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The Study of Chemistry

The Study of Chemistry. The Atomic and Molecular Perspective of Chemistry. • Matter is the physical material of the universe. • Matter is made up of relatively few (ca. 100) elements . • Elements are the building blocks of matter.

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The Study of Chemistry

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  1. The Study of Chemistry The Atomic and Molecular Perspective of Chemistry • Matter is the physical material of the universe. • Matter is made up of relatively few (ca. 100) elements. • Elements are the buildingblocks of matter. • On the nano (ultramicroscopic) level, matter consists of atoms. An atom is a “nano-basketball” -- nano = 10 -9. • Atoms usually are found in the combined state, either molecules, salts, or alloys. • Molecules may consist of the same type of atoms or different types of atoms.

  2. Classification of Matter Elements The next five elements are: Na 2%, K 2%, Mg 2%, H 1%, Ti 0.5%. The next six elements are: N 3%, Ca 1.5%, P 1%, K,S,Na 0.75%

  3. Elements in the Human Body – including trace elements

  4. The Periodic Table Bring your Periodic Table to each class!

  5. Properties of Matter Physical and Chemical Changes When a substance undergoes a physical change, its physical appearance changes, but its chemical nature does not. • Example: the melting of ice (physical change) results in a solid being converted into a liquid, but it is still water. Physical changes do not result in a change of composition. When a substance changes its composition, it undergoes a chemical change • Example: when pure hydrogen and pure oxygen react completely, they form pure water. In the flask containing water, there is no oxygen or hydrogen left over.

  6. Units of Measurement Powers of ten are used for convenience with smaller or larger units in the SI system. What is a GigaByte?

  7. Units of Measurement - Temperature • There are three temperature scales: • Kelvin Scale (used in science) • Same temperature increment as Celsius scale. • Lowest temperature possible (absolute zero) is zero Kelvin. • Absolute zero: 0 K = -273.15oC. • Celsius Scale (used in science) • Also used in science. • Water freezes at 0oC and boils at 100oC. • To convert: K = oC + 273.15. • Fahrenheit Scale (used in US engineering and commerce) • Water freezes at 32oF and boils at 212oF. • To convert:

  8. Units of Measurement - Temperature

  9. Units of Measurement - Temperature • A user-friendly way to view the Celsius Scale: • 0° - Cold! (coat) • 10° - Cool (sweat shirt) • 20° - Pleasant (long sleeves) • 25° - Room temperature (short sleeves) • 30° - Very warm (T-shirt) • 40° - Hot! (swimming pool!)

  10. Units of Measurement - Density • Used to characterize substances. • Defined as: density = mass /volume. • Units: g/cm3, also known as specific gravity. • Originally based on mass -- the density was defined as the mass of 1.00 g of pure water.

  11. The Atomic Theory of Matter • John Dalton: Elements are composed of atoms. • All atoms of an element are identical (chemically). (Dalton stressed “identical in weight” but he didn’t know about isotopes) • In chemical reactions, the atoms are not changed. • Compounds are formed when atoms of more than one element combine. (e.g., H2O, C6H6, C12H22O11 but not H2, Cl2)

  12. The Discovery of Atomic Structure The ancient Greeks were the first to postulate that matter consists of indivisible constituents. Later scientists realized that the atom consisted of charged (+ or -) entities. A charged particle will have its path bend in either an electric or magnetic field. Cathode Rays and Electrons A cathode ray tube (CRT) is a hollow vessel with an electrode at either end. A high voltage is applied across the electrodes.

  13. The Discovery of Atomic Structure The ancient Greeks were the first to postulate that matter consists of indivisible constituents. Later scientists realized that the atom consisted of charged (+ or -) entities. A charged particle will have its path bend in either an electric or magnetic field. Cathode Rays and Electrons A cathode ray tube (CRT) is a hollow vessel with an electrode at either end. A high voltage is applied across the electrodes.

  14. The Discovery of Atomic Structure • Three spots are noted on the detector: • a spot which is not affected by the electric field, • a spot in the direction of the positive (+) plate, • a spot in the direction of the negative (-) plate.

  15. b-radiation:Large deflection toward the positive plate corresponding to radiation which is negatively charged and of low mass. These b particles are light and of low mass. bparticles are electrons. g-radiation:No deflection; neutral (zero charge) radiation. a-radiation:Small deflection toward the negative plate corresponding to high mass, positively charged radiation.

  16. The Discovery of Atomic Structure The Nuclear Atom From the separation of radiation we conclude that the atom consists of neutral, positively, and negatively charged entities. J. J. Thomson assumed all these charged species were found in a sphere.

  17. The Discovery of Atomic Structure The Nuclear Atom Rutherford’s a-particle experiment:

  18. The Discovery of Atomic Structure The Nuclear Atom In order to get the majority of -particles through a piece of foil to be undeflected, the majority of the atom must consist of a low mass, diffuse negative charge - the electron. To account for the small number of high deflections of the -particles, the center or nucleus of the atom must consist of a dense positive charge.

  19. The Discovery of Atomic Structure The Nuclear Atom Rutherford modified Thomson’s model as follows: assume the atom is spherical but a massive positive charge must be located at the center, with a diffuse light negative charge surrounding it.

  20. The Modern View of Atomic Structure The atom consists of positive, negative, and neutral entities (protons, electrons, and neutrons). • Protons and neutrons are located in the nucleus • of the atom, which is small. Most of the mass of the • atom is due to the nucleus. Electrons are located outside of the nucleus. Most of the volume of the atom is due to electrons.

  21. The Nucleus Ångstrom unit: (1Å = 10-8cm =10-10 m)

  22. The Periodic Table Columns in the periodic table are called groups (numbered from 1A to 8A or 1 to 18). Rows in the periodic table are called periods. Metals are located on the left hand side of the periodic table (most of the elements are metals). Non-metals are located in the top right hand side of the periodic table. Elements with properties similar to both metals and non-metals are called metalloids and are located at the interface between the metals and non-metals.

  23. Some elements occur naturally as diatomicmolecules (Most elements can be viewed as uniatomic; but there are unusual elemental molecules, e.g., P4, S8, C60.)

  24. The Periodic Table Some of the groups in the periodic table are given special names. • These names indicate the similarities between • group members: • Group 1A: Alkali metals - “al kali” = “the ashes” (of a • fire) • Group 2A: Alkaline earth metals (“earths” historically were • oxides that were difficult to reduce to the metal). • Group 6A: Chalcogens - “ore formers” • Group 7A: Halogens - “salt formers” • Group 8A: Noble gases - “unreactive” gases • At the bottom are the lanthanides (“rare earths”) and the • actinides.

  25. Metals The Periodic Table Non-Metals Metalloids “semiconductors”

  26. COMPARISON OF METALS AND NONMETALS PROPERTY METALS NONMETALS Appearance Metallic luster Flat, matte appearance Conductivity of electricity High Low Conductivity of heat High Low Change of form Malleable Brittle Affinity for electrons Loses electrons Gains electrons Oxide chemistry Oxide forms bases Oxide forms acids MO + H2O → MOH XO + H2O → HXO Examples: Na2O + H2O → 2NaOH SO3 + H2O → H2SO4

  27. Halogens Chalcogens Alkaline Earths Alkali Metals Navigating the Periodic Table Transition Metals Noble or Inert Gases Lanthanides (rare earths) Actinides

  28. Different Kinds of Compounds A salt, formed by ionic bonding, is formed between a metal and a nonmetal, (e.g., NaCl, Ag2O).

  29. Different Kinds of Compounds A molecule, formed by covalent bonding, is formed between a nonmetal and a nonmetal, (e.g., CO2, PBr3, H2O).

  30. Different Kinds of Compounds An alloy, formed by metallic bonding, is formed between a metal and a metal, (e.g., brass or nickel-steel)

  31. The Wave Nature of Light

  32. Bohr’s Model of the Hydrogen Atom Line Spectra Colors from excited gases arise because electrons move between energy states in the atom. These are called line spectra. Na H

  33. Bohr’s Model of the Hydrogen Atom Absorption Emission E4 E3 E2 E1

  34. The Mole The mole connects the visible with the invisible. A fluorine molecule (F2) weighs 38.000 amu. A mole of fluorine molecules weighs 38.000 grams. The number of fluorine molecules in a mole is an incredibly large number, called Avogadro’s Number, N, which is 6.022 x 1023. We will be using the mole concept very often. Amedeo Avogadro 1776-1856

  35. The Mole Examples: A mole of H is 1.008 grams. A mole of H2 is 2.016 grams. A mole of CO2 is 44.011 grams. A mole of CO is 28.01 grams. A mole of octane (C8H18) is 114.22 grams. A mole of copper (Cu) is 63.54 grams. A mole of table salt (NaCl) is 58.44 grams. A mole of sodium bicarbonate (NaHCO3) is 84.01 grams. A mole of Ag2O is 231.74 grams. A mole of glucose (C6H12O6) is 180.16 grams. A mole of chlorophyll (C55H72MgN4O5) is 893.51 grams.

  36. Molecules and Molecular Compounds Picturing Molecules

  37. Ions and Ionic Compounds When an atom or molecule loses electrons, it becomes positively charged. • For example, when Na loses an electron, it becomes Na+. • Positively charged ions are called cations.

  38. Ions and Ionic Compounds When an atom or molecule gains electrons, it becomes negatively charged. • For example when Cl gains an electron it becomes Cl-. • Negatively charged ions are called anions. • An atom or molecule can lose more than one electron.

  39. lose 2 e- 12p+ 12 e- 12p+ 10 e- Ions and Ionic Compounds When an atom or molecule loses two electrons, it becomes doubly positively charged. For example, when Mg loses two electrons, it becomes Mg2+ Mg2+ion Mg atom “magnesium atom” “magnesium ion”

  40. gain 2 e- 16 p+ 16 e- 16p+ 18 e- Ions and Ionic Compounds When an atom or molecule gains two electrons, it becomes doubly negatively charged. For example, when S gains 2 electrons, it becomes S2- S2-ion S atom “sulfur atom” “sulfide ion”

  41. Ions and Ionic Compounds Important: note that there are no easily identified NaCl molecules in the ionic lattice. Therefore, we cannot use molecular formulas to describe ionic substances.

  42. Mg2+ N3- Ions and Ionic Compounds Ionic Compounds I.e., 3Mg atoms need to form 3Mg2+ ions (total 3x2+ charges) and 2 N atoms need to form 2N3- ions (total 2x3- charges). Therefore, the formula is Mg3N2. MgO Mg N 3 2 Be careful! what’s the ionic compound formed between magnesium and oxygen?)

  43. Reactants Product Chemical Equations • Lavoisier: mass is conserved in a chemical reaction. • Chemical equations: descriptions of chemical reactions. • Two parts to an equation: reactants and products: 2H2+ O2 2H2O

  44. H2 H2O O2

  45. Stoichiometric coefficients: numbers in front of the chemical formulas give numbers of molecules or atoms reacting (and numbers being produced).

  46. Law of Conservation of Mass: All reactions must be balanced CH4 + O2 CO2 + H2O is not balanced. (Why?) Count atoms: Reactants: Products: 1 C 1 C 4 H 2 H 2 O 3 O

  47. Balance reactions only by changing coefficients, not by altering chemical formula

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