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The Structure of an Atom

Imagine you have a chocolate ice-cream cone and you want to share it with all your friends. You keep on giving out scoop after scoop until you are left with too little to even get a proper scoop. Soon enough your other friends also gather around you and ask for a share and so the process goes on.

In the end you grind the ice cream cone into a small pile and start dividing the pile into even smaller piles. Soon you will be left with just a speck of the cone. Can this process go on or will it be curtailed at some point till you get a fundamental particle which can not be divided any further?. We will discuss this and more in this section.

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Atomic Structure Definition

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The atomic structure, theoretically, can be defined as a unit having a central nucleus with a positive charge and surrounded by negatively charged electrons which also help in neutralizing the positive charge of the nucleus.
These electrons revolve around the nucleus at definite orbits at varying distance from the nucleus.

The mass of the nucleus is enhanced by the presence of neutrons which along with the positively charged protons provide the atomic mass number of the respective element. The electrons revolving around the nucleus has negligible mass as compared to a proton and is exactly 1/1837 of the proton's mass.

History of Atomic Structure

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John Dalton was the first to discover, "all matter is composed of small particles called atoms." According to him, the atom is a hard solid individual particle incapable of subdivision. But it was discovered later that the atom consists of smaller particles called sub atomic particles.

Discovery of Subatomic Particles

1. Discovery of Electrons

An electron is a subatomic particle which carries a unit negative charge. It was discovered by Sir. J. J. Thomson in 1897 while working on cathode rays. These negatively charged particles coming out of the cathode ray tube were named as Electrons, since they were negatively charged.

2. Discovery of Neutrons

Proton, the positive sub atomic particle was discovered by Goldstein when working on Canal rays or the positive rays and Rutherford gave them their name.

3. Discovery of Neutrons

Neutron is a sub atomic particle, neutral in nature. It was discovered by James Chadwick in 1932.


Discovery of Atomic Structure

1. Rutherford’s Model

First model of atom was given by Rutherford in 1909. Rutherford and Marsden performed their historical Alpha particle scattering experiment by directing very highly energetic alpha particles from a radioactive source against a thin gold foil provided with a circular fluorescent Zinc sulphide screen around it. A tiny flash of light was produced whenever an alpha particle struck the screen.

Based on their experiments, Rutherford called their atomic model a nuclear atom and gave forth these postulates

  1. Atom has a tiny dense central core or the nucleus which contains practically the entire mass of the atom, leaving the rest of the atom almost empty.
  2. The entire positive charge of the atom is located in the nucleus while the electrons are distributed in the vacant space around it.
  3. The electrons are moving in orbits or closed circular paths around the nucleus like planets around the sun.

Rutherford’s model could not account for the electrons falling inside the nucleus a. According to the electromagnetic theory; any moving object will continuously be losing energy.

Determination of Atomic Number

Mosley brought forward the idea of atomic number. He called it the position number of a particular element in the periodic table. According to him,‘Atomic number of an element is equal to the number of protons in the nucleus of the atom of that element.’ Atomic number, therefore, was referred to as the ‘proton number' of an element.

Determination of Mass number

The total number of protons and neutrons in the nucleus of an atom is called it's mass number. Therefore, the mass number of an atom can be obtained by rounding off the experimental value of atomic mass to the nearest whole number.

With the discovery of mass number and atomic number, a new atomic model was proposed by Neil’s Bohr with the quantum theory.

Concepts that Paved the Way for Bohr’s Atomic Model


Photoelectric Effect

When a beam of light of sufficiently high frequency is allowed to strike a metal surface in vacuum, electrons are ejected from the metal surface. The ejected electrons were termed as photo-electrons. This was proposed by Einstein and he was awarded Nobel prize in 1905 for his work on photons.

He interpreted the photoelectric effect by quantum theory.

Compton Effect

In 1923, A. H. Compton provided one more proof to quantum theory. He was awarded the Nobel prize for his theory in 1927, which is called as the COMPTON EFFECT.
When x rays of a wavelength struck a sample of graphite, an electron was ejected and the x-rays scattered at an angle had longer wavelength.

  • Black Body Radiation

A black body is defined as an object that absorbs all the radiations falling on it and when heated up releases this radiation. The radiations released by such a body are called black body radiations.

  • Plank’s Quantum Theory

Max Plank proposed a theory known as Plank’s quantum theory of radiation to explain black body radiation.

According to his theory

  1. Radiant energy is emitted or absorbed discontinuously in the form of tiny bundles of energy known as "quanta".
  2. Each quantum is associated with a definite amount of energy E depending upon the frequency of incident radiation. This is given by: E = h $\nu $, where h is the Plank’s constant. Value of Plank’s constant is = 6.626 x 10-34 joules.
  3. A body can emit or absorb energy only in whole number multiples of quantum, i.e, 1 h $\nu $, 2 h $\nu $, 3 h $\nu $ etc.


2. Bohr’s Atomic Model

Applying Plank’s Quantum theory to the electrons revolving around the nucleus, Niels Bohr put forward his model in 1913. He retained Rutherford’s model of very small positively charged nucleus at the center which contains all the protons and neutrons present in an atom. Bohr also agreed to the concept of negatively charged electrons revolving round the nucleus in the same way as planets revolving around the sun.

Important postulates of Bohr’s theory were:

  • The electrons in an atom revolve around the nucleus only in certain selected circular orbits. These orbits are associated with definite energies and are called energy levels. These are numbered as 1, 2, 3, 4 etc., or K, L, M, N … etc.
  • Only those orbits are permitted in which the angular momentum of the electron is a whole number multiple of h/2p, where h is the Plank’s constant.
  • As long as the electron remains in a particular orbit, it does not lose or gain energy. This means that energy of an electron in a particular orbit remains constant. These orbits, therefore, are called as stationary orbits. Electrons moving in a stationary orbit, without losing energy, is called as Ground state.
  • Energy is emitted or absorbed only when an electron jumps from one orbit to another. Energy is absorbed when an electron jumps from inner to outer orbit and is emitted when it moves from outer to an inner orbit.
    •  $\Delta E$ = $E_{2}-E_{1}$ = hv = $\frac{hc}{\lambda}$ ; where, C = Velocity of light, v = frequency and = wavelength.
  • The electrostatic attraction between the electrons and protons are balanced by the centrifugal force of the orbiting electrons. 2, 3…

Merits of Bohr’s theory

  • Bohr’s theory successfully explains hydrogen emission spectrum and other one electron ions.
  • According to Bohr, the electrons revolve in stationery shells. Even if the electron emits energy, it will finally be in n=1.
  • The wave numbers of spectral lines determined experimentally are in good agreement with those calculated using Bohr’s integral values 1.

Bohr’s orbits were later changed with orbitals. Orbitals are three dimensional spaces, where the probability of finding the electron is maximum. Thus, an orbital is the most probable space in which the electron spends most of its time while in constant motion. So, the modern atomic theory or the atomic model of the present day has adopted most concepts from Bohr’s theory replacing orbits with orbitals.


Ernest Rutherford Atomic Theory

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Ernest Rutherford

Born in New Zealand, he was one among the twelve children of a farmer. His parents gave him university education with great difficulty. A brilliant student - he worked with J.J Thomson at Cambridge University to study the effect of electricity through air. He studied the spontaneous disintegration of radium.

He is well known for the scattering experiment of alpha-rays through matter, which led to the famous structure of atom in 1911. He was awarded the Nobel Prize in 1908.

The famous words of Rutherford on completing the alpha-rays scattering experiment were: “It was quite the most incredible event that has ever happened to me in my life. It was almost as if you fired a 15 inch shell into a piece of tissue paper and it came back and hit you”.

J J Thomson Atomic Theory

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J.J Thomson

He is usually credited with the first 'modern' structure of the atom, the so-called 'plum pudding' model. In it, he pictured a sphere of positive charges mixed together with an equal number of electrons. For his theoretical and experimental investigations into the electron and the conduction of electricity by gases he was awarded the 1906 Noble prize in Physics. He and his student Rutherford were the first to demonstrate the ionization of air by X-rays.

Neils Bohr Atomic Theory

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Neils Bohr

He was a great Danish Physicist who obtained his doctorate in theoretical Physics in 1911. He made a great contribution towards the understanding of atomic structure and quantum mechanics. He was awarded the Nobel prize in Physics in 1922, mainly for his work on atoms at the age of 37. During World War II, he went to USA and was one of the witnesses of the atomic explosion in New Mexico in 1945. Later he returned to Copenhagen and promoted the peaceful use of atomic energy. In 1957, he was awarded the prize for 'Atom for Peace'.

More topics in The Structure of an Atom
Balancing Equations Calculating Molar Mass
Mole Unit Conservation of Mass
Standard Temperature
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