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Ch 2. Elements, Compounds and Chemical Reactions REVIEW OF GENERAL CONCEPTS

Ch 2. Elements, Compounds and Chemical Reactions REVIEW OF GENERAL CONCEPTS. Adapted from Brady & Senese, 5 th 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

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Ch 2. Elements, Compounds and Chemical Reactions REVIEW OF GENERAL CONCEPTS

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  1. Ch 2. Elements, Compounds and Chemical ReactionsREVIEW OF GENERAL CONCEPTS Adapted from Brady & Senese, 5th Ed.

  2. 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

  3. + 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

  4. NaCl has a 1:1 atom ratio on the atomic level and larger 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 numericalratio. 2.1. Elements and atoms are described by Dalton’s atomic theory

  5. 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

  6. 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 Using The Law Of Multiple Proportions 2.1. Elements and atoms are described by Dalton’s atomic theory

  7. 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

  8. 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

  9. 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

  10. Determining The Charge On An e-: Millikan 2.2 Atoms are composed of subatomic particles

  11. 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

  12. 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. Rutherford’s Nuclear Model of the Atom 2.2 Atoms are composed of subatomic particles

  13. 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

  14. 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

  15. Subatomic Particles 2.2 Atoms are composed of subatomic particles

  16. Atomic Mass • Dalton’s atomic theory states that atoms of an element have a constant, characteristic atomic mass or atomic weightmeasured inamu (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

  17. 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

  18. Atomic Notation • 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 top-heavy • For charge neutrality, the number of electrons and protons must be equal 2.2 Atoms are composed of subatomic particles

  19. 235 U 92 Mass number, A (protons + neutrons)  Chemical Symbol  Atomic number, Z (number of protons)  Example: uranium-235 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

  20. Learning Check: Fill in the blanks: symbol neutrons protons electrons 60Co 81Br 36 29 29 33 27 27 46 35 35 2.2 Atoms are composed of subatomic particles

  21. Your Turn! How many neutrons are there in 52Fe? • 52 • 55 • 26 • none of these 2.2 Atoms are composed of subatomic particles

  22. (75.77×34.9689) + (24.23×36.9659) u 100 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 chlorine-37. 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. 35.45 u 2.2 Atoms are composed of subatomic particles

  23. 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? • 156 u • 44.5 u • 157. u • not enough information given • none of these 153.7 u 2.2 Atoms are composed of subatomic particles

  24. Periodic Table • arranged in numbered rows – “periods” • columns called “groups” or “families” 2.3. The periodic table is used to organize and correlate facts

  25. 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

  26. Some Important Classifications: • A groups = representativeelementsor main group elements I A = alkali metals II A = alkaline earth metals VII A = halogens 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

  27. The modern periodic table 2.3. The periodic table is used to organize and correlate facts

  28. Metals, Nonmetals, And Metalloids 2.4. Elements can be metals, non-metals, or metalloids

  29. Your turn! Which of the following is correct? • Cu is a representative transition element • Na is an alkaline earth metal • Al is a semimetal in group IIIa • F is a representative halogen • None of these are correct 2.4. Elements can be metals, non-metals, or metalloids

  30. Properties Of Metals • reflect light (have metallic luster) • Can be hammered or rolled into thin sheets (are malleable) andcan 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

  31. 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

  32. 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

  33. 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

  34. 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 CuSO4 2.5. Formulas and equations describe substances and their reactions

  35. 1 2 3 2 8 1 4 8 3 2 1 1 10 9 Learning Check: Count The Atoms In A Chemical Formula • Na2CO3 • (NH4)2SO4 • Mg3(PO4)2 • CuSO4•5H2O • ___Na, ___C, ___ O • ___N, ___H, ___S, ____O • ___Mg, ___P, ____O • ___Cu, ___S, ___O, ___H 2.5. Formulas and equations describe substances and their reactions

  36. Molecules Form When Nonmetallic Elements Combine • Molecules are neutral particles made of 2 or more atoms. • Many molecular compounds contain hydrogen: Group Noble Period IVA VA VIA VIIA Gas 2 CH4 NH3 H2O HF Ne 3 SiH4 PH3 H2S HCl Ar 4 GeH4 AsH3 H2Se HBr Kr 5 SbH3 H2Te HI Xe 2.6 Molecular compounds contain neutral particles called molecules

  37. 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) Ionic Compounds 3 2 Fe3+ O2- FeO 2 3 2.7 Ionic compounds are composed of charged particles called ions

  38. 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

  39. Learning Check: Fill in the blanks: Symbol neutrons protons electrons 60Co3+ 81Br- 36 29 27 33 27 24 36 46 35 2.7 Ionic compounds are composed of charged particles called ions

  40. 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? Na? Nonmetals form anions by gaining electrons What is the expected charge on: N? O? The Charges On Many Representative Elements Can Be Predicted 2+ + 3- 2- 2.7 Ionic compounds are composed of charged particles called ions

  41. 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 Rules For Writing Formulas Of Ionic Compounds 2.8 The formulas of many ionic compounds can be predicted

  42. + - + + - - 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

  43. 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+ O2- Mg2+ O2- NH4+ PO43- Al3+ O2- Al2O3 MgO Mg2+O2- (NH4)+ (PO4)3- (NH4)3PO4 2.8 The formulas of many ionic compounds can be predicted

  44. Your Turn! Which of the following is the correct formula for the formula unit composed of potassium and oxygen ions? • KO • KO2 • K2O • none of these

  45. Your Turn! Which of the following is the correct formula for the formula unit composed of Fe3+ and sulfide ions? • FeS • Fe3S2 • Fe2S3 • none of these

  46. Transition And Post-transition Metals Usually Have Multiple Charges Transition Metals Chromium Cr2+, Cr3+Zinc Zn2+ Manganese Mn2+, Mn3+Silver Ag+ Iron Fe2+, Fe3+Cadmium Cd2+ Cobalt Co2+, Co3+ Gold Au+, Au3+ Nickel Ni2+ Mercury Hg22+, Hg2+ Copper Cu+, Cu2+ Post-transition Metals Tin Sn2+, Sn4+ Lead Pb2+, Pb4+ Bismuth Bi3+ 2.8 The formulas of many ionic compounds can be predicted

  47. Some PolyatomicIons (Ions With Two Or More Atoms): 2.8 The formulas of many ionic compounds can be predicted

  48. 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

  49. Naming Binary Molecules The first element in the formula is identified by its English name, the second by appending the suffix –ideto its stem Chemical Name as Name as Symbol Stem First Element Second Element O ox- oxygen oxide N nitr- nitrogen nitride P phosph- phosphorus phosphide Cl chlor- chlorine chloride I iod- iodine iodide 2.9 Molecular and ionic comounds are named following a system

  50. 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) hexa- = 6 di- = 2 hepta- = 7 tri- = 3 octa- = 8 tetra- = 4 nona- = 9 penta- = 5 deca- = 10 2.9 Molecular and ionic comounds are named following a system

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