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Chapter 2 Atoms and Elements AP CHEMISTRY Copyright 2011 Pearson Education, Inc. Dalton’s Atomic Theory 1. Each element is composed of tiny, indestructible 2. 3. 4. particles called atoms All atoms of an element have the same mass and properties that distinguish them from atoms of other elements Atoms combine in simple, whole-number ratios to form molecules In a chemical reaction, atoms of one element cannot change into atoms of another element, they simply rearrange the way they are attached to each other 2 Decide which statement is correct according to Dalton’s model of the atom 1. Copper atoms can combine with 2. 3. 4. 3 zinc atoms to make gold atoms Bulk water is composed of many identical molecules, each having one oxygen atom and two hydrogen 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 Some carbon atoms weigh more than other carbon atoms 0% 1 0% 0% 2 3 0% 4 The Statements According to Dalton’s Theory are: 4 1. Copper atoms can combine with zinc atoms to make gold atoms incorrect; atoms of one element cannot turn into atoms of another element by a chemical reaction 2. Bulk water is composed of many identical molecules, each having one oxygen atom and two hydrogen atoms correct; 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. 3. Some carbon atoms weigh more than other carbon atoms incorrect; all atoms of the same element weigh the same 4. 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; atoms must combine in small whole-number ratios. 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 Fe2O3 Charge • • • • Two kinds of charge exist in nature, + and – Two opposite charges attract, + attracts – Two like charges repel, + repels +, – repels – To be neutral, either no charge is present, or equal amounts of + and – charges are present 5 Cathode Ray Tube (CRT) • An evacuated glass tube with metal electrodes connected to a high voltage power supply • A glowing ray emanates from the cathode and terminates at the anode 6 What is the ray of a cathode ray tube? light or charged particles • Thomson believed that the cathode ray was composed of tiny electrically charged particles He designed an experiment to demonstrate this by measuring the amount of force it took to deflect the particles path a given amount A force was exerted by placing a charged electric field around the CRT 1. charged matter is attracted to an electric field 2. light’s path is not deflected by an electric field 7 Thomson’s Experiment +++++++++++ Cathode Anode (+) (-) ------------- - Power Supply 8 + Thomson’s Results • the beam was deflected toward the + plate, Cathode rays are made of tiny negatively charged particles, not light • Every material tested had these same particles • The amount of deflection is related to two factors, the charge and mass of the particles The charge to mass ratio of these particles is −1.76 x 108 C/g (high charge/mass ratio) Compared to the charge to mass of the hydrogen ion, which is +9.58 x 104 C/g (lower charge/mass ratio) 9 Thomson’s Conclusions • Since atoms are neutral as elements, their + • • and – charges have to cancel If the – particle has the same amount of charge as a + hydrogen ion, then the – particle must have a mass almost 2000 times smaller than a hydrogen ion! The only way for this to be true is if these particles were pieces of atoms apparently, the atom is not unbreakable 10 Thomson’s Conclusions • 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.” • The cathode ray particles became known as electrons tiny, negatively charged particles found in all atoms 11 Millikan’s Oil Drop Experiment pinhole (X-rays) 12 Millikan’s oil droplet experiment • An oil droplet that falls into the electric field becomes negatively charged by the stream of ionizing radiation When the charge on the drop (in coulombs) / oil drop mass (g) = acceleration due to gravity / the applied electric field, the droplet stops falling q/m = g/E • The oil drop mass = oil density x oil drop volume This was used to calculate the C/g ratio. a whole # of electrons was assumed every drop had a C/g ratio that reduced to a charge of -1.60 x 1019 C, the charge of the electron the electron has a mass of 9.1 x 10-28 g electrons are particles found in all atoms 13 A New Theory of the Atom • Because of these experiments, the atom was no • longer believed to be indivisible Thomson proposed a new atomic model that replaced Dalton’s premise that each element is composed of tiny, indestructible particles (atoms) The rest of Dalton’s theory was still valid at this point • Thomson proposed that instead of being a hard, marble-like unbreakable sphere, the way Dalton described it, the atom actually had an inner structure 16 J.J. Thomson’s Plum Pudding Model • Atoms contain many negatively charged electrons held in the atom by their attraction for the atom’s positively charged electric field The mass of the atom is due to the mass of the electrons Thomson assumed there were no positively charged pieces in the atom, because none showed up in the cathode ray experiment the negatively charged particles should not be near each other because they would repel • Atoms are mostly empty space 17 Discovery of Radioactivity Marie Curie 1867-1934 • Henri Becquerel and Marie Curie discovered that certain elements constantly emit small, energetic particles and rays that could penetrate matter • Ernest Rutherford discovered that there were three different kinds of emissions alpha (a) particles with a mass 4x that of the H atom and a positive (+) charge beta (b) particles with a mass ~1/2000x that of the H atom and a negative (–) charge gamma (g) rays that are energy rays, not particles 18 Rutherford’s Experiment to determine if an atom is mostly empty space Devised a method to shoot alpha particles through atoms to see the result • a particles have a mass of 4 amu & a charge of +2 c.u. large compared to electrons, so electrons will not deflect them • used large target atoms of gold, mass = 197 amu used very thin sheets of gold so the “bullet” would not be absorbed 19 20 Rutherford’s Experimental Results and Conclusions • > 98% of the a particles went straight through Atoms are mostly empty space because >98% of the particles went straight through • ~ 2% of the a particles went through the foil but were deflected by large angles The dense particle is positively charged which explains the large deflections of ~2% of the particles • ~ 0.005% of the a particles bounced off the foil Atom contain a dense particle that is small in volume compared to the atom but large in mass because ~ 0.005% of the particles bounced back 21 Plum Pudding Atom • • • • • • • • • • A few of the a particles do not go through • • • • • • • • • • • • If atom was like a plum pudding, all the a particles should go straight through Nuclear Atom . . . Almost all a particles go straight through Some a particles go through, but are deflected due to +:+ repulsion from the nucleus 22 Rutherford’s Interpretation The Nuclear Model 1. Atoms contain a tiny dense center, the nucleus 2. The nucleus contains the atom’s entire mass the electron’s comparative weight/mass is negligible 3. The nucleus is positively charged and it balances 4. the negative charges of the electrons The electrons are dispersed in the empty space of the atom surrounding the nucleus 23 Drop a pea into any large sports stadium. The pea is to the nucleus as the stadium is to the atom. the scale of comparison is called a relative scale Structure of the Nucleus • Rutherford proposed that the nucleus had a particle • that had the same amount of charge as an electron, but opposite in sign, called protons Protons are found in the nucleus and have a charge=+1.60 x1019 C and mass=1.67262 x 10−24 g Each proton has a charge of +1 charge units (cu) protons and electrons have equal amounts of charge, but opposite in sign, therefore atoms must have equal numbers of protons and electrons to be neutral 25 Something is Missing • How can an atom’s nucleus have multiple protons with + charges placed in close proximity? they should repel each other • Beryllium atoms have four protons it should weigh 4 amu; but it actually weighs 9.01 amu the protons account for only 4 amu Be has four electrons which weigh 4x(~0.00055 amu), far less than the extra 5 amu needed for mass balance Where is the extra mass coming from? 26 Proposal to Explain the Difference • Rutherford and Chadwick proposed that there • • • was another particle in the nucleus – called a Neutron Neutrons are subatomic particles with a mass = 1.67493 x 10−24 g 1 amu (slightly heavier than a proton) They have no charge They are found in the nucleus 27 Atomic Mass Units • We take 1/12th the mass of the carbon atom (6 protons and 6 neutrons), and call it 1 atomic mass unit (amu) protons and neutrons have a mass of just over 1 amu electrons have a mass of 0.00055 amu too small to be relevant to the total mass of the atom 28 29 Why Does Matter Appear Continuous If the Atom Is Mostly Empty Space? The emptiness of the atom is on such a small scale that the variations in density cannot be seen Consider a ladder framework that is mostly empty (right). This framework, if large enough, will look solid from a distance. Also, it has some rigidity based on the interaction of adjacent latices. 30 Elements • Each element has a unique number of protons in its nucleus The number of protons in the nucleus is called the atomic number, “Z” The number of protons defines the element • All the elements are arranged on the Periodic Table in order of their atomic numbers Each element has a unique name and symbol Each symbol is either one or two letters – one capital letter alone, or – one capital letter followed by one lowercase letter 31 Some symbols come from the element ‘s name, like C for carbon. Others come from the Latin name, like gold (Au-aurum) and copper (Cu-cuprium), or the Greek, like chlorine (Cl-chloros) and argon (Ar-argos). The Periodic Table of the Elements The atomic number tells you how many protons are in the nucleus and how many electrons are in the atom 32 The Nucleus • Atomic number (Z) = # of protons “Z” does not change for each element Does change from element to element • Mass Number (A) = # protons + # neutrons “A” does change for elements and their isotopes • Both Z and A are whole numbers 33 Isotopes • It was discovered that the same element could have atoms with different masses called isotopes • All isotopes of an element have the same number • of protons and electrons All isotopes of an element are chemically identical undergo the exact same chemical reactions • Isotopes of an element have different masses Due to different numbers of neutrons Isotopes are identified by their mass numbers, the sum of all the protons and neutrons in the nucleus 34 Isotopes • The observed atomic mass of any element is a weighted average of the weights of all the naturally occurring isotopes there are two isotopes of chlorine found in nature, one that has a mass of about 35 amu and another that weighs about 37 amu the percentage of each isotope is called the isotope’s natural abundance the observed atomic mass of chlorine is 35.45 amu 35 Neon Symbol Number of Number of A, Mass Protons Neutrons Number Percent Natural Abundance Ne-20 or 20 10Ne 10 10 20 90.48% 21Ne Ne-21 or 10 10 11 21 0.27% 22Ne Ne-22 or 10 10 12 22 9.25% 36 Which is not an example of isotopes? 1. 2. 3. 4. Z=1, N=1 and Z=1, N=2 Z=9, N=9 and Z=8, N=9 Z=9, N=9 and Z=9, N=10 Z=1, N=3 and Z=1, N=2 0% 37 1 0% 2 0% 3 0% 4 Example 2.3b: How many protons, electrons, 52 Cr and neutrons are in an atom of 24 ? Given: Find: Conceptual Plan: 52 24 Cr therefore A = 52, Z = 24 # protons, # electrons, # neutrons atomic number symbol symbol atomic number & mass number # p+ # e− # n0 in neutral atom, # p+ = # eRelationships: mass number = # protons + # neutrons 0 A = Z + # n + − Solution: Z = 24 = # p = # e 52 = 24 + # n0 52-24 = # n0 28 = # n0 Check: for most stable isotopes, n0 ≥ p+ 38 Complete the table and add all the numbers in the Electrons column. They add up to ????? 27 13 Al 1. 2. 3. 4. 122 116 106 98 0% 39 1 0% 0% 0% 2 3 4 Complete the table 13 6C 96 42 Mo 27 13 Al 133 55 Cs 116 40 Reacting Atoms • Elements that undergo chemical reactions do not turn into other elements All the atoms present at the start will be there at the end The number of protons determines the element the number of protons don’t change in a chemical reaction • Reactions involve sharing or transfer of electrons • When atoms gain or lose electrons, they acquire a charge to become ions Atoms that gain electrons become negatively charged anions Atoms that lose electrons become positively charged cations 41 Ions and Compounds • Ions behave differently than neutral atoms metallic sodium, made of neutral Na atoms, is highly reactive and quite unstable sodium cations, Na+, are found in table salt and are very nonreactive and stable • Because materials such as table salt are neutral overall, there must be a balance between the charges of cations and anions 42 Atomic Structures of Ions • Nonmetals form anions • For each negative charge, the ion has one more electron than the neutral atom F = 9 p+ and 9 e− F− = 9 p+ and 10 e− P = 15 p+ and 15 e− P3− = 15 p+ and 18 e− Anions are named by changing the ending of the element’s name to -ide fluorine oxygen F + 1e− F− O + 2e− O2− 43 fluoride ion oxide ion Atomic Structures of Ions • Metals form cations • For each positive charge, the ion has one less electron than the neutral atom Na atom = 11 p+ and 11 e− Na+ ion = 11 p+ and 10 e− Ca atom = 20 p+ and 20 e− Ca2+ ion = 20 p+ and 18 e− • Cations are named the same as the elemental metal sodium calcium Na Na+ + 1e− sodium ion Ca Ca2+ + 2e− calcium ion 44 Complete the table, add the ion charge column, and provide your answer. A. B. C. D. E. +2 -2 0 +3 -3 Al 3 0% 45 A. 0% 0% B. C. 0% 0% D. E. Complete the table S 2 Mg 2 Al 3 Br 46 Mendeleev • Periodic Law – Saw a repeating pattern of properties when the elements are arranged in order of increasing atomic mass • Ordered elements in rows by increasing atomic mass from left to right • Put elements with similar properties in the same column, with heavier mass lower in the column • Used pattern to predict properties of undiscovered elements 47 Periodic Pattern 48 Most About A fewofelements ¾the of remaining the elements are classified elements are classified asare metalloids. classified as metals. as nonmetals. They have Their solidsaTheir have reflective solids somesurface, characteristics have a non-reflective conductofheat metals and surface, electricity and some dobetter of not nonmetals. conduct than other heatelements, and electricity and are well, and malleable are brittle. and ductile 49 Metals • All metals are solids at room temperature except Hg which is liquid • • • • • • • Have reflective, shiny surface Conduct heat & electricity Are malleable (can be shaped) Are ductile (can be drawn or pulled into wires) Lose electrons to form cations in reactions Comprise ~75% of the elements Found on the lower left of the periodic table 50 Nonmetals Sulfur, S(s) • Found in all three states • Poor conductors of heat & • • • electricity Are brittle Gain electrons in reactions to become anions Found in upper right quadrant of the Periodic table Hydrogen, a nonmetal) is an exception 51 Bromine, Br2(l) Chlorine, Cl2(g) 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 52 The Modern Periodic Table • Columns are called Groups Elements with similar chemical and physical properties are in the same group designated by a number and letter at top • Rows are called Periods Each period shows a pattern of properties repeated in the period above or below 53 The Modern Periodic Table • Main group elements have letter “A” designation • Transition elements have letter “B” designation All transition elements are metals • The 2 bottom rows are called inner transition elements aka rare earth elements are metals really belong in Period 6 & 7 54 55 = Alkali metals = Halogens = Alkali earth metals = Lanthanides = Noble gases = Actinides = Transition metals 56 Hydrogen • Nonmetal • Colorless, diatomic gas very low melting point, boilng point and density • Reacts with nonmetals to form molecular compounds Reacts with Cl2 to form HCl, an acidic gas HCl dissolves in water to form acids Reacts with O2 to form H2O, a liquid • Reacts with metals to form hydrides metal hydrides react with water to form H2 • Placed in Group IA but based on properties, it does not belong there 57 Group IA = Alkali Metals • • • • Low density Low melting point Soft Very reactive lithium never found in elemental form • React with water to form basic sodium (alkaline) solutions and H2 2 Na(s) + 2 H2O(l) 2 NaOH(aq) + H2(g) releases a lot of heat • Tend to form water-soluble salts • Salts isolated from seawater are • melted and electrolyzed to form elemental form Flame tests Li = red, Na = yellow, K = violet 58 potassium rubidium cesium Group IIA = Alkali Earth Metals • Harder, higher melting, and denser metals than • • alkali metals Reactive, but less reactive than alkali metals Form stable, insoluble oxides Metals are extracted from the oxides • Oxides are basic (alkaline earth) • Like IA, IIA metals react with water to form H2 Be = no rxn (NR); Mg = rxn with steam; Ca, Sr, Ba = rxn with cold water “rxn” abbreviation for reaction • Flame tests Ca = red, Sr = red, Ba = green 59 Group VIIA = Halogens • Nonmetals Very reactive React with metals to form ionic compounds Cl2, Br2 react slowly with water Br2 + H2O HBr + HOBr • All diatomic (X2) fluorine chlorine bromine F2 and Cl2 are gases Br2 liquid I2 solid iodine • HX all acids astatine HF weak < HCl strong< HBr < HI 60 Group VIIIA = Noble Gases • All gases at room temperature very low melting and boiling points • Very unreactive, practically inert • Very hard to remove an electron from or give an electron to a noble gas 61 Ion Charge and the Periodic Table • The charge on an ion can often be determined • from an element’s position in the Periodic Table Metals always form positively charged species, cations For many main group metals, the charge equals the group number • Nonmetals form negatively charged species, anions the charge equals the group number 62 63 potassium cation sulfide anion calcium cation bromide anion aluminum cation Group IA Group VIA Group IIA Group VIIA Group IIIA K+ S2− Ca2+ Br− Al3+ Counting Atoms • a mole is the mass of a material that contains 6.022 x 1023 atoms/particles/ things Avogadro’s Number • the mole’s value is referenced to the number of atoms in exactly 12 grams of pure carbon-12 1 atom of C-12 weighs exactly 12 amu the average atomic mass of C is12.01 amu, therefore 1 mole of C atoms weighs 12.01 g 12.01 g contains 6.022 x 1023 atoms • this provides a relationship between mass and the number of atoms 64 Counting Atoms by Moles The number of atoms, 6.022 x 1023, per mole can be used as a conversion factor. Given the number of moles of a substance, you can calculate the number of atoms, and visa versa 65 Example 2.6: Calculate the number of atoms in 2.45 mol of copper Given: Find: Conceptual Plan: Relationships: 2.45 mol Cu atoms Cu mol Cu atoms Cu 1 mol = 6.022 x 1023 atoms Solution: Check: because atoms are small, the large number of atoms makes sense 66 A silver ring contains 1.10 x 1022 silver atoms. How many moles of silver are in the ring? 1 mol = 6.022 x 1023 atoms 1. 6.62x1045 moles 2. 0.0183 moles 3. 54.7 moles 0% 67 1 0% 2 0% 3 A silver ring contains 1.1 x 1022 silver atoms. How many moles of silver are in the ring? Given: 1.1 x 1022 atoms Ag Find: moles Ag Conceptual Plan: Relationships: atoms Ag mol Ag 1 mol = 6.022 x 1023 atoms Solution: Check: because the number of atoms given is less than Avogadro’s number, the answer makes sense 68 Relationship Between Moles and Mass • Molar Mass = the mass of 1 mole of atoms 6.022x1023 particles • The molar mass of an element, in grams, is numerically equal to the element’s atomic mass, in amu the lighter the atom, the less a mole weighs the more atoms there are in 1 g • Molar mass can be used as a conversion factor to convert mass to moles: g x mol/g = mole • or moles to mass: mol x g/mol = g 69 Mole and Mass Relationships 1 mole carbon 12.01 g 1 mole sulfur 32.06 g 70 Example 2.7: Calculate the moles of carbon in 0.0265 g of pencil lead Given: 0.0265 g C Find: mol C Conceptual Plan: gC mol C Relationships: 1 mol C = 12.01 g Solution: Check: because the given amount is much less than 1 mol C, the number makes sense 71 Calculate the moles of sulfur in 57.8 g of sulfur 1 mol S = 32.07 g A. B. C. D. 1.80 0.555 1850 1.802 0% A. 72 0% 0% B. C. 0% D. Calculate the moles of sulfur in 57.8 g of sulfur Given: Find: Conceptual Plan: 57.8 g S mol S gS mol S Relationships: 1 mol S = 32.07 g Solution: Check: because the given amount is much less than 1 mol S, the number makes sense 73 Example 2.8: How many copper atoms are in a penny weighing 3.10 g? Given: Find: Conceptual Plan: Relationships: 3.10 g Cu atoms Cu g Cu mol Cu atoms Cu 1 mol Cu = 63.55 g, 1 mol = 6.022 x 1023 Solution: Check: because the given amount is much less than 1 mol Cu, the number makes sense 74 How many aluminum atoms are in a can weighing 16.2 g? Given: 16.2 g Al Find: atoms Al Conceptual Plan: g Al mol Al atoms Al Relationships: 1 mol Al = 26.98 g, 1 mol = 6.022 x 1023 Solution: Check: because the given amount is much less than 1 mol Al, the number makes sense 75 Mass Spectrometry • Mass and isotope abundance are measured with • a mass spectrometer Atoms or molecules are ionized, then accelerated down a tube some molecules are broken into fragments during the ionization process these fragments can be used to help determine the structure of the molecule • Their path is bent by a magnetic field, separating them by mass 76 Mass Spectrometer 77 Mass Spectrum • A mass spectrum is a chart that gives the relative mass and relative abundance of each particle present 78 Example 2.5: If copper is 69.17% Cu-63 with a mass of 62.9396 amu and the rest Cu-65 with a mass of 64.9278 amu, find copper’s atomic mass Given: Find: Conceptual Plan: Relationships: Cu-63 = 69.17%, 62.9396 amu Cu-65 = 100-69.17%, 64.9278 amu atomic mass, amu isotope masses, isotope fractions avg. atomic mass Solution: Check: the average is between the two masses, closer to the major isotope 79 Practice – Ga-69 with mass 68.9256 amu and abundance of 60.11% and Ga-71 with mass 70.9247 amu and abundance of 39.89%. Calculate the atomic mass of gallium. Given: Ga-69 = 60.11%, 68.9256 amu Ga-71 = 39.89%, 70.9247 amu Find: atomic mass, amu Conceptual isotope masses, avg. atomic mass Plan: isotope fractions Relationships: Solution: Check: the average is between the two masses, closer to the major isotope 80 81 Scanning Tunneling Microscope • Gerd Bennig and Heinrich Rohrer found that as you pass a sharp metal tip over a flat metal surface, the amount of current that flows varies with distance between the tip and the surface • Measuring this “tunneling” current allowed them to scan the surface on an atomic scale – essentially taking pictures of atoms on the surface 82 Operation of a STM 83 Scanning Tunneling Microscope • Later scientists found that not only can you see the atoms on the surface, but the instrument allows you to move individual atoms across the surface 84