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The Atom and Atomic Structure 1. Historical Ideas 2. The Structure of the Atom 3. The Size of Atoms 4. Periodic Table ©2013 Robert Chuckrow 1. Historical Ideas Ancient-Greek philosophers reasoned that if an element such as gold were cut in half repeatedly, at a certain point, something indivisible would be reached. In 1805, John Dalton called these particles atoms, from the Greek word atmos, meaning indivisible. 2. The Structure of the Atom Scientists now accept as a fact that ordinary matter consists of atoms. We know that atoms consist of protons, neutrons, and electrons plus other subatomic, elementary particles (positrons and mesons), which will not be discussed here. Protons are positively charged, electrons are negatively charged, and neutrons are neutral. The mass of a proton is approximately equal to that of a neutron and about 1,845 times that of an electron. Rutherford’s Experiment in 1911 Before about 100 years ago, it was not known how protons, electron and neutrons are arranged within the atom. Rutherford’s experiment (next slide) showed that atoms contain very small, heavy atomic nuclei. He used a radioactive source that emitted alpha particles (helium nucleii consisting of two neutrons and two protons). He then measured how much the alpha particles were deflected after passing through a very thin gold foil. Rutherford’s Apparatus From Linus Pauling, College Chemistry, W.H. Freeman and Co., San Fancisco, CA, 1954, p. 36. Conclusions Drawn from Rutherford’s Experiment The fact that most of the alpha particles went straight through the foil without being deflected showed that the atoms consisted mostly of empty space. That a small percentage of alpha particles were deflected through large angles showed that the nuclei were very dense. In fact, the atomic nuclei occupy only one million-millionth of that of the whole atom. If an atom were the size of a bushel basket, the nucleus would be only about a thousandth of an inch in diameter. Review Questions 1. 2. 3. What is an alpha particle? Why did Rutherford use gold foil? If the thickness of the foil were doubled, what effect would that have on the results? Answers 1. 2. 2. An alpha particle consists of two neutrons and two protons and is a helium nucleus. Gold foil can be rolled into extremely thin sheets. The thinnest foil Rutherford used was only 1,000 atoms thick. Doubling the thickness of the foil resulted in a doubling of the fraction of alpha particles being deflected into large angles. 4. The Size of Atoms Atoms are very small. Guess how many atoms there are in an ounce of lead: A) eighty million B) eighty billion C) eighty-thousand billion D) eighty-thousand million-million Answer The correct answer is (D). There are eighty-thousand million-million atoms in an ounce of lead. Model of a Carbon Atom The Periodic Table It began to be realized that the properties of chemical elements are not arbitrary but depend on the structure of the atom and vary systematically. After much experimentation, it was possible to create a systematic arrangement of the elements into a table called the periodic table. Current Periodic Table http://www.ptable.com