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450
Aristotle
Around 460 BC, another scientist named Aristotle came up with the idea of all substances being made out of four elements. These elements were fire, wind, earth, and water. If the element was hot it was made of fire and if it was wet it would be made of water, etc. Aristotle's theory was more apealing to people because they understood it better then Deomcritus'. His idea was followed for many years after. -
450
Democritus
Democritus came up with the idea of "cutting" any substance until it is no longer possble in 460 BC. He called this idea "Atomos", which means, indivisible. The theory states that atoms are all individually created and can't be seperated. Also, the atoms differ in shape and size only. The way an object looks is depended on the way atoms are arranged. This idea was not adopted by people in 460 BC because Aristotle's idea made more sense. -
John Dalton
John Dalton is known as the "Father of Atomic Theory." His theory stated: All elements are made up of tiny indivisible particles, atoms. Atoms of the same element are identical.
Atoms of different elements are all different from each other
Atoms can be combined/changed in chemical reactions but can't be destroyed. Atoms of 1 element can be combined with atoms of another to create compounds like (H2O). Dalton's theory explained many unexplained chemical phenominas. -
Becquerel (cont.)
still clear and strong. This proved that uranium made radiation without an external source of energy, like the sun. -
Henri Becquerel
Henri Becquerel discovered radioactivity, the emission of radiation in an atom. He did an expierement using flouresent materials, uranium and potassium. He exposed the materials to sunlight and placed them on photographic plates wrapped in black paper. When they developed, the plates showed a picture of uranium crystals. At first, he thought energy from sunlight was being absorbed and producing x-rays. A few days later he got the pictures from a closed drawer and realized the images were -
J.J. Thomson
J.J. Thomson discovered the electron in 1897 and proved his theory using the cathode ray tube expieriment. He put one positive and one negatively charged plate on the paths of the tube and noticed the particles bent towards the positively charged plate. He showed that atoms were made of smaller particles and weren't invisible. He thought the atom was a sphere with positive particles and electrons surronded them in electrostatic forces. -
Marie & Pierre Currie
Marie & Pierre Curie discovered two new radioactive elements and did alot of expierements on radioactivity in 1896. Which helped shape the understanding of radioactive atoms and their properties. -
Quantum model
The quantum model was created in the early 1900's. It's based on other scientist's proven theories and mathematics. It also has to do with the quantum theory, which says matter has properties associated with waves and it’s impossible to know the exact position and momentum of an electron at the same time. Basically, the theory gives the location of an electron as a probability instead of an exact. This model is the model we use for an atom today. -
Robert Millikan
Robert Millikan discovered the unit charge of the electron with his Oil Drop experiment. He sprayed tiny oil droplets into a small, charged, hole that led to a closed area. Since the droplets now had a charge on them, he created an electric field around them. The electric field balanced the force of gravity on the drops, and they hung in mid-air. His expierement discovered that electrons had a negative charge. -
Ernest Rutherford
Ernest Rutherford overturned Thomson's electric cloud model in 1911 with his gold foil experiment. This expierement demonstrated the atom with a tiny, dense nucleus. A thin piece of gold foil was hit with a beam of fast moving particles. He expected the raysto go straight through, but they bounced into different directions. This proved: most of the space within the atoms is empty because some rays went through. Some were deflected because of a positivley charged part in the atom, the nucleus. -
Niels Bohr
Niels Bohr created a planetary model that showed the negatively-charged electrons orbit a small, positively-charged nucleus like planets orbit the Sun. The gravitational pull of the solar system is the same as the electrical force between the nucleus and the electrons. He found this by using the quantom theory, showing when an atom absorbs or gives off energy, the electron jumps to higher or lower orbits. -
James Chadwick
in 1932 James Chadwick discovered the nuetron. Scientists kept realizing that the atomic number of an element was less then its atomic mass. Knowing this, Chadwick did an expierement tracking particle radiation in hopes to find a particle with the same mass as a proton, but no charge. His expierements were successful and he discovered that a nuetron has .1% more mass then a proton. James Chadwick's research on the nuetron increased the understanding of the structure of the nucleus.
