Atomic Structure

In 1803, Dalton put forward a theory which explained the composition of matter. According to Dalton, an atom may be defined as the smallest part of matter that can be take part in a chemical reaction. Dalton’s atomic theory explained the law of conservation of mass, law of constant composition and law of multiple proportion very successfully, but failed to explain the results of many experiments,like generation of  electricity when glass or ebonite when rubbed with silk or fur. Further modern research has shown that the atom is not indivisible but can be broken down.

The important constituents of an atom or the fundamental particles of an atom are – protons, electrons and neutrons. Other than hydrogen atom, every atom contains these three subsatomic particles. Structure of hydrogen atom shown that it has only one electron and one proton while the number of neutrons present in it is 0. The existence of electrons and protons in atoms was first suggested by Sir J.J. Thomson and Goldstein respectively based on discharge tube experiments and was further confirmed by the phenomenon of radioactivity discovered by Becquerel. The neutron was discovered by James Chadwick by bombarding beryllium with a rays.
 

Dalton’s  Atomic Theory

All the objects around you, this book, your pen or pencil and things of  nature such as rocks,  water and plant constitute the matter of the universe. Matter is any substance which occupies space and has mass.

Dalton, in 1808, proposed that matter was made up of extremely small, indivisible particles called atoms. (In Greek atom means which cannot be cut). This concept  was accepted for number of years.

The main postulates of Dalton’s  atomic theory are

• Matter is made up of small indivisible particles, called atoms.

• Atoms can  neither  be created nor destroyed. This means that a chemical reaction is just a simple rearrangement of atoms and the same number of atoms must be present before and after the reaction.

• Atom is the smallest particle of an element which takes part in a chemical reaction.

• Atoms of the same element are identical in all respects especially, size, shape and mass.

• Atoms of different elements have different mass, shape and size.

• Atoms of different elements combine in a fixed ratio of small whole numbers to form compound atoms, called molecules.

However, the researches done by various eminent scientists and the discovery of radioactivity have established beyond doubt, that atom was not the smallest indivisible particle but had a complex structure of its own and was made up of still smaller particles like electrons, protons, neutrons etc. At present about 35 different subatomic particles are known but the three particles namely electron, proton and neutron are regarded as the fundamental particles.

We shall now take up the brief study of these fundamental particles.  The existence of electrons in atoms was first suggested, by J.J. Thomson, as a result of experimental work on the conduction of electricity through gases at low pressures and high voltage, which produces cathode rays consisting of negatively charged particles, named as electrons. The e/m ratio for cathode rays is fixed whose value is 1.76 x 108 C/g

We know that an atom is electrically neutral, if it contains negatively charged electrons it must also contain some positively charged particles. This was confirmed by Goldstein in his discharge tube experiment with perforated cathode. On passing high voltage between the electrodes of a discharge tube it was found that some rays were coming from the side of the anode which passed through the holes in the cathode. These anode rays (canal rays) consisted of positively charged particles formed by ionization of gas molecules by the cathode rays.

The charge to mass ratio ( e/m value) of positively charge particles was found to be maximum when the discharge tube was filled with hydrogen gas as hydrogen is the lightest element. These positively charged particles are called protons. e/m varies with the nature of gas taken in the discharge tube. The positive particles are positive residues of the gas left when the gas is ionized.The neutral charge particle, neutron was discovered by James Chadwick by bombarding boron or beryllium with a–particles.
 

Fundamental Particles

Atoms are made up of three fundamental particles.The charge and mass of these fundamental particles are as follows:

One unit charge = 4.80298 × 10^-10 esu = 1.60210 × 10^-19 coulombs

one amu = 1/2× mass of ₆C¹² atom

We will gain an in depth knowledge of concepts of atomic structure i.e structure of an atom for  IIT JEE and other exams under following subtopics

1. Discovery of Electron,  Proton & Neutron

2. Atomic Models

3. Electromagnetic Radiations

4. Quantum Mechanical Model of Atom

5. Dual Nature of matter and Photoelectric Effect

6. Electronic Configuration

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