Subatomic Particles

Between 1803 and 1808, John Dalton proposed an atomic theory. According to this theory, an atom is the smallest indivisible particle, and it cannot be divided further. It almost explained everything related to atoms except their chemical activity. It could not conclude anything about atomic masses as well.

Finally, scientists J.J. Thomson and Ernest Rutherford proposed the accepted picture of atoms. After their experiment, they concluded that an atom is not the smallest indivisible particle. Atoms can be divided into subatomic particles: electrons, protons, and neutrons.

Electron

The study of electrons begins with the study of cathode ray discharge tubes. A cathode ray discharge tube is a thin glass tube with two metal plates on both ends, called electrodes. At a high voltage and low pressure, current flows through particles from cathode to anode. 

Results observed from this experiment are,

• The movement of rays is from cathode to anode.
• These are invisible rays, but their presence is observed through fluorescent or phosphorescent phenomena, as glass tubes glow when hit by cathode rays.
• In the case of zero external fields, this ray travels in a straight path.
• In the presence of external fields, cathode rays show similar behaviour as negatively charged particles. So these particles in cathode rays are termed electrons.

Further, with this cathode-ray tube experiment, J.J. Thomson evaluated the e/m ratio, where e represents electrical charge and m represents the mass of electrons in the presence of external fields. 

Upon close study of the deflection of cathode rays, the e/m ratio derived was1.75*1011 C kg-1.

General Characteristics of an Electron

• J.J. Thomson discovered electrons through the cathode ray experiment and further evaluation of the e/m ratio.
• Electrons have a negative charge.

• Electrons carry a charge of  -1.60 *10 -19 coulomb.
• The mass of electrons with respect to Hydrogen is 0.000549 atomic mass units.
• The electron has the highest mass of  9.1 * 10 -28 g.

• According to Bohr’s theory, electrons revolve around the nucleus in a circular orbit inside the atom.

Proton

Modified cathode ray tube resulted in the discovery of positively charged particles, i.e. canal rays. The e/m ratio depends on the gas present in the cathode ray tube. Canal rays show the opposite behaviour compared to cathode rays.

General Characteristics of a Proton

• Ernest Rutherford proposed the word proton for the hydrogen nucleus.
• Protons with neutrons form the nuclei inside an atom. 
• Protons are positive in charge.

• Proton carries a charge of  +1.60 * 10 -19 coulomb.
• At  1.672 * 10 -24 g, a proton’s mass is lesser than the electron’s mass, 
• Protons are obtained from nuclear reactions. That’s why protons are considered the basis of atoms.

Neutron

Chadwick discovered the neutron 20 years after the development of the structure of atoms.

• Neutrons are neutral, chargeless particles.
• Neutrons, together with protons form the main constituent of the nucleus.

• Neutron has the lowest mass of 1.65*10 -24 g.
• The neutron’s mass is nearly the same as the proton’s mass with zero charge.

Bohr’s Theory

After the development of subatomic particles, the matter of their arrangement was up for debate. Bohr’s theory eventually solved this. 

As per the theory, the nucleus is at the centre of the atom. It consists of neutrons and protons. Electrons revolve around the nucleus in a circular orbit. Each orbit can have a maximum of eight electrons.

Each orbit is related to a definite amount of energy. The orbit is represented by k, l, m, n etc. 

Quantum Number

There are three variables to express the action of an electron. These variables are n, l, m, or quantum numbers. 

Quantum numbers help predict the shape, size and orientation where electrons are present in maximum probability. They also help identify the energy levels of every orbital.

Spin quantum numbers refer to an electron’s spin with respect to its axis.

There are four quantum numbers to express the behaviour of an electron.

n- Principal Quantum Number

l- Azimuthal Quantum Number

m- Magnetic Quantum Number

s- Spin Quantum Number

Conclusion

According to Dalton’s atomic theory, an atom is the smallest particle that can’t be divided further. But after long efforts of scientists, they concluded that atoms can be divided into subatomic particles—electrons, protons and neutrons. 

Protons have a positive charge, and electrons are negative. Protons and neutrons together form the nuclei. 

Quantum numbers help predict the shape, size and orientation where electrons are present in maximum probability. They also help identify the energy levels of every orbital.