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 n. A unit of matter, the smallest unit of an 

element, consisting of a dense, central, positively 

charged nucleus surrounded by a system of 

electrons, equal in number to the number of 

nuclear protons, the entire structure having an 

approximate diameter of 10-8 centimeter and 

characteristically remaining undivided in 

chemical reactions except for limited removal, 

transfer, or exchange of certain electrons.

Democritus   thought that it ended at 

some point, a smallest possible bit of matter. 

He called these basic matter particles, atoms. 

For more than 2000 years nobody did 

anything to continue the explorations that 

the Greeks had started into the nature of 

matter. Not until the early 1600's did people 

begin again to question the structure of 

matter. 

Robert Boyle  Boyle published The 

Skeptical Chymist. In this book he 

overturned Aristotle’s conception of the four 

elements and replaced it with the modern idea 

of an element—namely that an element is a 

substance that cannot be separated into 

simpler components by chemical methods. The 

Skeptical Chymist is recognized as the 

foundation-stone of modern chemistry.

Antoine Lavoisier 

In 1777 he demonstrated the 

Conservation Of Matter( that being, 

Matter cannot be created nor can it be 

destroyed), in a chemical reaction.Then 

used this to help him state the 

difference between an element and a 

compound.

John Dalton- Proposed an "atomic 

theory" with spherical solid atoms based 

upon measurable properties of mass. 

Dalton also proposed that each chemical 

element is composed of atoms of a 

single, unique type, and though they 

cannot be altered or destroyed by 

chemical means, they can combine to 

form more complex structures (chemical 

compounds).

Demitri Mendeleev- Arranged 

elements into 7 groups with similar 

properties.   He discovered that the 

properties of elements   "were 

periodic functions of the their 

atomic weights".   This became 

known as the Periodic Law.

Eugen Goldstein -  was a 

German physicist. He was an 

early investigator of discharge 

tubes, the discoverer of anode 

rays, and is sometimes credited 

with the discovery of the proton.

Sir Joseph John "J. J." Thomson 

was a British physicist and Nobel 

laureate. He is credited for the 

discovery of the electron and of 

isotopes, and the invention of the 

mass spectrometer. Thomson was 

awarded the 1906 Nobel Prize in 

Physics for the discovery of the 

electron and for his work on the 

conduction of electricity in gases.

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