the first person to develop a theory that stated there was a limit to how small matter could be divided
the person who proved that Democritus was correct
What about Dalton's theory was not correct?
He said atoms were indivisible which we know now is not true
What did Democritus say about matter?
There is a certain point to which matter can be cut /reduced and then it cannot be cut. The smallest point to which matter can be reduced is the atom & it does not get any smaller than that.
What does Atomos mean?
Democritus' theory was ignored and forgotten for more than 2000 years. WHY?
At the time, the famous philosophers Aristotle and Plato had a more respected (but incorrect) theory.
Who's theory became one of the foundations of modern chemistry?
John Dalton's theory
Four parts of John Dalton's atomic theory
1. atoms of same elements are exactly alike
2. atoms of different elements are different
3.compounds are formed by the joining of 2 or more different elements
4 elements are made of atoms & are indivisible
father of modern chemistry
Four element theory
matter was made up of four elements:
earth, air, water and fire
Who promoted the four element theory
Plato and Aristotle
Who came up with the idea that the atom is made of even smaller particles?
English scientist J.J. Thomson
What was J.J. Thomson's model called?
the Plum Pudding Model
Why did Thomson compare his model to Plum Pudding ?
Atoms were made of a positively charged substance with negatively charged electrons scattered about "like raisins in a pudding"
How did JJ Thomson arrive at his big idea? What was the experiment?
He studied the passage of an electric current through a tube of gas. As the current passed through the gas, it gave off rays of negatively charged particles. Since the atoms of the gas were uncharged, he was confused by where the negative charges came from....hence atoms were made of smaller particles
What was Ernest Rutherford famous for?
His Gold Foil Experiment
Describe Rutherford's Gold Foil Experiment?
He fired a stream of tiny positively particles at a thin sheet of gold foil. Most of the positively charged "bullets" passed right through the gold atoms in the sheet of gold foil without changing course at all. Some or the positively charged bullets however did bounce away from the gold sheet as if they had hit something solid.. He knew that positively charges repel positive charges.
Rutherford's Gold Foil Experiment drawing
JJ Thomson Experiment
Cathode ray tube proved that atoms are made up of smaller parts.
Rutherford's Big Idea
Discovery of the nucleus--Atoms had a small dense positively charged center that repelled his positively charged "bullets" in the gold foil experiment. Atoms were mostly made up of open space. The Nucleus was tiny. The negatively charged particles were scattered outside the nucleus around the atom's edge.
What did scientist Niels Bohr propose?
In his model, he placed an electron in a specific energy level within the atom. Electrons move in definite orbits around the nucleus. Each energy level is located at certain distances from the nucleus.
diagram of the Bohr Model
Plum Pudding model diagram
Idea: electrons travel in specific energy levels
Big Idea: atoms have a nucleus and are mostly space
Big Idea: there is a limit to how small matter can be divided
Big Idea: all matter is made of four elements--air,earth, fire and water
Aristotle & Plato
Big Idea: 4 parts:
atoms of same elements are exactly alike
atoms of different elements are different
compounds are formed by the joining of atoms of 2 or more elements.
atoms are indivisible, cannot be cut.
Big Idea: atoms were indeed made up of smaller pieces thus disproving Dalton & Democritus idea that atoms cannot be cut (made smaller)