Chapter 2. Conservation of Mass. Law of Definite Proportions. Group Activity. Draw you idea of what an atom looks like.

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1 Group Activity Draw you idea of what an atom looks like. Chapter 2 Answer the following questions: How do you know what an atom looks like at the atomic level? How do scientists know what at atom looks like? Can scientists see the parts of an atom? Explain. Atoms, Molecules, and Ions Conservation of Mass Before chemistry there was Alchemy 1774: Priestley isolated oxygen by heating HgO (mercury (II) oxide). When carried out in a closed container the total mass of the contents within the container didn t change. 1785: Lavoisier formulated The Law of Conservation of Mass. In a chemical reaction, matter is neither created nor destroyed Mass of reactants must = mass of products + any unreacted material 6g H2 8g H2 Alchemy s demise 61: Robert Boyle, the father of modern chemistry, publishes The Sceptical Chymist, Law of Definite Proportions Joseph Louis Proust discovered that all samples of a specific substance will have the same ratio of elements by mass. For example, all samples of pure: water: 1 part H, 8 part O by mass carbon dioxide: 1 part C, 2.7 parts O by mass Elements within a compound combine in specific proportions Compounds are not a random mixture of elements + 34g O2 42g total reaction 36g H2O 42g total

2 Dalton s Atomic Theory Dalton's Theory: ❶All matter is composed of atoms; indivisible particles that are exceedingly (really, really) small. ❷All atoms of a given element are identical, both in mass and in chemical properties. However, they are different from atoms of other elements. ❸Atoms are not created or destroyed in chemical reactions. Reactions only change how atoms are arranged. ❹Atoms combine in small, wholenumber ratios to form compounds. 7 Plum Pudding? Based on J.J. Thompson s experiments, the plum pudding model of the atom was developed Nuclear Atom In 1911, Hand Geiger and Ernest Marsden carried out an experiment under the direction of Ernest Rutherford. Gold foil experiment: Positive alpha particles were shot to toward gold foil. (Radium) Large sphere of positive charge Negatively charged electrons scattered throughout 10 What was expected vs. what was observed Expected data: no large deflections. Plum pudding model fits. Actual data: large deflections. New model of the atom needed. 1 out of every 8000 α particle had a large deflection +? Discovery of the Neutron 1932! Sir James Chadwick (Irene JoliotCurie, Frederic JoliotCurie) Bombarded beryllium with α particles The radiation that was emitted was electrically neutral This radiation could eject protons from wax Used multiple elements Determined it had the same mass as a proton

3 Discovery of the neutron Can we see atoms? Can we see inside them? Simplified experimental setup Why did it take so long to discover the neutron? STM image (7 nm x 7 nm) of a single zigzag chain of cesium atoms (red) on a galliumarsenside surface (blue) Photo courtesy National Institute of Standards and Technology (NIST) 4 4 Number Nickel atoms STM

4 Relative Sizes Nucleus is to an atom what a pea is to a stadium Atomic #, Mass #, and Isotopes How many of each particle are in the following atoms (protons, electrons, neutrons)? 1 1 H H 3 1 H 12 6 C 13 6 C 14 6 C 18 8 O 17 8 O 8 O N 15 7 N O U U 21 Atomic Mass vs. Atomic Mass Number Atomic mass and atomic number are NOT the same Atomic mass is the mass found on the periodic table, which is an average of isotopes. This mass is not a whole number. It has units. Isotopic mass is the mass of a specific isotope. Must look up from a source. Except for one important exception ( 12 C), it is not a whole number. It has units. Mass number is the number of protons plus the number of neutrons for a specific isotope. This MUST be a whole number! It has no units. O Atomic mass amu O Isotopic mass amu O Mass number Molecules, Ions, and Chemical Bonds Which side of the periodic table gains electrons? Loses electrons? Metals tend to lose electrons, have a positive charge (cation) Nonmetals tend to gain electrons, have a negative charge (anion) Covalent Chemical Bonds Covalent bonds form when atoms share electrons; typically between nonmetals Hydrogen, oxygen and other elements exist as diatomic molecules H 2, N 2, F 2, O 2, I 2, Cl 2, Br 2 Na Na 1+ Cl Cl 1 Mg Mg 2+ S S 2

5 Ionic Chemical Bonds Ionic bonds form when atoms transfer electrons; typically between metal and nonmetal Ions are atoms or groups of atoms that have a positive or negative charge; they have gained or lost electrons. Representations of ionic compounds Do they appear different than covalent compounds? Cations have lost one or more electrons; cations have positive charges Anions have gained one or more electrons; since electrons are negative, anions have negative charges Ionic vs. Covalent Bonding Can you determine which is most likely ionic bonding and which is covalent bonding? Metallic Bonding Electrons are shared by all of the metal atoms. The are said to be delocalized electrons. Sometimes referred to as a sea of electrons. Terminology Compounds Atom: one sphere in an image H 2 O Molecule: individual unit made up of two or more atoms covalently bonded Compound: composed of one or more atoms of different elements bonded covalently or ionically. Are the following compounds? Molecules? Formula Unit: individual unit made up of two or more ions bonded NaCl O 2 O 3 NaCl

6 Classifications of Matter Pure substance: all atoms, compounds, or molecules are the same in a sample Mixture: blends of two or more different substances Homogeneous: uniform throughout (seawater) Heterogeneous: not uniform (water and oil) Describe each picture below using the terms mixture or pure substance atom, element, molecule, and/or compound Physical Separation Chemical Separation 32 Classifications of Matter How would you classify the following? Air Distilled water Ice Cream Rocks Gold jewelry Diamond Chemical Formulas Chemical (or molecular) formulas indicate the composition of molecules and ions Which atoms are present and the ratios in which they have combined Structural formulas show how atoms are bonded together Empirical formulas give the smallest wholenumber ratio of a molecular formula. (H 2 O 2 becomes HO.) These formulas are often used in analysis. 33 Chemical Formulas Why do we have different types of models? Sodium Chloride NaCl 35