Exploring Atoms: Landmark Subatomic Discoveries
From the universe to the basic unit of life "Cell," from biggest clusters, galaxies, stars, and planets to the unseen particles of air, each and everything is made up of atoms. As cell is the basic unit of life, so the atom is the fundamental unit of matter; even cell is also composed of atoms. That is not just only atom, there is even a universe inside an atom. So let's have a deep dive into the search of an atom.
The word "Atom" is derived from the Greek word "Atomos" meaning "indivisible." Atom, the basic unit of matter, is composed of 3 major types of particles which are called sub-atomic particles like electron, proton, and neutron.
Old Concept: An atom is the smallest indivisible particle of matter that cannot be further divided. This old concept was given by Democritus and John Dalton. But later in the 19th century, this concept was rejected.
New Concept: Atom is the smallest divisible particle of matter which can be further divided into small particles.
Discoveries of Sub-atomic Particles
Discovery of Electron:
Later in the 19th century, Sir J.J. Thomson conducted an experiment "Cathode Ray Tube" sometimes abbreviated as CRT. He used a vacuum tube, anode electrode (positively charged) and cathode electrode (negatively charged). Rays emitted from the cathode electrode passed through the anode electrode in straight direction and targeted on zinc sulfide. But still, Thomson was not clear about what types of rays these are. He put positively and negatively charged plates from the upside and downside of the vacuum tube; as a result, the unknown particles were bent to the positively charged plate.
Conclusion:
1. As a beam of particles was bent towards the positively charged plate, Sir Thomson called them "negatively charged particles," later known as electrons.
This experiment of J.J. Thomson rejected the old concept about atom as an indivisible particle.
Discovery of Proton:
In 1909, Ernest Rutherford conducted "Gold Foil Experiment," he used lead block as source of alpha rays and gold foil in the center of circled zinc sulfide screen. As beam of rays was targeted on the gold foil, as a result, some rays were passed straight, some were deflected at smaller and larger angles and some rays were bounced back at angle of 180 degrees.
Conclusion:
1. Most of rays were passed straight, it means atom has most empty space.
2. Atom has a dense and positively charged body in the center because of the deflection of rays at different angles, later known as protons.
3. Electrons orbit around the nucleus.
4. Nucleus is much smaller than atom. If an atom has size of earth then its nucleus will be a football stadium.
Discovery of Neutron:
Scientists compared the weight of helium (He) to the weight of two atoms of hydrogen (H), it was concluded that the weight of both sides was not equal, even there was equal number of protons on both sides but when two more atoms of hydrogen were added, the balance became equal. Scientists observed that there is something else inside atom that has almost similar mass to the mass of proton.
In 1932, James Chadwick, the student of Rutherford, conducted an experiment in the search of that particles. Chadwick used polonium inside lead block as a source of alpha particles, which were shot on beryllium; as a result, unknown particles emit from beryllium in straight direction. James used detector with positive and negative charges on both sides but the unknown particles neither attract to the positive nor negative.
Conclusion:
1. Particles have no any charge.
2. Particles just travelled in straight direction without bending to the positive or negative sides.
3. James Chadwick named that particles "neutron."
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