Detailed Information on "ATOMIC STRUCTURE" - Part1 and The Foundation of Chemistry.

Atomic Structure forms the base of chemistry. Everything in the universe is made up of atoms, the smallest units of matter called Atom. Understanding Atomic Structure is essential for having knowledge about the principles of chemical reactions, bonding, and properties of materials. So, overall we can say that Atom is the Foundation Of Chemistry.

The Smallest Unit Of Matter

Early Attempts For Smallest : Atomic Structure

• First of all, Maharishi Kanada proposed an atomic theory. In this theory, he says that everything is subdivided into parts but after repeated subdivision, there must be smallest unit of that thing which can not further divided, called Parmanu.
• Greek Philosophers Leucippus and Democritus also proposed an atomic theory in which he suggested the smallest particles that can not further divided, named as Atom.

Laws of Chemical Composition

In a chemical reaction, there are reactants and products and they together make a chemical reaction. There is an arrow in between reactant and product. 

• Law of Conservation Of Mass : This law is given by Antoine Lavoisier in 1789. Mass of reactants is equal to the mass of products in a chemical reaction. Mass can neither be created nor be destroyed. 

• Law of Constant Proportion :  This law is given by Joseph Proust in 1799. The ratio of mass of elements in a compound is always fixed. For example - in water ( H2O ) compound, mass of hydrogen has two units and mass of oxygen has sixteen units, there is fixed ratio of mass ( 1:8 ).

Molecules

Molecules are made up of different - different atoms or we can say, two atoms when combined together made Molecules.

There is two types of Molecules: 1. Homo-molecules 2. Hetero-molecules

Different - Different Discoveries On Atomic Structure

There are many discoveries on Atom and it's structures. In the below table, there are discoveries of different parts of atom with their scientists' name.

Dalton's Atomic Theory

Dalton's Atomic Theory is given by John Dalton in 1803 AD.

• All maters are made up of very small indivisible particles, these particles are known as 'Atom'.
• Atoms of similar elements have same size, same mass and same chemical properties.
• Atoms can not divided further or we can say it is indivisible.
• Atoms of different elements have different shape, size and mass.
• In a chemical reaction, the atoms of elements are combined, separated and rearranged.

Thomson's Model Of Atom 

Discovery of Electron is proposed by J.J. Thomson in 1897 AD. He proposed an experiment "Cathode Ray Tube".

Observations:

• The cathode rays are made up of very tiny particles of negatively charged. He called these particles as 'Corpuscles' and later named as 'Electron'.
• The mass of the electron is (1/1836)th part of the mass of hydrogen atom.
• It confirms that Atom is divisible.

J.J. Thomson's Water Melon Model

He suggested that an atom has spherical shape just like a water-melon having uniform reddish part (positively charged protons) and seeds are embedded into the reddish part and these seeds indicate negatively charged electrons. This model is given by J. J. Thomson. And this model is known as Water-Melon Model or Plum-Pudding Model.

In this model, J. J. Thomson suggested that an atom have equal number of protons and electrons in magnitude. So, atom is electrically neutral. 

Rutherford's Model

Alpha particles are strike on the thin gold foil. This alpha particles (+2) have two times the proton charge. So, we can also say that those particles, which are strike on the gold foil, are of Helium Particles. Some observations are noted after this experiment.

Rutherford's Thin Gold Foil Model 

Observations

• Atoms have a lot of empty space. So, alpha particles are passed through gold foil in straight direction.
• There is positively charged center, called Nucleus. Because, as alpha particles are passed through gold foil's center then we observe that alpha particles have diverted at some angles.
• Nucleus have equal quantity of positive and negative charge.
• Electrons are revolved around the proton.

Limitations

• Rutherford's Model can not explanation about the stability of atom. Because Electron, revolves around the protons, have some acceleration and as it revolve again and again then after some time electron looses it's energy and should be merged into the proton. Rutherford can not explain about this. 

Proton: Positively Charged Particles

Proton is discovered by E. Goldstein by the experiment of canal or anode rays in 1886 and stated that these are positively charged particles.

Bohr's Model

Bohr's Model is intruduced for correcting Rutherford's Atomic Model. The principle of Bohr's Model is based on Quantum Theory described by Max's Plan.

• He suggested that Electrons move around fix orbit and these orbits have fixed energy level. These electrons in the fixed orbit do not emit energy and merge with proton in the nucleus. 
• He also suggested that Energy losses happen not in continuous manner but if there will be any energy loss, electrons move to the lower energy level and if any electron absorbs energy, it moves to the upper energy level.
• We can say that all electrons revolve in fixed energy levels or orbits.
• He also say that Electrons are revolving in circular path having 100 percent acceleration, so due to centripital force, electrons try to move outside the orbit and it does not happen. This is because proton in the nucleus have also attract electrons, so electrons revolve around a fix orbit and this is the reason, Atom is stable.

Limitations of Bohr's Atomic Model

Bohr's Model is applicable only for simpler molecules like hydrogen and as we go to the higher or complex molecules, it does not work on it.

Conclusion

From the early philosophical ideas of Maharishi Kanada and Greek philosophers to modern scientific discoveries by Dalton, Thomson, Rutherford, and Bohr, our understanding of the atom has evolved gradually.

We have studied upto Bohr's Atomic Model. This is Atomic Structure Part-1. In next part, we will study about further evolution of Atom and its structure.