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340
Aristotle
Date is in B.C.
Aristotle was a philosopher, he believed that all matter was composed of combinations of fire, Earth, water, and air. -
440
Democritus
Date is in B.C.
Democritus, another Greek philosopher, proposed that matter was made up of tiny particles that could not be divided into smaller pieces. He called these particles atomos, meaning indivisible. -
Robert Boyle
An Irish scientist, Boyle measured relationships between volume and pressure of gases. He determined that gases are made up of tiny particles that group together to make different substances. -
Antoine Lavoisier
A French scientist, measured the masses of the substances that reacted together and the substances produced in many chemical reactions. He discovered that mass is neither produced or lost during a chemical reaction. Lavoisier called this the law of conservation of mass. -
John Dalton
An English chemist and physicist. He imagined that all atoms were like small spheres, but they could have different properties. They varied in size, mass, or color. Dalton used this model to explan matter:
-All matter is made up of small, indivisible particles called atoms.
-All atoms of an element are identical in properties like size and mass.
-Atoms of different elements have different properties.
-Atoms of different elements can combine in specific fixed ratios to form new substances. -
J.J. Thomson
An English physicist who discovered the electron. His experiments showed that a beam of particles was made of negative charges. He showed that all elements produced the same type of beam. He used this evidence to develop a new model of the atom. This model said that all atoms are made of smaller subatomic particles put together in different combinations to make different elements. He said that an atom was a sphere of positive charge in which negative particles were imbedded. -
Hantaro Nagaoka
A Japanese scientist, proposed another new model of the atom in which he placed the electrons on the outside of the sphere. They travelled around the central sphere in a pattern like the rings around planet Saturn. -
Ernest Rutherford
Discovered the nucleus of the atom with his gold foil experiment. -
Quantum Mechanical Model
Each electron is thought of as a cloud of negative charge, instead of a tiny negative particle. Electrons occupy the whole space all at once at different energy levels. The electron cloud surrounds a nucleus containing two types of particles called nucleons: protons and neutrons. Protons have a positive electrical charge and neutrons are neutral.
