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492 BCE
Democritus
Democritus really based his theory on observations and natural curiosity. His theory was that everything was made up of small and indestructible things called atoms. -
John dalton
He concluded that gases must consist of tiny particles in constant motion. He showed that a compound always consists of the same elements in the same ratio. only if elements are made of tiny particles that can combine in an endless variety of ways. -
John dalton
Discovery- All substances are made of atoms. Atoms are the smallest particles of matter. They cannot be divided into smaller particles. They also cannot be created or destroyed.
All atoms of the same element are alike and have the same mass. Atoms of different elements are different and have different masses.
Atoms join together to form compounds. A given compound always consists of the same kinds of atoms in the same ratio -
Joseph John Thomson
He did experiments in which he passed an electric current through a vacuum tube.
Discovery- Thomson’s experiments showed that an electric current consists of flowing, negatively charged particles. Thomson’s experiments also showed that the negative particles are all alike and smaller than atoms. Thomson concluded that the negative particles couldn’t be fundamental units of matter because they are all alike. Instead, they must be parts of atoms. The negative particles were later named electrons -
Ernest Rutherford
A beam of alpha particles was aimed at very thin gold foil and their passage through the foil detected. Most of the alpha particles passed straight through the foil as though they were moving through empty space. Surprisingly though, some of the alpha particle hit the foil and bounced off. It happened in a very low percentage of the atoms. -
Niels Bohr
Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. The motion of the electrons in the Rutherford model was unstable because, according to classical mechanics and electromagnetic theory, any charged particle moving on a curved path emits electromagnetic radiation; thus, the electrons would lose energy and spiral into the nucleus.
