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Chapter 2 Atoms, Molecules, and Ions

Chapter 2 Atoms, Molecules, and Ions. Postulates of Dalton’s Atomic Theory All matter is composed of indivisible atoms. An atom is an extremely small particle of matter that retains its identity during chemical reactions. An element is a type of matter composed of only one kind of atom.

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Chapter 2 Atoms, Molecules, and Ions

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  1. Chapter 2Atoms, Molecules, and Ions

  2. Postulates of Dalton’s Atomic Theory • All matter is composed of indivisible atoms. An atom is an extremely small particle of matter that retains its identity during chemical reactions. • An element is a type of matter composed of only one kind of atom.

  3. A compound is a type of matter composed of atoms of two or more elements chemically combined in fixed proportions. • A chemical reaction involves the rearrangement of the atoms present in the reacting substances to give new chemical combinations present in the substances formed by the reaction.

  4. Atomic Symbol • A one- or two-letter notation used to represent an atom corresponding to a particular element. The first letter must be UPPERCASE. The second letter, when present, must be lowercase. Symbols come from the element name; several names come from Latin.

  5. Law of Multiple Proportions • When two elements form more than one compound, the masses of one element in these compounds for a fixed mass of the other element are in small, whole-number ratios.

  6. As an example, compare the mass of oxygen in the two oxygen compounds of carbon: Carbon monoxide 12 g C 16 g O Carbon dioxide 12 g C 32 g O The ratio of oxygen in CO2 to CO:

  7. Like Dalton, today’s chemists model atoms using spheres. Modern models are often drawn using computer programs and use different colors to represent atoms of different elements. Which of these models depicts CO2? A B C D E E is CO2

  8. Structure of the Atom • The atom is composed of two parts: Nucleus Central core Positively charged Contains most of the atom’s mass Electrons Very light Negatively charged Exist in the region around the nucleus

  9. Discovery of the Electron • J. J. Thomson used an apparatus similar to the one shown on the next slide, a cathode ray tube. • He discovered that the particles that make up the cathode ray are negative and are part of all matter. This finding is illustrated on the following slides. • As a result, Thomson concluded that atoms are not indivisible, as Dalton had postulated.

  10. Cathode Ray Tube

  11. Cathode rays are attracted to the positive end of the magnet and repelled by the negative end.

  12. Thomson’s experiments allowed him to calculate the ratio of the electron’s mass to its charge. • Robert Millikan calculated the charge on the electron. He determined the charge on various drops of oil and found the smallest increment in charge—that is, the charge on the electron. This is illustrated on the next slide.

  13. Nuclear Model of the Atom • Experiments done in Ernest Rutherford’s laboratory used positively charged alpha particles to bombard very thin gold (and other metals) foil. • Most alpha particles passed through the foil, but a few were scattered at large angles, sometimes almost straight backward. • Based on these findings, Rutherford proposed that 99.95% of the atom’s mass is in the positively charged nucleus.

  14. Proton • A nuclear particle having a positive charge equal to that of the electron and a mass more than 1800 times that of the electron. • The number of protons in an atom is called the atomic number,Z. • An element is a substance whose atoms have the same number of protons and thus the same atomic number, Z.

  15. Neutron • A nuclear particle having a mass almost equal to that of the proton but no electrical charge • The mass number,A, is the total number of protons and neutrons in the nucleus. • Isotopes are atoms whose nuclei have the same atomic number (number of protons) but different numbers of neutrons (mass number).

  16. Nuclide • An atom characterized by a certain atomic number, Z, and mass number, A. • Nuclide symbol • Examples:

  17. Write the nuclide symbol for the atom that has 19 protons and 20 neutrons. Atomic number: Z = 19 The element is potassium, K. Mass number: A = 19 + 20 = 39 The nuclide symbol is

  18. Atomic Mass • The average atomic mass for the naturally occurring element expressed in atomic mass units (amu). • Atomic Mass Unit • Equal to exactly one-twelfth the mass of a carbon-12 atom

  19. An element has four naturally occurring isotopes. The mass and percentage of each isotope are as follows: What is the atomic weight and name of the element?

  20. To find the portion of the atomic weight due to each isotope, multiply the fraction by the mass of the isotope. The atomic weight is the sum of these products. The atomic weight is 207 amu; the element is lead.

  21. Periodic Table of the Elements • A tabular arrangement of elements in rows and columns, highlighting the regular repetition of properties of the elements.

  22. A period consists of the elements in one horizontal row. • A group consists of the elements in one vertical column. • Groups are numbered using two systems: IUPAC Numbers 1 through 18 Older system Roman numerals I–VIII and the letters A and B

  23. Metal • A substance or mixture that has a characteristic luster or shine and is generally a good conductor of heat and electricity. • Nonmetal • An element that does not exhibit the characteristics of a metal • Metalloid • An element having characteristics of both metals and nonmetals

  24. Chemical Formula • A notation that uses atomic symbols with numerical subscripts to convey the relative proportions of atoms of the different elements in the substance.

  25. Molecule • A definite group of atoms that are chemically bonded together (tightly connected by attractive forces). • Molecular Formula • A formula giving the exact number of different atoms of an element in a molecule.

  26. Polymer • A very large molecule that is made up of a number of smaller molecules repeatedly linked together. • Monomers • The small molecules that are linked together to form a polymer.

  27. An ion is an electrically charged particle obtained from an atom or chemically bonded group of atoms by adding or removing one or more electrons. • A cation is a positively charged ion formed by losing one or more electrons. A cation is named by its element name followed by the word ion. For example, Na+ is the sodium ion, etc. Under the definition of anion add the following: An anion is named by changing the last part of the element name to –ide followed by the word ion. For example, F- is the fluoride ion, O2- is the oxide ion, N3- is the nitride ion, etc. • An anion is a negatively charged ion formed by gaining one or more electrons.

  28. An ionic compound is composed of cations and anions. • Ions are arranged in a repeating three-dimensional pattern, forming a crystal. • The formula of an ionic compound gives the smallest possible integer number of ions in the substance (without writing charges) so that the combination is electrically neutral. • The formula gives the formula unit of the compounds. A formula unit is not a molecule!

  29. What is formula of the ionic compound of Mg2+ and N3-? The common multiple of the charges is 6, so we need three Mg2+ and two N3-. The resulting formula is Mg3N2

  30. What is the formula of the ionic compound of Ca2+ and PO43-? The common multiple of the charges is 6, so we need three Ca2+ and two PO43-. The resulting formula is Ca3(PO4)2

  31. Organic Compounds • An important class of molecular substances; they contain carbon combined with other elements – notably hydrogen, oxygen, and nitrogen. • Hydrocarbons contain only carbon and hydrogen.

  32. Functional Group Name of Group Examples Alcohol Methyl alcohol Ether Dimethyl ether Carboxylic acid Acetic acid • A functional group is a reactive portion of a molecule that undergoes predictable reactions.

  33. Chemical nomenclature is the systematic naming of chemical compounds. • Compounds that are not organic are called inorganic compounds. Carbon monoxide, carbon dioxide, carbonates, and cyanides are also classified as inorganic compounds.

  34. Naming Inorganic Compounds • The cation is always named first with the name of the cation without the word ion. • The anion is named second with the name of the anion without the word ion. • Examples: • Cation Anion Name • Na+ Br- Sodium bromide • Mg2- O2- Magnesium oxide • Al3+ Cl- Aluminum chloride

  35. Monatomic ions • Ions formed from a single atom. • Common monatomic ions are listed on the next slide by group and by period.

  36. Period IA IIA IIIA IVA VA VIA VIIA 1 2 3 4 5 6 Common Monatomic Ions of the Main-Group Elements H- Li+ Be2+ N3- O2- F- Na+ Mg2+ Al3+ S2- Cl- K+ Ca2+ Ga3+ Se2- Br- Rb+ Sr2+ In3+ Sn2+ Te2- I- Cs+ Ba2+ Tl3+, Tl+ Pb2+ Bi3+

  37. Rules for Predicting the Charge on a Monatomic Ion • 1. Most main-group metals have one monatomic ions with a charge equal to the group number. Al, in Group IIIA(3), has one ion, Al3+. Sr, in Group IIA(2), has one ion, Sr2+. K, in Group IA(1), has one ion, K+.

  38. 2. Some main-group metals with high atomic number have more than one cation. One cation will have the charge of the group number minus 2; the second cation will have a charge equal to the group number Pb in Group IVA(14) has two ions: Pb2+ and Pb4+ Tl in Group IIIA(13) has two ions: Tl+ and Tl3+

  39. 3. Most transition metals form more than one cation, of which one is +2. Zn and Cd form only the +2 ion. Ag forms only the +1 ion. • 4. Nonmetal main-group elements form one monatomic anion with a charge equal to the group number minus 8. F in Group VIIA(17) forms the F- ion. S in Group VIA(16) forms the S2- ion. N in Group VA(15) forms the N3- ion.

  40. Naming Monatomic Ions • Monatomic cations are named after the element if the element forms only one cation.

  41. If more than one cation forms: • In the Stock system, the charge is written using a Roman numeral and is enclosed in parentheses. Cu2+ is copper(II). Cu+ is copper(I). • In an older system, the suffic –ic (for the higher-charged cation) or –ous for the lower-charged cation) is added to the element’s stem. Cu2+ is cupric. Cu+ is cuprous.

  42. Fe3+ is iron(III) or ferric ion. • Fe2+ is iron(II) or ferrous ion. • Hg2+ is mercury(II) or mercuric ion. • The second ion mercury forms is diatomic: • Hg22+ is mercury(I) or mercurous ion.

  43. Cr3+ is chromium(III) or chromic ion. • Cr2+ is chromium(II) or chromous ion. • Mn2+ is manganese(II) or manganous ion. • Co2+ is cobalt(II) or cobaltous ion. • Zinc forms only Zn2+, so it is called zinc ion. • Cadmium forms only Cd2+, so it is called cadmium ion. • Silver forms only Ag+, so it is called silver ion.

  44. Polyatomic Ion • An ion consisting of two or more atoms chemically bonded together and carrying an electrical charge. • Table 2.5 lists common polyatomic ions.

  45. Cations • mercury(I) or mercurous Hg22+ • ammonium NH4+ • Anions • peroxide O2- • hydroxide OH- • cyanide CN-

  46. phosphate PO43- • monohydrogen phosphate HPO42- • dihydrogen phosphate H2PO4- • carbonate CO32- • hydrogen carbonate (bicarbonate) HCO3- • sulfate SO42- • hydrogen sulfate (bisulfate) HSO4- • sulfite SO32- • hydrogen sulfite (bisulfite) HSO3-

  47. acetate C2H3O2- oxalate C2O42- chromate CrO42- dichromate Cr2O72- permanganate MnO4- nitrate NO3- nitrite NO2-

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