1800--1875

John Dalton (1766-1844) Country of Origin: England Year of Discovery: 1803

John Dalton was born to a poor Quaker Family in Eaglelsfield, England.In her earlier years, he taught at a Quaker boarding school and later he began teaching in Manchester.While in Manchester he joined the Manchester Literary and Philosophical Society which helped stimulate many of his ideas on atoms as well as his surroundings.Before becoming involved in atomism, he focused on his first true love, meteorology.Being a Quaker caused him to be rather modest, but this did not hold him back from becoming one of the most important scientists is the atomism field.

John Dalton’s Atomic Theory (1803) states that elements are made of tiny particles called atoms. He formed this theory that states that every atom with in an element are the exact same in mass as well as other properties, atoms of a different element are different, and that atoms can be distinguished from each other by their weight. His theory states that atoms of one element can combine with atoms of another element to form a new compound.His theory states that atoms combine in whole ratios and that atoms cannot be broken down smaller, destroyed, or magically created. Instead, when a new compound is formed, atoms are grouped together in a new manner but they are still all present.According to Dalton, a chemical reaction is simply a rearrangement of atoms. John Dalton also formed an element set that was new to its time but basic compared to the periodic table we have today. His elemetal set was the basis for the periodic table, though he did not know it at the time.

**Daloton' Elemental Set...**



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**//Sir William Crooke (1832-1919)//**

Country of Origin: England

Year of Discovery: 1861

Sir William Crooke was born in 1832 in England.He studied chemistry at Royal College of Chemistry and soon became an influential chemist and physicist.After graduating, he taught at Oxford University.

Crooke is mostly known for his discoveries surrounding this Crooke’s tube.The Crooke’s tube contained a current that moved through the tube in a similar fashion to light.He discovered that magnets could deflect it.In his years he was able to isolate a new element called thallium.His discoveries with the tube were important because the rays he found were negatively charged and were deflected by magnetic fields. With his Crooke's tube he showed that cathode rays traveled in straight lines and produced heat when the rays hit certain materials.

**Crooke's Tube...**



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**//Dmitri Mendeleev (1837-1907)//** Country of Origin: Siberia Year of Discovery: 1889

Dmitri Mendeleev was the last of fourteen children born in Siberia to a working family.As a young boy, his father went blind and his mother had to support the family.She moved the family to St. Petersburg and worked endlessly to get her youngest boy into college.After studying in St. Petersburg, he became a professor in 1866.

In 1889, Mendeleev formed his “law of periodicity” and found that “certain properties of the elements can be foretold from their atomic weights.”He is best known for his work with his periodic table and he arranged elements onto a table.At the time, only 63 elements had been discovered but he placed all of these onto a periodic table and arranged them according to their atomic masses.He was smart enough to leave spaces available for elements that had not yet been discovered, but he knew they were out there.His table did not contain any Noble Gasses, but it was arranged in a proper manner that could be expanded upon easily once they were discovered.

**Mendeleev's Periodic Table...**




 * Today's Periodic Table...**



Mendeleev was around during a time when only 63 elements had been discovered. He put these elements in the correct spots that still for the most part remain today. He categorized them in ways that are still used and the spaces he left open were eventually filled in. Although at the time he probably did not expect as many elements that have been discovered as of today, he still knew other elements existed and began with a very basic but important step with his periodic table.