Chapter 5. Early Atomic Theory and Structure

Transcription

1 Chapter 5 Early Atomic Theory and Structure 1

2 5.1 Early thoughts ~ 400 B.C. Greek philosophers Empedocles all matter was composed of four elements earth, air, water, fire Democritus all forms of matter were divided into tiny individual particles atoms Aristotle opposed the theory of Democritus endorsed the Empedocles theory 2

3 5.2 Dalton s s model of the atom Dalton atomic model (1803 ~ 1810) 1. elements are composed of minute, indivisible particles called atoms (atoms are composed of subatomic particles) (atoms, under certain circumstances, can be decomposed) 2. atoms of the same element are alike in masses and sizes (not all the atoms of a specific element have the same mass) 3. atoms of different elements have different masses and sizes 4. compounds are formed by the union of two or more atoms of different elements 5. atoms combine to form compounds in simple numerical ratios 6. atoms of two elements may combine in different ratios to form more than one compound 3

4 5.3 Composition of compounds water hydrogen peroxide 11.2% H 5.9% H 88.8% O 94.1% O composition 2 H + 1 O 1 H + 1 O law of definite composition a compound always contains two or more elements combined in a definite proportion by mass law of multiple proportion atoms of two or more elements may combine in different ratios to produce more than one compound 4

5 5.4 The nature of electric charge 5.4 The nature of electric charge the properties of electric charge: 1. charge may be positive or negative 2. unlike charges attract, like charges repel 3. charge may be transferred from one object to another, by contact or induction 4. the less the distance between two charges, the greater the force of attraction (or repulsion) the force can be expressed kq 1 q 2 F = r 2 q 1,q 2 : charge, r: distance, k: constant 5

6 5.5 Discovery of ions Faraday certain substances conduct an electric current when dissolved in water ions Arrhenius ion is an atom carrying a positive or negative charge NaCl Na + + Cl - Na + ions move toward the cathode Cl - ions move toward the anode positive ion cation negative ion anion Stoney some fundamental unit of electricity associated with atoms electron Thomson discovery of electron 6

7 5.6 Subatomic parts of the atom the diameter of an atom 0.1 ~ 0.5 nm STM Cu atoms subatomic particles electrons, neutrons, protons 1875 Crookes tube generates cathode rays 1897 Thomson cathode rys 1. travel in straight lines 2. are negative in charge 3. are deflected by electric and magnetic fields 4. produce a sharp shadow 5. are capable of moving a small paddle wheel 7 discovery of electrons

8 electron (e - ) mass: g 1/1837 the mass of H atom amu (1 amu = g) electric charge of -1 diameter less than cm proton (p) first observed by Goldstein in 1886 mass ~1837 times of the mass of electron g ~ 1 amu Thomson model of atom the electrons and the protons (in equal number) are negatively charged particles embedded in the atomic sphere a neutral atom could become an ion by gaining or losing electrons 8

9 neutron (n) discovered by Chadwick in 1932 mass ~ 1 amu g slightly greater than proton ex. 5.1 mass of helium atom = g how many atoms in 4.0 g of helium? 4.0 g = atoms He g/atom 9

10 5.7 The nuclear atom 1896 Becquerel discovery of radioactivity emission of α, β, γ rays from nuclei of radioactive elements 1907 Rutherford α particles are He 2+ ions 1911 Rutherford α particle scattering experiment positively charged mass occupies a relatively tiny volume nucleus of the atom nucleus is very heavy and dense (density of nucleus ~10 12 g/cm 3 ) nucleus contains all the protons and neutrons 99.9% of the mass of the atom 10

11 general arrangement of subatomic particles each atom consists of a nucleus (containing protons and neutrons) surrounded by electrons a neutral atom contains the same number of protons and electrons atomic numbers of the elements atomic number = number of protons in the nucleus = number of electrons the atomic number determines the identity of an atom ex. atomic number = 1 H atom atomic number = 6 C atom atomic number = 92 U atom 11

12 5.8 Isotopes of the elements all atoms of a given element have the same number of protons (and electrons), but, may have different numbers of neutrons isotopes atoms of element having the same number of protons but different numbers of neutrons (different atomic masses) three isotopes of hydrogen isotopic notation mass number A ex. 12 C 13 C O 17 O 18 O atomic number Z 12

13 5.9 Atomic mass mass spectrometer instrument determining the masses of individual atoms 1 atomic mass unit (amu) defined as equal to exactly 1/12 of the mass of a 12 C atom 1 amu = g average atomic mass ex. Cu 13

14 isotope isotopic mass abundance average atomic mass (amu) (%) (amu) 63 Cu Cu ( )(0.6909) + ( )(0.3091) = amu number of neutrons = mass number atomic number ex. 5.2 how many protons, electrons, neutrons 14 in an atom of 6C? 6 protons 6 electrons 14 6 = 8 neutrons ex. 5.3 two chlorine isotopes: 37 17Cl 24.47% amu 35Cl 75.53% amu 17 average atomic mass? (0.2447) ( ) + (0.7553) ( ) 14 = amu amu