Chapter 2

Chapter 2Atoms and ElementsVisualizing AtomsBinnig & Rohrer development of the Scanning Tunneling Microscope (STM).Produce images on the atomic level.Iodine atoms on the surface of platinum metal

Modern Atomic TheoryLaw of Conservation of MassAntoine Lavoisier 1789In a chemical reaction, matter is neither created nor destroyed.

7.7 g Na + 11.9 g Cl2 19.6 g NaClModern Atomic TheoryLaw of Definite ProportionsProust 1797All samples of a given compound will have the same proportions of their constituent elements.Mass of WaterMass of OxygenMass of HydrogenRatio O:H18.0 g16.0 g2.0 g8:145.0 g40.0g5.0 g8:1Modern Atomic TheoryThe atomic theory of matterDalton, 1808Four postulates (main themes)Each element is composed of tiny, indestructable particles called atoms.

All atoms of a given element are identical; The atoms of different elements are different and have different properties.

Modern Atomic TheoryAtoms combine in simple, whole number ratios to form compounds.

Atoms of one element cannot change into atoms of another element. In a chemical reaction, atoms only change the way they are bound together.

The evidence for the existence of atoms is overwhelming!Discovery of Atomic StructureDiscovery of the electron.J.J. Thompson cathode ray tube 1897 determined that an electron was negatively charged.

Discovery of Atomic StructureRobert Milliken oil drop 1909 determined the charge value of an electron as well as its mass.

Oil Drop ExperimentDiscovery of Atomic Structure RadioactivityCurie 1900Rutherford 1905 Three main types of radioactive particles: Alpha, Beta, and Gamma.Alpha particlesEssentially a helium nuclei.Sources many, but Curies used radium.Nuclear Model of the AtomErnest Rutherford 1911.Gold foil experiment.Gold can be smashed into very thin sheets that are only a few atoms thick.Alpha particles, from an alpha source, beamed at the gold foil.Photographic paper placed as a detector in front and behind alpha source.Gold Foil Experiment

Gold Foil ExperimentNuclear Model of the AtomOnly 1 in 8000 alpha particles is scattered.Scattering occurs when an alpha particle encounters a massive gold nuclei.Rutherford proposed that:Most of the atoms mass and all of its positive charge were found in the small core of the atom called the nucleus.Most of the volume of the atom is empty space.Nuclear Model of the AtomRutherfords model still had one problem.H = 1 proton in nucleus.He = 2 protons in nucleus.He mass 4x mass of H mass.Final piece of the puzzle is the neutron.Neutrons have no charge and a mass of 1amu.Discovered in 1932 by James Chadwick.Modern View of Atomic Structure Three subatomic particles exist in an atom.Protons, neutrons, and electrons.

Modern View of Atomic StructureElectrons have a negative charge of 1.602 x 10-19 C and a negligible mass.Protons and neutrons reside in the nucleus which is extremely small. Protons have a positive charge, equal in magnitude to an electron and a mass of about 1amu .Neutrons have no charge and a mass of 1amu.Over 99.9% of the mass of an atom resides in the nucleus.Modern View of Atomic StructureThe atom is 100,000 times larger than the nucleus. If a golf ball represented the size of the nucleus, the atom would be about 3 miles in diameter.Diameter of nucleus 10-15 m.Diameter of atom 10-10 m.The density of the nucleus is roughly 1013 to 1014 g/cm3.Atomic and Mass NumbersThe Atomic Number (Z) is equal to the number of protons in the nucleus.Each element has a unique atomic number and hence a unique number of protons.The Mass Number (A) is the sum of the protons and neutrons found in the nucleus.Isotopes & SymbolsIsotopes for an element occur when they have more than one mass number.Isotopes of an element have the same number of protons, but a different number of neutrons. An isotope can be designated by its mass number in the upper left corner AX. Or it can be designated after the symbol XA.Example - 12C or C-12IonsAtoms quite often will gain or lose electrons when forming compounds.When Lithium metal reacts, it loses one electron forming a +1 charge. Li+1.When Fluorine gas reacts, it gains one electron forming a -1 charge. F-.Positive ions are called cations.Negative ions are called anions.Periodic LawMendeleev (1869) first to group the elements by similar properties.First, he listed them in order of increasing atomic mass.He then started a new row when elements had similar properties. Thus, they are arranged according to horizontal rows which highlight the repeating properties of the elements in the vertical columns.periods the elements in a horizontal row constitutes a period. groups the elements in a vertical column constitutes a group. The Periodic Table Groups are numbered and labeled with A and B's.Different conventions of numbering are used, however, we will use the traditional N.A. method with the As for the first two and last six groups.The Periodic TableThe A groups are called main group elements and the B groups are called transition elements. Some groups have special names:Group 1A = Alkali metalsGroup 2A = Alkaline Earth metalsGroup 7A = HalogensGroup 8A = Noble gases or inert gases.The Periodic Table

The Periodic Table Metals a substance that has a characteristic of luster or shine, and is a good conductor of heat and electricity. Metals are solids at room temperature and tend to lose electrons easily. Metals are found to the left and below the diagonal line that runs through the right side of the main group elements.The greatest majority of elements are metals.The Periodic TableNonmetals either a gas at room temperature or a brittle solid, they are nonconductors of heat and electricity. Nonmetals tend to gain electrons easily. Non-metals are found to the right and above the diagonal line. Metalloids are elements that have the properties of both metals and nonmetals.These fall along the diagonal line and include B, Si, Ge, As, Sb, and TeHydrogen the one oddball on the periodic table.Describing An ElementHow would you describe the element:C

Al

Sr

FeIons Metals form cations.Ex) Na Na+ + 1e-Non-metals form anions.Ex) Cl + 1e- Cl-Predicting the charge of a species based on the periodic table.Ions and Ionic Compounds Main group elements usually form only one charge (valence)Transition metals usually form many different charges.Polyatomic ions are special groups of atoms that chemically combine to form a charged species.Atomic WeightsThe Atomic Mass Unit Scale is based on the 12C atom.1 amu = 1/12 the mass of the 12C atom.Average atomic masses are based on the masses of each type of isotope a well as their abundance in nature. Relationship of an amu to grams:1 amu = 1.66054 x 10-24 g1 g = 6.012214 x 1023 amuDetermining An Average MassA.W. = S(fract. abundance) x (isotopic mass)ExampleChlorine has two isotopes, Cl-35 and Cl-37. Cl-35 has a mass of 34.969amu and an abundance of 75.78% Cl-37 has a mass of 36.966amu and an abundance of 24.22%. What is the atomic weight of Chlorine?

Determining An Average MassA.W. = (34.969amu) x (0.7578) + (36.966amu) x (0.2422)

A.W. = 35.45amuMolar MassThe MoleUnit of quantity used in Chemistry.Not convenient to count atoms.1 atom of C-12 = 12 amu (exact)One mole of C-12 = 12 grams (exact)Avogadros Number Represents the number of C-12 atoms in 12 grams.=6.02 x 1023 atoms.