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ACP Chemistry 2011/12 Atoms and Atomic Theory The History of Atomic Theory Circa. 400-5 BC. Greek philosopher Democritus proposes the idea of matter being made up of small, indivisible particles (atomos). Late 18th Century. Lavoisier proposes the Law of conservation of mass and Proust proposes the Law of constant composition. Early 19th Century. Using the previously unconnected ideas above, John Dalton formulates his Atomic Theory. Dalton’s Atomic Theory (i) Elements are made from tiny particles called atoms. (ii) All atoms of a given element are identical. (N.B. see isotopes below). (iii) The atoms of a given element are different to those of any other element. (iv) Atoms of different elements combine to form compounds. A given compound always has the same relative numbers and types of atoms. (Law of constant composition). (v) Atoms cannot be created or destroyed in a chemical reaction they are simply rearranged to form new compounds. (Law of conservation of mass). Structure of the Atom and The Periodic Table Several experiments were being carried out in the 19th and 20th centuries that began to identify the subatomic particles that make up the atom. A summary of those experiments is given below. Scientist Experiment Knowledge gained Relating to Crookes Cathode Ray Tube Electron J.J. Thomson Cathode Ray Deflection Negative particles of some kind exist Mass/charge ratio of the electron determined Milikan Oil Drop Experiment Charge on the electron Electron Rutherford, Marsden, and Geiger Gold Foil Experiment Nucleus present in atom The nucleus of an atom and the proton Electron In the first part of the 20th Century, following Chadwick’s discovery of the neutron, Bohr proposed the idea that the atom was made up of the nucleus containing protons and neutrons that was being orbited by electrons in specific, allowed orbits. This particle model of the electron and atom was expanded a few years after Bohr’s original ideas to incorporate the wave nature of the electrons. PARTICLE CHARGE MASS POSITION in atom PROTON +1 1 amu* Nucleus NEUTRON 0 1 amu* Nucleus ELECTRON -1 1/1836 amu* Outside of the nucleus *atomic mass units 1/4 ACP Chemistry 2011/12 The atomic numbers (in the table below printed above the symbol and sometimes referred to as Z) and mass numbers (in the table below printed below the symbol and sometimes referred to as A) that appear on the periodic table have specific meanings. Atomic number the number of protons in the nucleus of one atom of the element Since all atoms are neutral it also tells us the number of electrons surrounding the nucleus. (N.B. When atoms lose or gain electrons they become charged and form ions). Mass number the number of protons the number of neutrons in one atom of the element Group 1A 2A THE PERIODIC TABLE 1 2 Period 3 Period 4 Period 5 Period 6 Period 7 4A Group 5A 6A 5 B 11 13 Al 27 31 Ga 70 49 In 115 81 Tl 204 6 C 12 14 Si 28 32 Ge 73 50 Sn 119 82 Pb 207 7 N 14 15 P 31 33 As 75 51 Sb 122 83 Bi 209 7A 1 H 1 Period Period 3A 3 Li 7 11 Na 23 19 K 39 37 Rb 86 55 Cs 133 87 Fr 223 4 Be 9 12 Mg 24 20 Ca 40 38 Sr 88 56 Ba 137 88 Ra 226 T R A 21 22 Sc Ti 45 48 39 40 Y Zr 89 91 57 72 La* Hf 139 178 89 104 Ac** Rf 226 58 Ce 140 90 Th 232 *Lanthanides **Actinides N S 23 V 51 41 Nb 93 73 Ta 181 105 Db 59 Pr 141 91 Pa 231 I T 24 Cr 52 42 Mo 96 74 W 184 106 Sg 60 Nd 144 92 U 238 I O 25 Mn 55 43 Tc 99 75 Re 186 107 Bh 61 Pm 147 93 Np 237 N 26 Fe 56 44 Ru 101 76 Os 190 108 Hs 62 Sm 150 94 Pu 242 M E T A L S 27 28 29 30 Co Ni Cu Zn 59 59 64 65 45 46 47 48 Rh Pd Ag Cd 103 106 108 112 77 78 79 80 Ir Pt Au Hg 192 195 197 201 109 110 111 Mt Ds Rg 63 Eu 152 95 Am 243 64 Gd 157 96 Cm 247 65 Tb 159 97 Bk 251 66 Tb 163 98 Cf 251 67 Ho 165 99 Es 254 68 Er 167 100 Fm 253 69 Tm 169 101 Md 256 8 O 16 16 S 32 34 Se 79 52 Te 128 84 Po 210 70 Yb 173 102 No 254 9 F 19 17 Cl 35.5 35 Br 80 53 I 127 85 At 210 71 Lu 175 103 Lr 257 KEY: Metal 13 Al 27 14 Si 28 Semi Metal Non-metal 15 P 31 In this example Al is a metal, Si is a semi-metal (metalloid) and P is a non-metal. 2/4 8A 2 He 4 10 Ne 20 18 Ar 40 36 Kr 84 54 Xe 131 86 Rn 222 ACP Chemistry 2011/12 Isotopes Atoms with the same number of protons and electrons, but different numbers of neutrons are called isotopes. All Atoms of the same element contain the same number of protons and electrons but may have different numbers of neutrons. Since it is the electrons in atoms that affect the chemical properties of a substance, isotopes of the same element have the same chemical properties. Practice: 1. Consider the following pairs; does either pair represent a pair of isotopes? (a) 40 19 K and 40 18 (b) 90 38 Sr and 2. Determine the number of protons, electrons and neutrons in, (a) 210 82 (b) 34 16 Ar 94 38 Sr Pb S On some periodic tables you will find atomic mass numbers that are not integers. What does this mean? A good starting point is to analyze what it does not mean. For example, the atomic mass of Cl is often quoted on a periodic table as 35.5 and can be represented by the following symbol; 35.5 17 Cl This does not mean that there are 17 protons, 17 electrons and 18.5 neutrons in an atom of chlorine. It is not possible to have a fraction of a neutron, there can only be a whole number of neutrons in an atom. So what does it mean, and where does the 0.5 come from? Here is the explanation. The non integer values mean that there is more than one isotope of chlorine that exists in nature, in this case 35Cl and 37Cl. A quick calculation will tell you that these two species have the same number of protons and electrons, but different (whole) numbers of neutrons (18 and 20 respectively). That is, they are isotopes of one another. These isotopes happen to exist naturally in the following abundance; 35 Cl 75% and 37Cl 25%. A simple calculation can be applied to work out the average atomic mass when considering all the isotopes present in a natural sample. Average atomic mass (% of each isotope)(atomic mass of each isotope) 100 3/4 ACP Chemistry 2011/12 In this case Average atomic mass = ((35)(75) ((37)(25)) 35.5 (A non-integer in this case) 100 Another example is provided by Boron. Boron has two isotopes 18.7% and 81.3% respectively. Average atomic mass = 10 B and 11B. They have the abundance ((10)(18.7) ((11)(81.3)) 10.8 (Again a non-ineger) 100 Practice: 1. The Noble gas, Neon has three isotopes of masses 22, 21 and 20. If the isotopes have the abundance 8.01%, 1.99% and 90.00% respectively, what is the average atomic mass of neon atoms? 2. A naturally occurring sample of an element consists of two isotopes, one of mass 85 and one of mass 87. The abundance of these isotopes is 71% and 29%. Calculate the average atomic mass of an atom of this element. 3. If 69Ga and 71Ga occur in the %’s 62.1 and 37.9, calculate the average atomic mass of gallium atoms. 4. Naturally occurring Chlorine molecules, Cl2 have masses of 70, 72 and 74. They occur in the percentages 56.25%, 37.50% and 6.250%. What is the average atomic mass of chlorine atoms? What is the relative abundance of 35Cl and 37Cl isotopes? 4/4