December 28, 2015


1. History of Atom

In 460 B.C., a Greek philosopher, Democratism, developed a kind of idea of atoms. He looked at things around him and felt that if you go on breaking any material into parts and further parts, you will have to reach something which can’t be broken further. This he called an atom. It was purely speculation and no experimental evidence was available at that time to prove anything.

More than 2000 years had to pass before people began to consider the idea of atoms seriously. In the 1800’s, an English chemist, John Dalton did lot of experiments with several chemicals and he found a strange thing. Elements always combined with each other in fixed proportions. Any extra amount added would come out as residue. An element A for example would combine with 2 B’s by weight. This will always remain true. How can this be possible unless some fundamental particle in one element is combining with some fundamental particles in another, the number being always the same? Thus he proposed that every element was made up of fundamental particles and he called them by the same name atoms.

Thomsons ‘Rasin in the Pudding’ model of the atom

In 1897, the next great event took place. A negatively charged particle was discovered by J.J.Thomson in his experiments with discharge of gases. He called the negative charge particle electron. He also proposed a model for the atom. Since matter is not negatively charge he felt that there was a positive charge that balanced the negative charge and the electrons were put like raisins on a lump of pudding that was the positive charge.


Berlin proved that when the energy is released when heated until it glows, this energy always comes out as discrete units, that is it takes only multiples of certain basic values. The energy was not continuous. He called these energy packets quanta.

At that time light was considered to be a wave but Albert Einstein thought otherwise. He felt that light was made up of quanta and he called this quantum of light Photon. Einstein also discovered the photo electric effect, the phenomenon by which a material gives out electrons when light is put on it and absorb. By this time people wondered about light. What was it really? A wave or a particle? Finally they came to the conclusion that light sometimes behaves as a wave and at other times behaves as a particle (but never both). The behavior which actually comes out depends upon the set up of the experiment. During the same time, alpha particles were discovered which had a positive charge. Scientists felt that this was the positive charge in the atom. In 1911, a land mark experiment was performed by Earnest Rutherford. He felt that it would be interesting to bombard atoms with alpha rays. He just wanted to see what would happen. He used radium to generate the alpha particles and shown them on to atoms in a thin gold foil. Behind the foil sat a fluorescent screen for which he could observe the alpha particles impact.

A strange thing happened. Most of the alpha particles passed through but occasionally some were hit back as if striking something hard! The only explanation of this was that most of the atoms had empty space in between and with hard points here and there.

Rutherford proposed his planetary model of the atom. He said that the positive charge was concentrated at the centre called the nucleus and the electrons moved round it like planets. But there was one terrible problem with this model. By the time a lot had been discovered about electricity and magnetism in general. A negative charge moving around the nucleus should lose energy by generating electro magnetic waves. Bohr used the principle energy comes in dissect packets of energy called Quanta to come with his model of the atom. He proposed that only some special orbits are allowed for electrons to move around. These orbits are called energy level. When electron gains energy it jumps from lower to higher orbit. When it looses energy, it jumps from higher to lower. Electron can’t have an intermediary between two levels. The energy difference between two levels is the quanta.

In 1932, J. Chadwick another sub atomic partial called the neutron. The neutron has mass nearly equal to the proton and has no charge. The mass of the nucleus is equal to the sum of the masses of the protons and the neutrons.

2. How are the electrons distributed in the difference orbits

To keep an atom electrically stable, it has the same number of negatively charged electrons in orbit around the nucleus as there are positively charged protons in the nucleus. In situations where there are either more or less electrons in orbit than there are protons in the nucleus, the atom is called an ion. This happens in static electricity and in some chemical solutions.

Electron shells

Electrons are arranged in shells or orbits around the nucleus.

Maximum number

There is a definite arrangement of the electrons in these shells and a maximum number of electrons possible in each shell.

Shell or Orbit Number 1 2 3 4 5

2 8 18 32 50

The most electrons possible in the first shell are 2. After the first shell is filled, the second shell starts filling up, according to the number of positive charges in the nucleus. The most allowed in the second shell is 8 electrons. Then the third shell starts to fill.

Electron shells for a Sodium atom (atomic number 11, with 11 electrons)


Filling order complicated

After the second orbit or shell is filled, things start to get complicated. The third shell fills until it gets to 8, and then the fourth shell starts adding electrons until it too has 8 electrons. Then the third shell fills until it gets to 18.

Outer shell basis of Chemistry or valency

The number of electrons in each shell is the basis of chemical combinations.

Energy levels

Electrons have potential energy, depending on their shell or orbit. Each orbit has it’s own energy level. The each energy levels are further broken into sub orbits. There are some situations when an electron will jump from one orbit—or energy level—to another. When that happens, the electron gives off electromagnetic energy of one wave length or colour of light.

Since each element has its own electron shell or energy level configuration, it has own set of colours called its spectrum. This is used to identify any element.

The power of the Atomic theory :

What a reality, the atomic structure is! It is a deep fact about the whole physical world and explains so much! As we already saw in the earlier chapters, it explains sound (propagation of waves due to spring like disturbances of medium.) It explains heat. Heat is nothing but the jiggling and wiggling of atoms. It explains the giving off of light of a certain colour. This happens when electrons jump from one orbit to another.

Electric current is nothing but free electrons moving in metals due to a potential difference. Magnetism in magnets is the result of the molecular atoms in matter that are aligned. All the macroscopic properties can be explained by the atomic theory. Examples are state (solid, liquid or gas), pressure of gases, strength of materials (more bounding of the atoms inside, more the strength), temperature (average energy of movement of atoms in a body), density (more packed the atoms, more the density) etc. The whole of chemistry that is chemical combination depends upon valency as mentioned earlier. Hence chemistry fundamentally is based on atomic theory. The foundation of biology is chemistry and hence even biology depends upon atomic theory (the formation of complex molecules).