CHEM 161: Chapter 2 – Independent Review v0911 page 1 of 3 CHAPTER 2: ATOMS, IONS, AND COMPOUNDS (Topics to Review) Early Models Democritus (462-370 B.C.): proposed that all matter was made up of tiny, indivisible particles called atomos (meaning “not to cut) or atoms. Empedocles (490-430 B.C.): suggested all matter was composed of four basic elements: air, water, fire, and earth. Aristotle (384-321 B.C.): accepted Empedocles idea and added a fifth element, heavenly ether, which is perfect, eternal, and incorruptible. Aristotle’s idea of five basic elements was accepted for 2000 years. John Dalton (1766-1844), an English chemist and physicist, established the foundation for modern chemistry with the following: The Modern Model of the Atom 1. All matter is composed of tiny indivisible* particles called atoms. 2. An element is composed of only one type of atom – All atoms of one type of an element are identical*. – Atoms of different elements behave in different ways. Note: Parts of these statements (indicated with *) were later proven to be wrong. 3. Two or more elements combine to form compounds. law of definite proportions (also the law of constant composition): – A compound always has same elements in the same proportion by mass – i.e., a compound always has the same formula → Water is always H2O. law of multiple proportions: – Two or more elements can combine to form different compounds – e.g. C and O can combine to form CO or CO2 – For every compound the element’s masses are always in a fixed ratio. – because always have whole #’s of atoms of each element in the compound 4. A chemical reaction involves only the rearrangement of atoms—NEVER their creation or destruction. This is called the law of conservation of mass. Chemical Reaction: REACTANTS → PRODUCTS (starting materials) (substances after reaction)CHEM 161: Chapter 2 – Independent Review v0911 page 2 of 3 The mass of the product(s) in a reaction must equal the mass of the reactant(s). For example, 11.2 g hydrogen + 88.8 g oxygen = 100.0 g water Ex. 1: Methane burns by reacting with oxygen present in air to produce steam and carbon dioxide gas. Calculate the mass of methane that reacts if 199.4 g of oxygen react to produce 112.3 g of steam and 137.1 g of carbon dioxide. methane + oxygen = steam + carbon dioxide x + 199.4 g = 112.3 g + 137.1 g = 249.4 g 249.4 g – 199.4 g 50.0 g methane 2.5 AVERAGE ATOMIC MASSES Atoms are too small to weigh directly – e.g. one carbon atom has a mass of 1.99 x 10-23 g–too inconvenient! → need more convenient unit for mass → atomic mass unit (amu) Carbon-12 was chosen and given a mass value of 12 amu → 1 amu = 1/12 the mass of carbon-12 → Mass of all other atoms measured relative to mass of carbon-12 Average Atomic Mass of an Element – Why is carbon’s mass on Periodic Table 12.01 amu, NOT 12.00 amu?! – Atomic masses reported on the Periodic Table are weighted averages of all the naturally occurring isotopes for each element. Ex. 1 If 98.892% of carbon exists as carbon-12, with a mass of 12.00000, while 1.108% exists as carbon-13, with a mass of 13.00335, calculate the average atomic mass for carbon. average atomic mass = (0.98892)(12.00000 amu) = 11.86704 amu + (0.01108)(13.00335 amu) = 0.144077.. amu 12.011117… amu →places decimal 3 to 12.011 amuCHEM 161: Chapter 2 – Independent Review v0911 page 3 of 3 Ex. 2 The atomic masses of the three naturally occurring isotopes or argon, Ar-36 (0.3365%), Ar-38 (0.0632%) and Ar-40 (99.6003%), are 35.96754552 amu, 37.9627325 amu, and 39.9623837 amu, respectively. Calculate the average atomic mass for argon. (0.003365) (35.96754…amu) = 0.1210307… amu (0.000632) (37.96273…amu) = 0.0239924… amu (0.996003) (39.9623837…amu) = 39.80265… amu 39.94767… amu →places decimal 4 to 39.9477 amu 2.4 THE PERIODIC TABLE OF THE ELEMENTS – A vertical column is called a group or family. – Elements belonging to the same group exhibit similar chemical properties – A horizontal row is called a period or series. Main-Group (Representative or A Group) Elements Those elements in groups 1, 2, 13, 14, 15, 16, 17, 18 (or IA to VIIIA) – Group 1 or IA: alkali metals – Group 2 or IIA: alkaline earth metals – Group 17 or VIIA: halogens – Group 18 or VIIIA: noble gases (because they are all gases that do not react) Transition Metals (or B Group Elements) – Elements in Groups 3 to 12 (middle of the Periodic Table) Inner Transition Elements (beneath the main body of Periodic Table) – Lanthanide series: Ce-Lu, also called rare earth metals, make up <0.005% of Earth's crust – Actinide series: Th-Lr, also called transuranium elements, generally all man-made and exist for only very short periods of time before decaying to other elements Periodic Law: Elements can be arranged to display recurring properties. → We can use the Periodic Table to predict the properties of elements. Dimitri Mendeleev (1834-1907), a Russian chemist, published his periodic table in 1869, before many elements were even discovered. – He organized elements to display recurring properties according to increasing atomic mass. Henry G. J. Moseley (1887-1915), an English physicist, carried out high-energy X-ray radiation experiments on atomic nuclei. → Repeating properties of elements are more clearly reflected if elements are arranged according to increasing atomic number (not increasing atomic mass). → Periodic Table’s arrangement today – Trends for increasing atomic mass are identical with those for increasing atomic number, except for Ni & Co, Ar & K, Te &