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Ch 2. Atoms and Elements Atom Nucleus (Rutherford’s experiment) Electrons (Thomson’s experiment)

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Presentation on theme: "Ch 2. Atoms and Elements Atom Nucleus (Rutherford’s experiment) Electrons (Thomson’s experiment)"— Presentation transcript:

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2 Ch 2. Atoms and Elements

3 Atom Nucleus (Rutherford’s experiment) Electrons (Thomson’s experiment)

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5 Atomic Nucleus

6 Nucleus carries positive charges Each electron carries one negative charge Nucleus carries almost all the mass of an atom

7 Atom Nucleus Electrons (each carries a negative charge) Protons (each carries a positive charge) Neutrons (neutral) number of electrons = number of protons = atomic number Atoms are neutral

8 Atoms that have the same number of protons belong to one kind of element.

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10 isotopes: same number of protons, different number of neutrons mass of a proton ≈ mass of a neutron >> mass of an electron number of protons + number of neutrons = mass number

11 X A Z Chemical symbol Mass number Atomic number or X-A

12 Practice – Complete the table 11 Tro: Chemistry: A Molecular Approach, 2/e

13 Practice – Complete the table 12 Tro: Chemistry: A Molecular Approach, 2/e

14 No change occurs inside a nucleus in chemistry Atoms can lose or gain electrons Na − e −  Na + positive ion = cation Cl + e −  Cl − negative ion = anion Mg − 2e −  Mg 2+ O + 2e −  O 2−

15 Practice – Complete the table 14 Tro: Chemistry: A Molecular Approach, 2/e

16 Practice – Complete the table 15 Tro: Chemistry: A Molecular Approach, 2/e

17 Symbol Number of Protons in Nucleus Number of Neutrons in Nucleus Number of Electrons Net charge 87 Rb −

18 Symbol Number of Protons in Nucleus Number of Neutrons in Nucleus Number of Electrons Net charge 87 Rb S 2− − 65 Cu

19 isotopes: same number of protons, different number of neutrons mass of a proton ≈ mass of a neutron >> mass of an electron number of protons + number of neutrons = mass number

20 1 H x 10 −24 g 16 O x 10 −23 g One atomic mass unit (amu) is defined as 1/12 of the mass of a 12 C atom. 12 C atom: 6 protons, 6 neutrons, 6 electrons. 1 amu = x 10 −24 g

21 1 H x 10 −24 g 16 O x 10 −23 g 1 amu = x 10 −24 g

22 21 Mass Spectrometer Tro: Chemistry: A Molecular Approach, 2/e

23 Mass Spectrum of Natural Copper % 63 Cu % 65 Cu natural abundance: percent of an isotope in nature

24 Mass Spectrum of Natural Copper % 63 Cu: amu % 65 Cu: amu Average atomic mass of Cu = ?

25 How to find the average? 1, 1, 1, 1, 2 Average = ( ) / 5 = 1.2

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27 Mass Spectrum of Natural Copper % 63 Cu: amu % 65 Cu: amu Average atomic mass of Cu = ? amu listed in the periodic table

28 35 Cl amu75.78 % 37 Cl amu24.22 % Average atomic mass of Cl = amu x % amu x % = amu listed in the periodic table

29 mass of a proton ≈ mass of a neutron ≈ 1 amu mass number of an isotope ≈ atomic mass of the isotope in amu 35 Cl amu75.78 % 37 Cl amu24.22 %

30 One atomic mass unit (amu) is defined as 1/12 of the mass of a 12 C atom. The number of carbon atoms in exactly 12 g of 12 C is called Avogadro’s number: x One Avogadro’s number of particles is called a mole. 1 mol = x particles 1 pair = 2 particles 1 dozen = 12 particles (exact number)

31 Example 2.6, page 67 Calculate the number of atoms in 2.45 mol of Cu. 1 mol = x particles

32 Practice 2.6, page 67 A pure Ag ring contains 2.80 x Ag atoms. How many moles of Ag does it contain? 1 mol = x particles

33 For an element X, its atomic mass is x amu. What is the mass of 1 mol of X in grams? The mass of 1 mol of X is x g. The molar mass of an element is the mass in grams per mole of the element. Unit: g/mol

34 Two ways to find molar mass 1) Read from the periodic table 2) Use the definition of molar mass: (recall d = m/V)

35 1 mol = x particles Unit: g/mol molar mass and Avogadro’s number are exact numbers

36 A piece of Cu has a mass of 200. g. How many copper atoms are present? 1 mol = x particles

37 A silicon chip has a mass of 5.68 mg. How many silicon atoms are present in the chip? 1 mol = x particles

38 Compute the mass in grams of a sample of six Americium atoms. 1 mol = x particles

39 Calculate the number of moles in a sample of cobalt (Co) containing 5.00 x atoms and the mass of the sample. 1 mol = x particles

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41 40 Mendeleev Ordered elements by atomic mass Saw a repeating pattern of properties Periodic Law – when the elements are arranged in order of increasing atomic mass, certain sets of properties recur periodically Put elements with similar properties in the same column Used pattern to predict properties of undiscovered elements Where atomic mass order did not fit other properties, he re-ordered by other properties –Te & I Tro: Chemistry: A Molecular Approach, 2/e

42 41 Periodic Pattern Tro: Chemistry: A Molecular Approach, 2/e

43 Periodic Patterns LiBeBCNOF NaMgAlSiPSCl KCa H NMH 2 O a/b H2H2 1.0 MLi 2 O b LiH6.9 MNa 2 O b NaH23.0 MK2ObK2Ob KH39.1 MBeO a/b BeH MMgO b MgH MCaO b CaH NMB2O3aB2O3a BH M Al 2 O 3 a/b AlH NMCO 2 a CH M/NM SiO 2 a SiH NMN2O5aN2O5a NH NMP 4 O 10 a PH NM H2SH2S 32.1 SO 3 a NM H2OH2O16.0 O2O2 NMCl 2 O 7 a HCl 35.5 NM HF19.0 a = acidic oxide, b = basic oxide, a/b = amphoteric oxide M = metal, NM = nonmetal, M/NM = metalloid 42 Tro: Chemistry: A Molecular Approach, 2/e

44 43 About ¾ of the elements are classified as metals. They have a reflective surface, conduct heat and electricity better than other elements, and are malleable and ductile Most of the remaining elements are classified as nonmetals. Their solids have a non-reflective surface, do not conduct heat and electricity well, and are brittle. A few elements are classified as metalloids. Their solids have some characteristics of metals and some of nonmetals. Tro: Chemistry: A Molecular Approach, 2/e

45 44 Metals Solids at room temperature, except Hg Reflective surface –shiny Conduct heat Conduct electricity Malleable –can be shaped Ductile –can be drawn or pulled into wires Lose electrons and form cations in reactions About 75% of the elements are metals Lower left on the table Tro: Chemistry: A Molecular Approach, 2/e

46 45 Nonmetals Found in all three states Poor conductors of heat Poor conductors of electricity Solids are brittle Gain electrons in reactions to become anions Upper right on the table –except H Sulfur, S(s) Bromine, Br 2 (l) Chlorine, Cl 2 (g) Tro: Chemistry: A Molecular Approach, 2/e

47 46 Metalloids Show some properties of metals and some of nonmetals Also known as semiconductors Properties of Silicon shiny conducts electricity does not conduct heat well brittle Tro: Chemistry: A Molecular Approach, 2/e

48 47 The Modern Periodic Table Elements with similar chemical and physical properties are in the same column Columns are called Groups or Families –designated by a number and letter at top Rows are called Periods Each period shows the pattern of properties repeated in the next period Tro: Chemistry: A Molecular Approach, 2/e

49 48 The Modern Periodic Table Main group = representative elements = “ A ” groups Transition elements = “ B ” groups –all metals Bottom rows = inner transition elements = rare earth elements –metals –really belong in Period 6 & 7 Tro: Chemistry: A Molecular Approach, 2/e

50 49 Tro: Chemistry: A Molecular Approach, 2/e

51 50 = Alkali metals = Alkali earth metals = Noble gases = Halogens = Lanthanides = Actinides = Transition metals Tro: Chemistry: A Molecular Approach, 2/e

52 51 Important Groups - Hydrogen Nonmetal Colorless, diatomic gas –very low melting point and density Reacts with nonmetals to form molecular compounds –HCl is acidic gas –H 2 O is a liquid Reacts with metals to form hydrides –metal hydrides react with water to form H 2 HX dissolves in water to form acids Tro: Chemistry: A Molecular Approach, 2/e

53 lithium sodium potassium rubidium cesium 52 Important Groups – Alkali Metals Group IA = Alkali Metals Hydrogen usually placed here, though it doesn ’ t really belong Soft, low melting points, low density Flame tests  Li = red, Na = yellow, K = violet Very reactive, never find uncombined in nature Tend to form water-soluble compounds, therefore salt is crystallized from seawater then molten salt is electrolyzed colorless solutions React with water to form basic (alkaline) solutions and H 2 2 Na + 2 H 2 O  2 NaOH + H 2 releases a lot of heat Tro: Chemistry: A Molecular Approach, 2/e

54 53 Important Groups – Alkali Earth Metals Group IIA = Alkali earth metals Harder, higher melting, and denser than alkali metals –Mg alloys used as structural materials Flame tests  Ca = red, Sr = red, Ba = green Reactive, but less than corresponding alkali metal Form stable, insoluble oxides from which they are normally extracted Oxides are basic = alkaline earth Reactivity with water to form H 2 –Be = none; Mg = steam; Ca, Sr, Ba = cold water Tro: Chemistry: A Molecular Approach, 2/e

55 54 Important Groups – Halogens Group VIIA = halogens Nonmetals F 2 and Cl 2 gases; Br 2 liquid; I 2 solid All diatomic Very reactive Cl 2, Br 2 react slowly with water Br 2 + H 2 O  HBr + HOBr React with metals to form ionic compounds HX all acids –HF weak < HCl < HBr < HI bromine iodine chlorine fluorine astatine Tro: Chemistry: A Molecular Approach, 2/e

56 55 Important Groups – Noble Gases Group VIIIA = Noble Gases All gases at room temperature –very low melting and boiling points Very unreactive, practically inert Very hard to remove electron from or give electron to Tro: Chemistry: A Molecular Approach, 2/e

57 56 Ion Charge and the Periodic Table The charge on an ion can often be determined from an element ’ s position on the Periodic Table Metals always form positively charged cations For many main group metals, the charge = the group number Nonmetals form negatively charged anions For nonmetals, the charge = the group number − 8 Tro: Chemistry: A Molecular Approach, 2/e

58 57 Tro: Chemistry: A Molecular Approach, 2/e

59 Practice – What is the charge on each of the following ions? 58 potassium cation sulfide anion calcium cation bromide anion aluminum cation K + S 2− Ca 2+ Br − Al 3+ Tro: Chemistry: A Molecular Approach, 2/e

60 Law of Conservation of Mass 59 Antoine Lavoisier Tro: Chemistry: A Molecular Approach, 2/e In a chemical reaction, matter is neither created nor destroyed Total mass of the materials you have before the reaction must equal the total mass of the materials you have at the end total mass of reactants = total mass of products

61 Reaction of Sodium with Chlorine to Make Sodium Chloride g Na g Cl 2  19.6 g NaCl Tro: Chemistry: A Molecular Approach, 2/e The mass of sodium and chlorine used is determined by the number of atoms that combine Because only whole atoms combine and atoms are not changed or destroyed in the process, the mass of sodium chloride made must equal the total mass of sodium and chlorine atoms that combine together

62 Law of Definite Proportions All samples of a given compound, regardless of their source or how they were prepared, have the same proportions of their constituent elements 61 Joseph Proust Tro: Chemistry: A Molecular Approach, 2/e

63 62 Proportions in Sodium Chloride A g sample of sodium chloride contains 39.3 g of sodium and 60.7 g of chlorine A g sample of sodium chloride contains 78.6 g of sodium and g of chlorine A g sample of sodium chloride contains g of sodium and g of chlorine Tro: Chemistry: A Molecular Approach, 2/e

64 Law of Multiple Proportions When two elements (call them A and B) form two different compounds, the masses of B that combine with 1 g of A can be expressed as a ratio of small, whole numbers 63 John Dalton Tro: Chemistry: A Molecular Approach, 2/e

65 Oxides of Carbon 64 Carbon combines with oxygen to form two different compounds, carbon monoxide and carbon dioxide Carbon monoxide contains 1.33 g of oxygen for every 1.00 g of carbon Carbon dioxide contains 2.67 g of oxygen for every 1.00 g of carbon Because there are twice as many oxygen atoms per carbon atom in carbon dioxide of in carbon monoxide, the oxygen mass ratio should be 2 Tro: Chemistry: A Molecular Approach, 2/e

66 Dalton ’ s Atomic Theory 65 Tro: Chemistry: A Molecular Approach, 2/e Dalton proposed a theory of matter based on it having ultimate, indivisible particles to explain these laws 1. Each element is composed of tiny, indestructible particles called atoms 2. All atoms of a given element have the same mass and other properties that distinguish them from atoms of other elements 3. Atoms combine in simple, whole-number ratios to form molecules of compounds 4. In a chemical reaction, atoms of one element cannot change into atoms of another element they simply rearrange the way they are attached

67 Practice – Decide if each statement is correct according to Dalton’s model of the atom Copper atoms can combine with zinc atoms to make gold atoms Water is composed of many identical molecules that have one oxygen atom and two hydrogen atoms 66 Tro: Chemistry: A Molecular Approach, 2/e

68 Practice – Decide if each statement is correct according to Dalton’s model of the atom Copper atoms can combine with zinc atoms to make gold atoms – incorrect; according to Dalton, atoms of one element cannot turn into atoms of another element by a chemical reaction. He knew this because if atoms could change it would change the total mass and violate the Law of Conservation of Mass. Water is composed of many identical molecules that have one oxygen atom and two hydrogen atoms – correct; according to Dalton, atoms combine together in compounds in small whole-number ratios, so that you could describe a compound by describing the number of atoms of each element in a molecule. He used this idea to explain why compounds obey the Law of Definite Proportions. 67 Tro: Chemistry: A Molecular Approach, 2/e

69 Practice – Decide if each statement is correct according to Dalton’s Model of the Atom Some carbon atoms weigh more than other carbon atoms Because the mass ratio of Fe:O in wüsite is 1.5 times larger than the Fe:O ratio in hematite, there must be 1.5 Fe atoms in a unit of wüsite and 1 Fe atom in a unit of hematite 68 Tro: Chemistry: A Molecular Approach, 2/e

70 Practice – Decide if each statement is correct according to Dalton’s model of the atom Some carbon atoms weigh more than other carbon atoms – incorrect; according to Dalton, all atoms of an element are identical. Because the mass ratio of Fe:O in wüsite is 1.5 times larger than the Fe:O ratio in hematite, there must be 1.5 Fe atoms in a unit of wüsite and 1 Fe atom in a unit of hematite – incorrect; according to Dalton, atoms must combine in small whole-number ratios. If you could combine fractions of atoms, that would mean the atom is breakable and Dalton’s first premise would be incorrect. You can get the Fe:Fe mass ratio to be 1.5 if the formula for wüsite is FeO and the formula for hematite is Fe 2 O Tro: Chemistry: A Molecular Approach, 2/e

71 Cathode Ray Tube 70 Tro: Chemistry: A Molecular Approach, 2/e Glass tube containing metal electrodes from which almost all the air has been evacuated When connected to a high voltage power supply, a glowing area is seen emanating from the cathode

72 J.J. Thomson Believed that the cathode ray was composed of tiny particles with an electrical charge Designed an experiment to demonstrate that there were particles by measuring the amount of force it takes to deflect their path a given amount –like measuring the amount of force it takes to make a car turn 71 Tro: Chemistry: A Molecular Approach, 2/e

73 72 Thomson ’ s Experiment Power Supply - + Cathode Anode Investigate the effect of placing an electric field around tube 1. charged matter is attracted to an electric field 2. light’s path is not deflected by an electric field (-) (+) Tro: Chemistry: A Molecular Approach, 2/e

74 73 Tro: Chemistry: A Molecular Approach, 2/e

75 Thomson ’ s Results The cathode rays are made of tiny particles These particles have a negative charge –because the beam always deflected toward the + plate The amount of deflection was related to two factors, the charge and mass of the particles Every material tested contained these same particles The charge:mass ratio of these particles was −1.76 x 10 8 C/g –the charge/mass of the hydrogen ion is x 10 4 C/g 74 Tro: Chemistry: A Molecular Approach, 2/e

76 Thomson’s Conclusions If the particle has the same amount of charge as a hydrogen ion, then it must have a mass almost 2000x smaller than hydrogen atoms! –later experiments by Millikan showed that the particle did have the same amount of charge as the hydrogen ion The only way for this to be true is if these particles were pieces of atoms –apparently, the atom is not unbreakable 75 Tro: Chemistry: A Molecular Approach, 2/e

77 76 Millikan ’ s Oil Drop Experiment Tro: Chemistry: A Molecular Approach, 2/e

78 Thomson’s Conclusions, cont’d Thomson believed that these particles were therefore the ultimate building blocks of matter –“We have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further... a state in which all matter... is of one and the same kind; this matter being the substance from which all the chemical elements are built up.” These cathode ray particles became known as electrons 77 Tro: Chemistry: A Molecular Approach, 2/e

79 78 Electrons Electrons are tiny, negatively charged particles found in all atoms Cathode rays are made of streams of electrons The electron has a charge of −1.60 x C The electron has a mass of 9.1 x 10 −28 g Tro: Chemistry: A Molecular Approach, 2/e

80 A New Theory of the Atom Because the atom is no longer indivisible, Thomson must propose a new model of the atom to replace the first statement in Dalton ’ s Atomic Theory –rest of Dalton ’ s theory still valid at this point Thomson proposes that instead of being a hard, marble-like unbreakable sphere, the way Dalton described it, the atom actually had an inner structure 79 Tro: Chemistry: A Molecular Approach, 2/e

81 80 Thomson ’ s Plum Pudding Atom The structure of the atom contains many negatively charged electrons These electrons are held in the atom by their attraction for a positively charged electric field within the atom –there had to be a source of positive charge because the atom is neutral –Thomson assumed there were no positively charged pieces because none showed up in the cathode ray experiment Tro: Chemistry: A Molecular Approach, 2/e

82 Predictions of the Plum Pudding Atom The mass of the atom is due to the mass of the electrons within it –electrons are the only particles in Plum Pudding atoms, therefore the only source of mass The atom is mostly empty space –should not have a bunch of negatively charged particles near each other as they would repel 81 Tro: Chemistry: A Molecular Approach, 2/e

83 82 Radioactivity In the late 1800s, Henri Becquerel and Marie Curie discovered that certain elements would constantly emit small, energetic particles and rays These energetic particles could penetrate matter Ernest Rutherford discovered that there were three different kinds of emissions –alpha, , rays made of particles with a mass 4x H atom and + charge –beta, , rays made of particles with a mass ~1/2000 th H atom and – charge –gamma, , rays that are energy rays, not particles Marie Curie Tro: Chemistry: A Molecular Approach, 2/e

84 Rutherford ’ s Experiment How can you prove something is empty space? Put something through it! –use large target atoms use very thin sheets of target so it will not absorb “bullet” –use very small particle as bullet with very high energy but not so small that electrons will affect it Bullet = alpha particles, target atoms = gold foil –  particles have a mass of 4 amu & charge of +2 c.u. – gold has a mass of 197 amu & is very malleable 83 Tro: Chemistry: A Molecular Approach, 2/e

85 84 Tro: Chemistry: A Molecular Approach, 2/e

86 Rutherford ’ s Results Over 98% of the  particles went straight through About 2% of the  particles went through but were deflected by large angles About 0.005% of the  particles bounced off the gold foil – “...as if you fired a 15 ” cannon shell at a piece of tissue paper and it came back and hit you. ” 85 Tro: Chemistry: A Molecular Approach, 2/e

87 Rutherford ’ s Conclusions Atom mostly empty space –because almost all the particles went straight through Atom contains a dense particle that is small in volume compared to the atom but large in mass –because of the few particles that bounced back This dense particle is positively charged –because of the large deflections of some of the particles 86 Tro: Chemistry: A Molecular Approach, 2/e

88 87... Nuclear Atom Plum Pudding Atom If atom was like a plum pudding, all the  particles should go straight through Almost all  particles go straight through Some  particles go through, but are deflected due to +:+ repulsion from the nucleus A few of the  particles do not go through Tro: Chemistry: A Molecular Approach, 2/e

89 88 Rutherford ’ 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 give 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 Tro: Chemistry: A Molecular Approach, 2/e

90 Structure of the Nucleus Rutherford proposed that the nucleus had a particle that had the same amount of charge as an electron but opposite sign – these particles are called protons –based on measurements of the nuclear charge of the elements protons are subatomic particles found in the nucleus with a charge = x C and a mass = x 10 −24 g Because protons and electrons have the same amount of charge, for the atom to be neutral there must be equal numbers of protons and electrons 89 Tro: Chemistry: A Molecular Approach, 2/e

91 Relative Mass and Charge It is sometimes easier to compare things to each other rather than to an outside standard When you do this, the scale of comparison is called a relative scale We generally talk about the size of charge on atoms by comparing it to the amount of charge on an electron, which we call −1 charge units –proton has a charge of +1 cu –protons and electrons have equal amounts of charge, but opposite signs We generally talk about the mass of atoms by comparing it to 1/12 th the mass of a carbon atom with 6 protons and 6 neutrons, which we call 1 atomic mass unit –protons have a mass of 1 amu –electrons have a mass of amu, which is generally too small to be relevant 90 Tro: Chemistry: A Molecular Approach, 2/e

92 Some Problems How could beryllium have four protons stuck together in the nucleus? –shouldn ’ t they repel each other? If a beryllium atom has four 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 amu and Be has only four electrons – it can ’ t account for the extra 5 amu of mass 91 Tro: Chemistry: A Molecular Approach, 2/e

93 There Must Be Something Else! To answer these questions, Rutherford and Chadwick proposed that there was another particle in the nucleus – it is called a neutron Neutrons are subatomic particles with a mass = x 10 −24 g and no charge, and are found in the nucleus 1 amu  slightly heavier than a proton no charge 92 Tro: Chemistry: A Molecular Approach, 2/e

94 93 Tro: Chemistry: A Molecular Approach, 2/e

95 Problems for Chapter , 75-78, 79-90


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