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A T O M - Coggle Diagram
A T O M
Development
Joseph Priestly, Antoine Lavoisier, and others set the stage for the foundation of chemistry. They demonstrated that substances could combine to form new materials.
It was the English chemist, John Dalton, who put the pieces of the puzzle together and developed an atomic theory in 1803. He held that all the atoms of an element are of exactly the same size and weight.
The Greeks preferred to believe in Plato and Aristotle's idea of four elements - that everything was made from earth, fire, water, and air.
The atomic theory, which holds that matter is composed of tiny, indivisible particles in constant motion, was proposed by the Greek philosophers Leucippus and Democritus.
. A major development was the periodic table, devised simultaneously by Dmitri Mendeleev and J. L. Meyer, which arranged atoms of different elements in order of increasing atomic weight so that elements with similar chemical properties fell into groups.
Models
Physicist Ernest Rutherford proposed the nuclear model of the atom, with electrons orbiting around a massive nucleus, and as mostly empty space, with the nucleus occupying only a very small part of the atom. Rutherford's gold foil experiment showed that the atom is mostly empty space with a tiny, dense, positively-charged nucleus.
The Bohr Model is a planetary model in which the negatively charged electrons orbit a small, positively charged nucleus similar to the planets orbiting the sun (except that the orbits are not planar).
J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. Thomson proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged "soup."
James Chadwick played a vital role in the atomic theory, as he discovered the neutron in atoms. Neutrons are located in the center of an atom, in the nucleus along with the protons. They have neither a positive nor negative charge, but contribute the the atomic weight with the same effect as a proton.
Erwin Schrödinger proposed the quantum mechanical model of the atom, which treats electrons as matter waves. Electrons have an intrinsic property called spin, and an electron can have one of two possible spin values: spin-up or spin-down. Any two electrons occupying the same orbital must have opposite spins.
John Dalton's model explained that atoms were tiny invisibly, indestructible particles; And that each had a certain mass, size, and chemical behavior that determined by what kind of element they were. He called his model the billiard ball model because he thought that atoms looked like billiard balls from pool.
Werner Heisenberg contributed to atomic theory through formulating quantum mechanics in terms of matrices and in discovering the uncertainty principle, which states that a particle's position and momentum cannot both be known exactly.
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Structure
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Protons and neutrons have approximately the same mass, about 1.67 × 10-24 grams, which scientists define as one atomic mass unit (amu) or one Dalton.
An atom is composed of two regions: the nucleus, which is in the center of the atom and contains protons and neutrons, and the outer region of the atom, which holds its electrons in orbit around the nucleus.
Structure
The number of neutrons is variable, resulting in isotopes, which are different forms of the same atom that vary only in the number of neutrons they possess.
Together, the number of protons and the number of neutrons determine an element’s mass number.
The number of protons determines an element’s atomic number and is used to distinguish one element from another.
