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Atoms and Elements

Atoms and Elements
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Dr.SamuelHunt,United Arab Emirates,Teacher
Published Date:21-07-2017
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Chapter 4 Atoms and Elements www.ThesisScientist.comExperiencing Atoms • Atoms are incredibly small • All of our senses, sense atoms www.ThesisScientist.comExperiencing Atoms • There are about 91 elements found in nature. Over 20 have been made in laboratories. • Each kind of atom is unique Carbon is not Hydrogen They have different properties Structure, magnetic meaning they can attract and repel other atoms, melting, boiling, electrical, stability, reactivity (attract and repel), etc… www.ThesisScientist.comThe Divisibility of Matter • Ultimate particle Upon division, eventually a particle is reached which can no longer be divided. -11 Atoms are 10 m -15 We detect particles at 10 m -35 In theory particles 10 m exist, we don’t have instruments that sensitive “Nothing exists except atoms and empty space; everything else is opinion.” - Democritus 460–370 B.C. www.ThesisScientist.comModern Evidence for Atoms IBM's Almaden Research Center in San Jose, California, April 1990 www.ThesisScientist.comSizes of Atoms • Using compositions of compounds and assumed formulas, Dalton was able to determine the relative masses of the atoms. Dalton based his scale on H = 1 amu. We now base it on C-12 = 12 amu exactly. Unit = atomic mass unit. Amu or dalton. • Absolute sizes of atoms: -24 Mass of H atom= 1.67 x 10 g. -25 3 Volume of H atom = 2.1 x 10 cm . www.ThesisScientist.comThe Atom Is Divisible • Work done by J. J. Thomson and others proved that the atom had pieces called electrons. • Thomson found that electrons are much smaller than atoms and carry a negative charge. th The mass of the electron is 1/1836 the mass of a hydrogen atom. The charge on the electron is the fundamental unit of charge that we call –1 charge unit. www.ThesisScientist.comPlum Pudding Atom • • • • •• • •• •• • • • •• • • • • • • www.ThesisScientist.comRutherford’s Experiment • How can you prove something is empty? • Put something through it. Use large target atoms. Use very thin sheets of target so they do not absorb ―bullet‖. Use very small particles as ―bullet‖ with very high energy. But not so small that electrons will effect it. • Bullet = alpha particles; target atoms = gold foil a particles have a mass of 4 amu & charge of +2 c.u.  Gold has a mass of 197 amu and is very malleable. www.ThesisScientist.comRutherford’s Experiment Alpha particles striking screen Radioactive sample Lead box Fluorescent screen Gold foil www.ThesisScientist.comPlum Pudding Atom • • If atom was like • • •• • a plum pudding, •• •• • all the a particles • • •• • should go • • • • straight through. • Very few of the a particles Nuclear Atom do not go through. . Most a particles go straight through. . . Some a particles go through, but are deflected. www.ThesisScientist.comRutherford’s Interpretation— The Nuclear Model 1. The atom contains a tiny dense center called the nucleus.  The amount of space taken by the nucleus is only about 1/10 trillionth the volume of the atom. 2. The nucleus has essentially the entire mass of the atom.  The electrons weigh so little they contribute practically no mass to the atom. 3. The nucleus is positively charged.  The amount of positive charge balances the negative charge of the electrons. 4. The electrons are dispersed in the empty space of the atom surrounding the nucleus.  Like water droplets in a cloud. www.ThesisScientist.comSome Problems • How could beryllium have 4 protons stuck together in the nucleus? Shouldn’t they repel each other? • If a beryllium atom has 4 protons, then it should weigh 4 amu, but it actually weighs 9.01 amu Where is the extra mass coming from? Each proton weighs 1 amu. Remember: The electron’s mass is only about 0.00055 amu and Be has only 4 electrons—it can’t account for the extra 5 amu of mass. www.ThesisScientist.comThere Must Be Something Else There • To answer these questions, Rutherford proposed that there was another particle in the nucleus—it is called a neutron. • Neutrons have no charge and a mass of 1 amu. The masses of the proton and neutron are both approximately 1 amu. www.ThesisScientist.comSubatomic Mass Mass Location Charge Symbol particle g amu in atom + + Proton 1.67262 1.0073 nucleus 1+ p, p , H -24 x 10 - Electron 0.00091 0.00055 empty space e, e 1 -24 x 10 0 Neutron 1.67493 1.0087 nucleus 0 n, n -24 x 10 www.ThesisScientist.comThe Modern Atom • We know atoms are composed of three main pieces—protons, neutrons, and electrons. • The nucleus contains protons and neutrons. -13 • The nucleus is only about 10 cm in diameter. • The electrons move outside the nucleus with an average distance -8 of about 10 cm.  Therefore, the radius of the atom is 5 about 10 times larger than the radius of the nucleus. www.ThesisScientist.comSome Notes on Charges • There are two kinds of charges, called positive and negative. • Opposite charges attract.  + attracted to –. • Like charges repel.  + repels +.  – repels –. • To be neutral, something must have no charge or equal amounts of opposite charges. www.ThesisScientist.comThe Nature of Electrical Charge • Electrical charge is a fundamental property of protons and electrons. • Positively and negatively charged objects attract each other. • Like charged objects repel each other. + to +, or  to . • When a proton and electron are paired, the result is a neutral charge. Because they have equal amounts of charge. www.ThesisScientist.comPractice—An Atom Has 20 Protons. Determine if Each of the Following Statements Is True or False? • If it is a neutral atom, it will have True 20 electrons. • If it also has 20 neutrons, its mass True will be approximately 40 amu. • If it has 18 electrons, it will have False a net 2 charge. www.ThesisScientist.comElements • Each element has a unique number of protons in its nucleus.  All carbon atoms have 6 protons in their nuclei. • The number of protons in the nucleus of an atom is called the atomic number.  Z is the short-hand designation for the atomic number.  Because each element’s atoms have a unique number of protons, each element can be identified by its atomic number.  The elements are arranged on the Periodic Table in order of their atomic numbers. • Each element has a unique name and symbol.  The symbol is either one or two letters One capital letter or one capital letter + one lower case letter. www.ThesisScientist.com