In the 1800 s, John Dalton proposed the first atomictheory. Since then, scientists have learned much more aboutthe structure of an atom, and therefore the atomic theoryhas changed. Dalton had to have a basis for his ideas about atoms. The first law he based his ideas on was the law orconservation of mass. This theory was discovered by Frenchchemist Antoine Lavoisier. He was the first to corectlydemonstrate how something burns. The law basically saysthat atoms can t be destroyed or divided, they havedefinite mass, and the total mass should be the same afterand before a chemical reaction takes place.
Another theoryDalton based his works on was the law of definiteproportions. This law was discovered by another FrenchChemist, Joseph Proust. In 1799, he showed that teh;proportion of mass of elementsw in a given compouind willalways be identical. Dalton felt that with his atomictheory, together with these theorys, gave strong proof forthe exsistance of atoms.
Dalton s theory stated that all elements are made ofatoms, indestructible parts, the smallest parts of allatoms.
He also stated that all elemnts are exactly the sameand compounds are formed by joining together two or moreatoms. This first atomic theory, however, was not excactlycorrect. There are many ideas along the way that helpedform the new atomic theory as it is today. The first ideathat helped contradict Dalton s theory was provided by William Crookes. In the 1870 s, Crookes created a Crookestube, which pumps out high voltage which is applied to twoelectrodes. The opposite of the glass tube is turnedyellow.
If an object is placed in the middle of the tube,the shape of the object is on the other side of the tube. This suggested that there are particles smaller than atoms, because electricity is not an atom, and that s what producedthe yellow shadow at the end of the glass tube.
Another new law that caused the further contradictionof Dalton s theory was Rutherford s model of an atom. Rutherford s experiment showed clearly that every atom has apositively charged center. Rutherford first believed thatthe positively charged helium atoms were called alphaparticles. With further experimentation, he reliezed thatnot only did atoms have a positively charged center, but amostly empty exterior from the center (nucleus), that alsohad negativly charged rather small particles (electrons orbiting the nucleus. However, Rutherford s model did have flaws. Forinstance, in his model, if electrons orbited the nucleus, then the electrons would lose energy and spiral in towardsthe nucleus. Danish physicist Niels Bohr made improvementon faults like these in Rutherford s model. In Bohr smodel, electrons do not lose energy when orbiting andtherfore do not spiral in toward the nucleus. Another newfactor in Bohr s model is energy levels. Electrons orbitaround energy levels.
In each energy level, the fartheraway the level from the nucleus, the more energy anelectron. In Bohr s model, the only way an electron canloose energy is by dropping an energy level, or moving onemore energy level closer to the nucleus. At the lowestenergy level, electrons can not lose energy because they areat a ground state, they can not move any lower. Another important theory is the quantum theory. Hundreds of years ago, during the 1600 s, there was adebate on wether light travels in waves or in particles. Then, in 1864 the idea that light travels in waves was onceagain brought to attention. In the early 1900 s, Max Planckrevived the theory of particles. Planck did experimentsthat could only be explained if light traveled in particles, what he called quanta. How much energy each quanta haddepended on what color the light is. The quantum theorystates that quanta, the fundamental unit of light, travelsin discreet packets.