Before we start with the concept of an atom, it is extremely important to clear all our doubts about an atom. An atom is the smallest particle that makes up matter. It can not be divided further. An atom consists of a proton, neutron, and electron. A proton has a positive charge, a neutron has no charge, and an electron carries a negative charge. All these together make up an atom. Today, we will discuss the discovery of protons, neutrons, and electrons. We will also talk about the various models of atoms in detail for a clear understanding of every concept for your convenience. This will provide you with the concept of atom study material. 

Discovery of Electron – Cathode Rays 

In 1879, William Crooks conducted an experiment that led to the discovery of cathode rays. The experiment was performed in a discharge tube about 60 cm in length. In this tube, the ends are completely sealed with two metal electrodes fitted at the two ends of the discharge tube. 

When a high voltage flows through the electrodes, we see the movement of current from the negative electrode (cathode) to that of the anode or positive electrode. The rays involved during this process are termed cathode rays. 

Properties of Cathode Rays 

• They travel linearly. 
• Cathode rays have negatively charged particles in them.
• Cathode rays do not have anything to do with the nature of gases or the material of electrodes. It is independent of these things. 
• They are invisible to the naked eyes.
• The behaviour of cathode rays can be visible by using certain materials like fluorescent light. 
• They move from cathode to anode. 

Determination of Charge/Mass Ratio for Electrons

J.J. Thompson was the first to discover the charge/mass ratio or e/m ratio for electrons. He used the electrons present in the discharge tube to act as cathode rays. These rays were to influence electric and magnetic fields in the tube. 

The factors that influenced it were: 

• The lighter the particle is, the more deflection will be seen.
• The more the magnitude of the charge, the more the interaction and the greater deflection will be provided. 
• The deflection also grows stronger with the voltage applied through the tube.

Thus, Thomson was able to determine: 

e/me = 1.758820 x 1011 C kg-1 

me  = Mass of the electron (in kg) 

e = Magnitude of Charge on the electron (in C)

Electron Charge

Through an oil drop experiment, R.A. Millikan was able to determine the charge on the electron: 

Charge of an electron (e) = -1.6022 × 10-19 C 

Discovery of Proton – Anode Rays 

Goldstein discovered anode rays in 1886. He modified the discharge tube used earlier by using a perforated cathode. He reduced the pressure and discovered a light-emitting ray passing through the cathode holes. They were moving in a direction opposite to that of cathode rays. They are anode rays or positive rays.

Characteristics of Anode Rays 

• The positive charge value depends on the nature of the gas present in the tube.
• The behaviour of anode rays is opposite to that of cathode rays in terms of their influence on the electric and magnetic fields.
• The charge to mass ratio of protons depends on the gas they originate from.
• Some protons carry multiple charges. 

Charge on Proton 

Charge on a proton = 1.602 x 10-19 C

Neutron 

Chadwick discovered neutrons in 1932. It has no charge. It is a neutral particle. He found neutrons by bombarding thin sheets of beryllium with a-particles moving very fast. In doing so, he discovered neutrally charged particles called neutrons.

Thomson Model of Atom 

Next, the concept of an atom consists of one of the most important models of atoms: the Thomson Model. 

J.J Thomson was a renowned scientist who gave his concept of the atom. He regarded an atom as a sphere with a fixed radius. In this sphere, the protons and electrons are embedded in it. 

In simple words, the atom is visualised as a pudding or cake that has a positive charge. This cake has electrons embedded in them, like raisins or nuts. The atom’s mass is spread evenly throughout the atom.

Drawbacks 

• This concept of the atom was not satisfactory enough. It did not do justice to the findings of Rutherford. 

Rutherford’s Model 

It was in 1911 that Rutherford performed his scattering experiment. In this experiment, he bombarded very thin foils of gold, copper, or silver with a-particles and recorded his findings. The gold foil also consisted of a fluorescent zinc sulphide screen around it. On contact with the a-particles, a flash of light was seen.

He observed that: 

• Most of the particles passed through the thin foil without any signs of deflection.
• A few of them deflected at very small or negligible angles.
• Rarely, some of them deflected at nearly an angle of 180°.

He concluded that: 

• There is enough space in an atom.
• The total volume of the atom is very large, and the volume of the nucleus is negligible in front of it.
• The positive charge is concentrated at very small volumes in the atom.

Drawbacks 

• It does not explain the stability of an atom.
• It gives no idea about how the electrons are distributed in an atom.

Bohr’s Model 

In 1913, Bohr proposed the most renowned concept of the atom. It forms the basis of the study material notes on the concept of the atom. He observed that: 

• Electrons revolve around the nucleus in a predetermined path known as orbits.
• Every orbit has an energy level of its own. They are energy shells or energy levels.
• If an electron is present in a particular orbit, it tends to remain constant.
• On supplying energy to electrons, it can jump to a higher state of energy. It is termed an excited state. 
• Energy orbits should have an angular momentum of h/2π. (h = Planck’s constant)

Achievements

• It explained and proved the stability of an atom.
• It helped in understanding the energy of electrons in hydrogen atoms.
• It helped in understanding the electron spectrum of the hydrogen atom.

Limitations

• It did not clarify spectral lines.
• It had nothing to do with Heisenberg’s uncertainty principle.
•  It had no explanation for the formation of chemical bonds. 
• Bohr’s model provided no clarification for multielectron atoms.

Conclusion 

To understand the working of an atom, it is extremely important to go through all the developments that have helped us better understand the functioning of an atom. Atoms make up almost everything around us. With time and the advancement of technology, our perspective towards atoms has also changed. We’ve tried to cover all the major scientific developments that have shaped our perspective towards the concept of the atom. This makes for an important concept of atom study material.