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Ch 2. Elements, Compounds and Chemical Reactions REVIEW OF GENERAL CONCEPTS Adapted from Brady & Senese, 5th Ed. Chemical Laws • The law of conservation of mass - in a chemical reaction, the mass of the reactants (starting materials) will equal the mass of resulting products Implication: reactions involve the re-organization of materials. • The law of definite proportions- the ratio of masses of each element is fixed for a given compound Implication: Each atom has a fixed specific mass, thus in unique combinations, the mass ratio is specific 2.1. Elements and atoms are described by Dalton’s atomic theory 2 Dalton’s Atomic Theory • • Matter consists of tiny particles called atoms Atoms are indestructible. In chemical reactions, the atoms rearrange but they do not themselves break apart + 2.1. Elements and atoms are described by Dalton’s atomic theory 3 Dalton’s Atomic Theory (Cont.): • • • In any sample of a pure element, all the atoms are identical in mass and other properties. The atoms of different elements differ in mass and other properties. In a given compound the constituent atoms are always present in the same fixed numerical ratio. NaCl has a 1:1 atom ratio on the atomic level and larger 2.1. Elements and atoms are described by Dalton’s atomic theory 4 The Law Of Multiple Proportions When two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers. • cements the idea that atoms react as complete (whole) particles. • chemical formulas indicate whole numbers of atoms- not fractions 2.1. Elements and atoms are described by Dalton’s atomic theory 5 Using The Law Of Multiple Proportions sulfur sulfur dioxide trioxide Mass S 32.06 g 32.06 g Mass O 32.00 g 48.00 g Use these data to prove the law of multiple proportions 2.1. Elements and atoms are described by Dalton’s atomic theory 6 Proof Of Atoms • Since the early 1980’s, the Scanning Tunneling Microscope (STM) has been used • A surface can be scanned for topographical information • The image for all matter shows spherical regions of matter-- atoms 2.1. Elements and atoms are described by Dalton’s atomic theory 7 Cathode Ray Tube (CRT) A gas filled glass tube that has electrical charge applied at both ends. Such a tube glows with light and is the precursor of the modern-day television screen. 2.2 Atoms are composed of subatomic particles 8 Discovery Of The Electron By JJ Thomson • In 1897, Thomson placed a magnet near CRT and noted deflection of the beam • Repeated experiment with an electrical field and noted that the discharge was deflected by an electrical field toward the (+) plate • Announced discovery of (-) particle, later named “electron” by Stoney 2.2 Atoms are composed of subatomic particles 9 Determining The Charge On An e-: Millikan 2.2 Atoms are composed of subatomic particles 10 Rutherford’s Alpha Scattering Experiment Most alpha rays passed right through the Au A few were deflected off at an angle 1 in 8000 bounced back towards the alpha ray source 2.2 Atoms are composed of subatomic particles 11 Rutherford’s Nuclear Model of the Atom • Since most of the alpha particles were not deflected, most of the atom is empty space. • Since some of the particles were deflected, they encountered small particles of the same charge. • Since some particles were reflected, there must be a small dense area. 2.2 Atoms are composed of subatomic particles 12 Discovery Of The Proton • Discovered in 1918 in Ernest Rutherford’s lab • Detected using a Mass Spectrometer 2.2 Atoms are composed of subatomic particles 13 Discovery Of The Neutron • Chadwick determined that the nuclei of light atoms could be caused to disintegrate by being bombarded by alpha particles. • In collision of alpha particles with Be, a free neutron was created • the presence of the neutron confirmed in 1932 2.2 Atoms are composed of subatomic particles 14 Subatomic Particles Particle Symbol electron proton neutron 0 1 1 1 e or e - p or 11H 1 0 n or n 0 Mass (u) Location Charge 5.48579903(10-4) orbital 1- 1.007276470 nucleus 1+ 1.008664904 nucleus 0 2.2 Atoms are composed of subatomic particles 15 Atomic Mass • Dalton’s atomic theory states that atoms of an element have a constant, characteristic atomic mass or atomic weight measured in amu (u) • Atomic masses are based on a standard mass, that of an atom of C • 1 atom of Carbon-12 = 12 u • Thus 1 u = 1/12 the mass of a Carbon-12 atom 2.2 Atoms are composed of subatomic particles 16 Isotopes • Most elements in nature are uniform mixtures of two or more kinds of atoms with slightly different masses • Atoms of the same element with different masses are called isotopes For example: there are 3 isotopes of hydrogen and 4 isotopes of iron • Chemically, isotopes have virtually identical properties 2.2 Atoms are composed of subatomic particles 17 Atomic Notation A Z Sy • An element is a substance whose atoms all contain the identical number of protons, called the atomic number (Z) • Isotopes are distinguished by mass number (A): Atomic number, Z = number of protons Mass number, A = (number of protons) + (number of neutrons) Note that for atoms, A is greater than Z: the symbol is topheavy • For charge neutrality, the number of electrons and protons must be equal 2.2 Atoms are composed of subatomic particles 18 Example: uranium-235 Mass number, A (protons + neutrons) 235 Chemical Symbol U Atomic number, Z (number of protons) 92 This information can be summarized: Number of protons = 92 ( = number of electrons) Number of neutrons = 143 Atomic number (Z) = 92 Mass number (A) = 92 + 143 = 235 Chemical symbol = U 2.2 Atoms are composed of subatomic particles 19 Learning Check: Fill in the blanks: symbol neutrons 60Co 81Br 65 29 Cu 33 46 36 protons 27 35 29 2.2 Atoms are composed of subatomic particles electrons 27 35 29 20 Your Turn! How many neutrons are there in 52Fe? A. 52 B. 55 C. 26 D. none of these 2.2 Atoms are composed of subatomic particles 21 Learning Check: Atomic Mass Naturally occurring chlorine is a mixture of two isotopes. In every sample of this element, 75.77% of the atoms are chlorine-35 and 24.23% are chlorine37. The measured mass of chlorine-35 is 34.9689 u and that of chlorine-37 is 36.9659 u. Calculate the average atomic mass of chlorine. (75.77×34.9689) + (24.23×36.9659) u 100 35.45 u 2.2 Atoms are composed of subatomic particles 22 Your Turn There are 2 isotopes of element Z. The first is 56.5% in abundance and has a mass of 152.3 u. If the atomic mass is 155.5 u, what is the mass of the other isotope? A. 156 u B. 44.5 u C. 157. u D. not enough information given E. none of these 153.7 u 2.2 Atoms are composed of subatomic particles 23 Periodic Table 1A 2A 1 H arranged in numbered rows – “periods” columns called “groups” or “families” 3 4 Li Be 11 12 Na Mg 3B 4B 5B 6B 7B 8B 8B 8B 1B 19 20 21 22 23 24 25 26 27 28 K Ca Sc Ti V Cr Mn Fe Co 37 38 39 40 41 42 43 44 Rb Sr Y Zr Nb Mo Tc 55 56 57 72 73 74 Cs Ba La Hf Ta 87 88 89 104 Fr Ra Ac 3A 4A 5A 6A 7A 8A 2 He 5 6 7 8 9 10 B C N O F Ne 13 14 15 16 17 18 2B Al Si P S Cl Ar 29 30 31 32 33 34 35 36 Ni Cu Zn Ga Ge As Se Br Kr 45 46 47 48 49 50 51 52 53 54 Ru Rh Pd Ag Cd In Sn Sb Te I Xe 75 76 77 78 79 80 81 82 83 84 85 86 W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 105 106 107 108 109 110 111 112 114 Rf Db Sg Bh Hs Mt Ds Uuu Uub Uuq 58 59 60 61 62 63 64 65 66 67 68 69 70 71 Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 90 91 92 93 94 95 96 97 98 99 100 101 102 103 Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr 2.3. The periodic table is used to organize and correlate facts 24 Periodic Table • Summarizes chemical and physical properties of the elements • Mendeleev first arranged atoms by increasing atomic mass. Noted repeating (periodic) properties • Modern table is arranged by increasing atomic number (Moseley) 2.3. The periodic table is used to organize and correlate facts 25 Some Important Classifications: • A groups = representative elements or main group elements I A = alkali metals VII A = halogens II A = alkaline earth metals VIII = noble (also inert) gases • B groups = transition elements • Inner transition elements = elements 58 – 71 and 90 – 103 58 – 71 = lanthanide elements 90 – 103 = actinide elements 2.3. The periodic table is used to organize and correlate facts 26 The modern periodic table 2.3. The periodic table is used to organize and correlate facts 27 Metals, Nonmetals, And Metalloids 2.4. Elements can be metals, non-metals, or metalloids 28 Your turn! Which of the following is correct? A. Cu is a representative transition element B. Na is an alkaline earth metal C. Al is a semimetal in group IIIa D. F is a representative halogen E. None of these are correct 2.4. Elements can be metals, non-metals, or metalloids 29 Properties Of Metals • reflect light (have metallic luster) • Can be hammered or rolled into thin sheets (are malleable) and can be drawn into wire (are ductile) • Are solids at room temperature (except Hg) • conduct electricity and heat 2.4. Elements can be metals, non-metals, or metalloids 30 Nonmetals And Metalloids • Nonmetals Lack the properties of metals Tend to pulverize when struck with a hammer Non-conductors of electricity and heat Many are gases, a few solids, and one liquid (Br) React with metals to form (ionic) compounds • Metalloids Have properties between metals and nonmetals 2.4. Elements can be metals, non-metals, or metalloids 31 Chemical Formulas • Are symbols used to describe other elements in a compound • elements and compounds • Free elements are not combined with another element in a compound. Examples: Fe (iron), Na (sodium), and K (potassium) Many non-metals occur in groups of 2 (as diatomic molecules)- H, O, N, F, Cl, I, Br Some elements occur as molecules: P4, S, S8, P10, O3 , etc… 2.5. Formulas and equations describe substances and their reactions 32 Chemical Formulas (Cont.) • Specify the composition of a substance • Fe2O3 is composed of the elements iron and oxygen in a 2:3 ratio • CO(NH2)2 expands to CON2H4, but parentheses often group atoms to show the compound’s structure 2.5. Formulas and equations describe substances and their reactions 33 Hydrates • Hydrates are crystals that contain water molecules, for example plaster: CaSO4 • 2H2O When all the water is removed (by heating), the solid that remains is said to be anhydrous (without water) CuSO4 •5H2O 2.5. Formulas and equations describe substances and their reactions CuSO4 34 Learning Check: Count The Atoms In A Chemical Formula • • • • • Na2CO3 (NH4)2SO4 • Mg3(PO4)2 • CuSO4•5H2O • ___Na, ___ 1 C, ___ 2 3 O 2 8 1 ____O 4 ___N, ___H, ___S, 8 3 2 ____O ___Mg, ___P, ___Cu, ___S, ___H 1 1 ___O, 10 9 2.5. Formulas and equations describe substances and their reactions 35 Molecules Form When Nonmetallic Elements Combine • Molecules are neutral particles made of 2 or more atoms. • Many molecular compounds contain hydrogen: Group Period IVA VA VIA 2 CH4 NH3 H2O 3 SiH4 PH3 H2S 4 GeH4 AsH3 H2Se 5 SbH3 H2Te VIIA HF HCl HBr HI Noble Gas Ne Ar Kr Xe 2.6 Molecular compounds contain neutral particles called molecules 36 Ionic Compounds • Positively charged ions are called cations • Negatively charged ions are called anions • subscripts in the formula always specify the smallest whole-number ratio of the ions needed to make a neutral combination (formula unit) 2 Fe3+ 3 O2- Fe2O 3 2.7 Ionic compounds are composed of charged particles called ions 37 What About Ions? • • • • Number of p+ = number of e- if neutral Number of p+ < number of e- if negative Number of p+ > number of e- if positive The number of p+ never changes when ions form How does Ca form Ca2+? Ca loses 2 electrons How is N3- formed? N gains 3 electrons 2.7 Ionic compounds are composed of charged particles called ions 38 Learning Check: Fill in the blanks: Symbol neutrons 60Co3+ 33 81Br- Cu 2 65 29 46 36 protons electrons 27 24 35 36 29 27 2.7 Ionic compounds are composed of charged particles called ions 39 The Charges On Many Representative Elements Can Be Predicted • Noble gases are especially stable • Main group elements will often gain or lose electrons to have the same number of electrons as the nearest noble gas • Metals form cations by losing electrons What is the expected charge on: Ca? 2+ Na? + • Nonmetals form anions by gaining electrons What is the expected charge on: N? 3O? 2- 2.7 Ionic compounds are composed of charged particles called ions 40 Rules For Writing Formulas Of Ionic Compounds • • • • The cation is given first in the formula The subscripts in the formula must produce an electrically neutral formula unit The subscripts should be the set of smallest whole numbers possible The charges on the ions are not included in the finished formula of the substance 2.8 The formulas of many ionic compounds can be predicted 41 Ionic Compounds Are Neutral • The positive charge must balance the negative charge • We could use trial and error to find the least common charge + - + - 2.8 The formulas of many ionic compounds can be predicted + - 42 Determining The Formula Of An Ionic Compound • Practically, we can often accomplish this by making the charge magnitude (not the charge) of one ion into the subscript for the other. (The “Criss-cross” rule) • If you choose this approach, make sure that the subscripts are reduced to the lowest whole number. Al3+ O2Al2O3 Al3+ O2MgO Mg2+ O2Mg2+ O2+ (PO )3+ 3(NH ) (NH4)3PO4 NH4 PO4 4 4 2.8 The formulas of many ionic compounds can be predicted 43 Your Turn! Which of the following is the correct formula for the formula unit composed of potassium and oxygen ions? A. KO B. KO2 C. K2O D. none of these Your Turn! Which of the following is the correct formula for the formula unit composed of Fe3+ and sulfide ions? A. FeS B. Fe3S2 C. Fe2S3 D. none of these Transition And Post-transition Metals Usually Have Multiple Charges Transition Metals Chromium Cr2+, Cr3+ Manganese Mn2+, Mn3+ Iron Fe2+, Fe3+ Cobalt Co2+, Co3+ Nickel Ni2+ Copper Cu+, Cu2+ Post-transition Metals Tin Sn2+, Sn4+ Bismuth Bi3+ Zinc Zn2+ Silver Ag+ Cadmium Cd2+ Gold Au+, Au3+ Mercury Hg22+, Hg2+ Lead Pb2+, Pb4+ 2.8 The formulas of many ionic compounds can be predicted 46 Some Polyatomic Ions (Ions With Two Or More Atoms): NH4+ Ammonium ion CO32- carbonate ion OH- hydroxide ion H3O+ hydronium ion NO2- nitrite ion SO32- sulfite ion NO3- nitrate ion SO42- sulfate ion ClO2- chlorite ion CrO42- chromate ion ClO3- chlorate ion Cr2O72- dichromate ion PO43- phosphate ion 2.8 The formulas of many ionic compounds can be predicted 47 The Stock System Of Naming Ionic Compounds • Cations: If the metal forms only one positive ion, the cation name is the English name for the metal If the metal forms more than one positive ion, the cation name is the English name followed, without a space, by the numerical value of the charge written as a Roman numeral in parentheses • Anions: monatomic anions are named by adding the “–ide” suffix to the stem name for the element polyatomic ions use the names in Table 2.5 2.9 Molecular and ionic comounds are named following a system 48 Naming Binary Molecules The first element in the formula is identified by its English name, the second by appending the suffix –ide to its stem Chemical Symbol O N P Cl I Name as Stem First Element oxoxygen nitrnitrogen phosph- phosphorus chlorchlorine iodiodine Name as Second Element oxide nitride phosphide chloride iodide 2.9 Molecular and ionic comounds are named following a system 49 Naming Binary Covalent Molecules • Format: number prefix + 1st element name number prefix + stem_ide for 2nd element. • Greek prefixes mono- = 1 (omitted on 1st atom) di=2 tri=3 tetra- = 4 penta- = 5 hexaheptaoctanonadeca- 2.9 Molecular and ionic comounds are named following a system =6 =7 = 8 = 9 = 10 50 Learning Check: Name The Following • PF5 = •phosphorus pentafluoride • HCl = •hydrogen chloride • N2O5 = •dinitrogen tetraoxide or dinitrogen tetroxide 2.9 Molecular and ionic comounds are named following a system 51 Your Turn! Which is the correct formula for nitrogen triiodide? A. N3I B. NI3 C. NIO3 D. N(IO3)3 E. none of the above Your Turn! Which is the correct name for P4O10? A. phosphorus decoxide B. tetraphosphorous decoxide C. tetraphosphorus decoxide D. tetraphosphorus oxide E. none of these 2.9 Molecular and ionic comounds are named following a system 54 Learning Check: Name The Following • • • • • • Na2O K2O NH4ClO3 Mg(C2H3O2)2 Cr2O3 ZnBr2 •sodium oxide •potassium oxide •ammonium chlorate •magnesium acetate •chromium(III) oxide •zinc bromide 2.9 Molecular and ionic comounds are named following a system 55 Learning Check: Determine The Formula • calcium hydroxide Ca(OH)2 • mercury(I) nitride (Hg2)3N2 • ammonium phosphate (NH4)3PO4 2.9 Molecular and ionic comounds are named following a system 56 Your Turn! Which is the correct name for Cu2S? A. copper sulfide B. copper(II) sulfide C. copper(I) sulfide D. none of these Your Turn! Which is the correct formula for ammonium sulfite? A. B. C. D. NH4SO4 (NH4)2S NH4S none of these (NH4)2SO3 Overview: Molecules vs. Formula Units • electrically neutral, discrete particles called molecules • Neutral groups of charged particles called formula units 59 Summary of Properties Hardness and brittleness Molecular compounds tend to be soft and easily crushed because the attractions between molecules are weak and molecules can slide past each other Ionic compounds are hard and brittle because of the strong attractions and repulsions between ions 60 Melting Points To melt the a solid, there must be sufficient kinetic energy to overcome the attractions between particles Molecular compounds have weak attractions between particles and so tend to have low melting points Many molecular compounds are gases at room temperature Ionic compounds tend to have strong attractions so they have high melting points Nearly all ionic compounds are solids at room temperature 61 Electrical Conductivity • Requires the movement of electrical charge • Ionic compounds: Do not conduct electricity in the solid state Do conduct electricity in the liquid and aqueous states-the ions are free to move • Molecular compounds: Do not conduct electricity in any state Molecules are comprised of uncharged particles 62 Your Turn! Which of the following is likely true of NO2? A. it conducts electricity well B. It has a low melting point C. It is likely a solid in its pure form D. None of these