spectrum.

Limitations of Bohr’s Theory:

<ol>
	<li>It is unable to explain the spectrum of atoms other than hydrogen-like doublets or multielectron atoms.</li>
	<li>It could not explain the ability of atoms to form molecules by chemical bonds. Hence, it could not predict the shape of molecules.</li>
	<li>It is not in accordance with the Heisenberg uncertainty principle and could not explain the concept of the dual character of matter.</li>
	<li>It is unable to explain the splitting of spectral lines in the presence of magnetic field (Zeeman effect) and electric field (Stark effect).</li>
</ol>

In this blog, we came to know about the discovery of the electron, proton and neutron and their characteristics. We studied Thomson, Rutherford, and Bohr atomic models and their formulations. We understood the nature of electromagnetic radiation and Planck’s quantum theory. We explained the photoelectric effect and described features of atomic spectra. For more such informative blogs stay connected with us.

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