Discovery of Subatomic Particles and Atomic Models
Chemist John Dalton was born September 6, 1766, in Eaglesfield, England. He was the first scientist to explain the behavior of atoms in terms of the measurement of weight.
J.J. Thomson was born on December 18, 1856, in Cheetham Hill, England.His research in cathode rays led to the discovery of the electron, and he pursued further innovations in atomic structure exploration.
J.J Thomson
He concluded that:
Electrons are negatively charged.
Atom is like a plump cake.
He did the Cathode Ray Tube experiment
He found the mass over charge ratio.
How did he found the ratio and the statement that electrons are negatively charged?
First, he applied the electric field then he observed that a deflection came out. From the deflection he understand that there is charge but he couldn't decide the charge. That's why with magnetic field the ray deflected to the anode. So, he understood the electrons are negatively charged.
After that, he know that it is negatively charged. He measured the mass of electron so he found out the mass over charge ratio.
Rutherford
Gold-Foil Experiment
He did "gold foil experiment". He directed alpha particles (α) at a thin sheet of gold foil. Most of the particles passed through the foil, but a small fraction were deflected, and a few even bounced backwards.
• The atom is mostly empty space.
• There is a positively charged, very small, dense region in the atom.
Rutherford named this region as 'nucleus '.
• Most of the mass and all of the positive charges of an atom are concentrated in the nucleus.
• Electrons are distributed around the nucleus randomly like the solar system.
Milikan's Oil-Drop Experiment
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Milikan set out to find the charge of an electron using this apparatus. As the voltage of the electric field increases, the velocity of oil drops decreases.
BOHR
Born on October 7, 1885, in Copenhagen, Denmark, Niels Bohr went on to become an accomplished physicist who came up with a revolutionary theory on atomic structures and radiation emission.
•Electrons in an atom can only occupy in certain orbits with definite radii to the nucleus. These orbits are referred to as the energy levels or shells.
• Letters or numbers can be given to these shells (energy levels) like K,L,M,N,Q or 1, 2, 3, 4, 5
• The electron in a given orbit has specific quantity of energy (quantized energy)
James Chadwick
In 1932 a British physicist James Chadwick discovered
that the nucleus was not made of only one particle – the
proton – but of two particles. That is NEUTRON
Neutrons were determined to have a mass slightly greater
than protons.
- All matter is made up of extremely small particles called atoms.
- Atoms are indivisible.
- Atoms make up all matter.
- Atoms are similar to billiard balls.