History of the atom….
Elements
• All of the material that we know of is made of only about 100 atoms • In their purest form we call these elements
Element’s Symbols
• All known atoms not only have names, but symbols • Often derived from Greek or Latin • The first letter of the symbol is always capitalized!!!
Atomic structure
– – – – – – excellent example of the scientific method experimental observations were made on the nature of matter a model was proposed to fit these observations and theory was developed to explain the model as available observations increased the model and the theory had to adjust to fit current theory is quantum theory but we’ll start a few steps before this
Greek Model
• • • Democritus (470-400 BC) philosophized what would happen if you kept dividing matter into smaller and smaller pieces. Was there a limited when matter could not be divided further? Democritus concluded that there was a limit, eventually the smallest particle would be reached Democritus called this particle an atomos
– The Greek word atomos means “indivisible” or “ not to be cut”
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The theory of atoms was mostly ignored for about 2100 years until…
John Dalton’s (1766-1844)Model
• • • 1803, English scientist Dalton began to experiment with Democritus’ ideas in mind. He studied gases. He discovered that gases behave as if there were made of individual particles. He came up with his atomic theory, the basic ideas are :
– – – All elements are composed of atoms, atoms are indivisible and indestructible Atoms of the same element are exactly alike/atoms of different elements are different Chemical reactions involve the combination of atoms, not the destruction of atoms. Compounds are formed by the joining of two or more atoms
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Thompson theorized that atoms were made of even smaller particles Thompson passed an electric current through gas, and the gas gave off negatively charged rays, since the gas itself was neutral, Thompson hypothesized that the atoms must be made of smaller particles that had charges. Thompson’s theory
– the atom is made of a puddinglike positively charged material throughout which negatively charged electrons are scattered, like plum pudding
J.J. Thompson’s model
Ernest Rutherford’s (1871-1937) model
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He fired a stream of positively charged particles through a thin sheet of gold foil. Most of the particle passed right through the sheet He hypothesized that atoms were mostly empty space Rutherford’s theory
– All of an atom’s positively charged particles were contained in the nucleus (the center) of the atom. And the negatively charged particles are scattered near the edge of the atom.
Niels Bohr’s (1885-1962) Model
• • Improved on Rutherford’s model Theory
– Electrons move in definite orbits around the nucleus, like planets around the sun. These orbits or energy levels are located at certain distances from the nucleus. 2 on the 1st, 8 on the 2nd… etc
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The atom has a small positively charged nucleus, surround by regions where there are enough electrons to make the atom neutral. These regions are called orbitals.
» The number of protons= the number of electrons
Quantum or wave model
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The mass number = the number of protons or electrons + the number of neutrons
Homework
• Read pages 47-58, problems 1, 6, 7, 10, 12 on pages 80 &81