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Atoms, Molecules and Ions Chapter 2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

The Atom The Development of the atomic Structure Democritus BC John Dalton, 1800

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.

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.

Copyright © Cengage Learning. All rights reserved. 2 | 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 Dalton’s Atomic Theory Law of Multiple Proportions

7 8 X 2 Y 16 X8 Y + Law of Conservation of Mass

8 Cathode Ray Tube Discovery of the Electron by J.J Thomson

Discovery of the Electron J. J. Thomson used an apparatus similar to the one shown on the next slide, a cathode ray tube. - particles that make up the cathode ray are negative and are part of all matter. - Thomson concluded that atoms are not indivisible, as Dalton had postulated

10 Thomson’s Model

Copyright © Cengage Learning. All rights reserved. 2 | 11 Cathode rays are attracted to the positive end of the magnet and repelled by the negative end.

12 e - charge = x C Thomson’s charge/mass of e - = x 10 8 C/g e - mass = 9.10 x g Measured mass of e - (1923 Nobel Prize in Physics) Millikan’s Experiment

Copyright © Cengage Learning. All rights reserved. 2 | 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.

Copyright © Cengage Learning. All rights reserved. 2 | 14

Copyright © Cengage Learning. All rights reserved. 2 | 15

Copyright © Cengage Learning. All rights reserved. 2 | 16 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.

Copyright © Cengage Learning. All rights reserved. 2 | 17 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).

18 atomic radius ~ 100 pm = 1 x m nuclear radius ~ 5 x pm = 5 x m Rutherford’s Model of the Atom “If the atom is the Houston Astrodome, then the nucleus is a marble on the 50-yard line.”

Copyright © Cengage Learning. All rights reserved. 2 | 19 Nuclide An atom characterized by a certain atomic number, Z, and mass number, A. Nuclide symbol Examples:

Copyright © Cengage Learning. All rights reserved. 2 | 20 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 = = 39 The nuclide symbol is

21 6 protons, 8 (14 - 6) neutrons, 6 electrons 6 protons, 5 (11 - 6) neutrons, 6 electrons How many protons, neutrons, and electrons are in C 14 6 ? How many protons, neutrons, and electrons are in C 11 6 ?

Copyright © Cengage Learning. All rights reserved. 2 | 22 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

Copyright © Cengage Learning. All rights reserved. 2 | 23 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

Copyright © Cengage Learning. All rights reserved. 2 | 24 An element has four naturally occurring isotopes. The mass and percentage of each isotope are as follows: Percentage AbundanceMass (amu) What is the atomic weight and name of the element?

Copyright © Cengage Learning. All rights reserved. 2 | 25 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. Fractional Abundance Mass (amu)Mass From Isotope The atomic weight is 207 amu; the element is lead.

26 Common Ions Shown on the Periodic Table

Copyright © Cengage Learning. All rights reserved. 2 | 27 Periodic Table of the Elements A tabular arrangement of elements in rows and columns, highlighting the regular repetition of properties of the elements.

Copyright © Cengage Learning. All rights reserved. 2 | 28 Periodic Table of the Elements A tabular arrangement of elements in rows and columns, highlighting the regular repetition of properties of the elements.

29 Chemistry In Action Natural abundance of elements in Earth’s crust Natural abundance of elements in human body

Copyright © Cengage Learning. All rights reserved. 2 | 30 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.

31 The most reactive metals (green) and the most reactive nonmetals (blue) combine to form ionic compounds.

32 A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical forces H2H2 H2OH2ONH 3 CH 4 A diatomic molecule contains only two atoms H 2, N 2, O 2, Br 2, HCl, CO A polyatomic molecule contains more than two atoms O 3, H 2 O, NH 3, CH 4 diatomic elements

33 An ion is an atom, or group of atoms, that has a net positive or negative charge. cation – ion with a positive charge If a neutral atom loses one or more electrons it becomes a cation. anion – ion with a negative charge If a neutral atom gains one or more electrons it becomes an anion. Na 11 protons 11 electrons Na + 11 protons 10 electrons Cl 17 protons 17 electrons Cl - 17 protons 18 electrons

34 A monatomic ion contains only one atom A polyatomic ion contains more than one atom Na +, Cl -, Ca 2+, O 2-, Al 3+, N 3- OH -, CN -, NH 4 +, NO 3 - A cation is a positively charged ion formed by losing one or more electrons. An anion is a negatively charged ion formed by gaining one or more electrons.

35 Common Ions Shown on the Periodic Table

36 13 protons, 10 (13 – 3) electrons 34 protons, 36 (34 + 2) electrons How many protons and electrons are in ?Al How many protons and electrons are in ?Se

37 A molecular formula shows the exact number of atoms of each element in the smallest unit of a substance An empirical formula shows the simplest whole-number ratio of the atoms in a substance H2OH2O H2OH2O molecularempirical C 6 H 12 O 6 CH 2 O O3O3 O N2H4N2H4 NH 2

Copyright © Cengage Learning. All rights reserved. 2 | 38 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!

39 ionic compounds consist of a combination of cations and an anions The formula is usually the same as the empirical formula The sum of the charges on the cation(s) and anion(s) in each formula unit must equal zero The ionic compound NaCl

40 Formula of Ionic Compounds Al 2 O 3 2 x +3 = +63 x -2 = -6 Al 3+ O 2- CaBr 2 1 x +2 = +22 x -1 = -2 Ca 2+ Br - Na 2 CO 3 1 x +2 = +21 x -2 = -2 Na + CO 3 2-

Copyright © Cengage Learning. All rights reserved. 2 | 41 What is formula of the ionic compound of Mg 2+ and N 3- ? The common multiple of the charges is 6, so we need three Mg 2+ and two N 3-. The resulting formula is Mg 3 N 2

Copyright © Cengage Learning. All rights reserved. 2 | 42 What is the formula of the ionic compound of Ca 2+ and PO 4 3- ? The common multiple of the charges is 6, so we need three Ca 2+ and two PO The resulting formula is Ca 3 (PO 4 ) 2

Copyright © Cengage Learning. All rights reserved. 2 | 43 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.

Copyright © Cengage Learning. All rights reserved. 2 | 44 A functional group is a reactive portion of a molecule that undergoes predictable reactions.

Copyright © Cengage Learning. All rights reserved. 2 | 45 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.