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Chapter 2 Atoms, Molecules and Ions The atomic theory Dalton’s Atomic Theory (1808) 1. Elements are composed of extremely small particles called atoms. 2. All atoms of a given element are identical, The atoms of one element are different from the atoms of all other elements. 3. Compounds are formed from atoms of more than one element in a fixed ratio. 4. A chemical reaction involves only the separation, combination, or rearrangement of atoms. Law of definite proportions Atoms of any element are alike, atoms of different elements are different. Law of multiple proportions CO, carbon monoxide: C/O = 1:1 CO2, carbon dioxide: C/O = 1:2 The ratio of atoms of different elements in a compound is an integer. Law of conservation of mass 2X+Y X2Y Rutherford’s Model of the Atom atomic radius ~ 100 pm = 1 x 10-10 m nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m Mass and Charge Electron cloud outside nucleus Relative Charge Relative Mass Proton 1 ~ 1 amu Neutron 0 ~ 1 amu Electron -1 1/1840 amu Location nucleus nucleus outside nucleus Atomic number, Mass number and Isotopes Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons number of protons = number of electrons Mass Number A Atomic Number Z X Element Symbol Isotopes Isotopes: same element, different number of neutrons The Isotopes of Hydrogen Hydrogen deuterium tritium 1H 2H 3H 1 1 1 How many protons, neutrons, and electrons are in 14C ? 6 protons, 6 electrons, 14 – 6 = 8 neutrons, 6 The periodic table: Column: group 1A: alkali metal Row: period 2A: alkaline earth metal 7A: halogen 8A: noble gas Different colors: metals, metalloids, nonmetals Molecules and ions compounds are of two types: - Molecular: involves shared electrons and consists of electrically neutral, discrete particles (molecules) H2O, O2 - Ionic compounds: involves electron transfer and charged particles (ions) NaCl 2.2 Molecular Compound 2.3 As a general rule, molecular compounds are formed when nonmetallic elements combine Period 2 3 4 5 IVA CH4 VA NH3 VIA H2O H2S VIIA HF HCl HBr HI Noble Gas Ne Ar Kr Xe A diatomic molecule contains only two atoms:H2, N2, O2, Br2, HCl, CO A polyatomic molecule contains more than two atoms: O3, H2O, NH3, CH4 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. Na+:11 protons, 10 electrons Na:11 protons, 11 electrons anion – ion with a negative charge. If a neutral atom gains one or more electrons, it becomes an anion. Cl:17 protons, 17 electrons Cl-:17 protons, 18 electrons A monatomic ion contains only one atom: Na+, Cl-, Ca2+, O2-, Al3+ A polyatomic ion contains more than one atom: OH-, CN-, NH4+, NO327 3+ How many protons and electrons are in 13 Al ? How many protons and electrons are in78 34Se 2- ? 13 protons, 13 – 3 = 10 electrons 34 protons, 34 + 2 = 36 electrons Common ions: 1A: Li+, Na+, K+ 2A: Mg2+, Ca2+, Ba2+ 3A: Al3+ 6A: O2- S2- 7A: F-, Cl-, Br-, I- Other: Ag+, Fe2+, Fe3+, Co2+, Zn2+, Mn2+ Simple ionic compounds: formed from metals + nonmetals Ionic compounds consist of a combination of cations and an anions. The most reactive metals and the most reactive nonmetals combine to form ionic compounds. NaCl, ZnO, Al2O3 Molecular formula shows the exact number of atoms of each element in the smallest unit of a substance Empirical formula shows the simplest whole-number ratio of the atoms in a substance Molecular H2O C6H12O6 O3 N2H4 Empirical H2O CH2O O NH2 Naming Molecular compounds • • • common names H2O, NH3, CH4, element further left in periodic table is 1st if more than one compound can be formed from the same elements, use prefixes to indicate number of each kind of atom (The prefix of “mono” may be omitted for the first element.) • Name first element first and last element ends in ide Prefix: mono- di- tri- tetra- penta- hexa- hepta- octa- nona- decaMeaning: 1 2 3 4 5 6 7 8 9 10 Examples: PF5 phosphorus pentafluoride SO2 sulfur dioxide HCl hydrogen chloride NF3 nitrogen trifluoride N2O4 dinitrogen tetraoxide N2O dinitrogen oxide Formula of Ionic Compounds The formula is always the same as the empirical formula because molecules don’t exist in ionic compounds Rules for writing formulas of ionic compounds: 1. The positive ion is given first in the formula 2. The subscripts in the formula must produce an electrically neutral formula unit 3. The subscripts should ne the set of smallest whole numbers possible 4. The charges on the ions are not included in the finished formula of the substance Cations: name of element Anions: change ending of element spelling to -ide 2.4 Formula of Ionic Compounds Al3+ Al2O3 +3 x 2 = +6, 3 x (-2) = -6 Aluminum oxide O2The sum of charges on the cations and anions in each formula unit must be zero. The subscript of cation is numerically equal to the charge on the anion; The subscript of anion is numerically equal to the charge on the cation; CaBr2 +2 x 1 = +2, 2 x (-1) = -2 Na2CO3 Mg(OH)2 +1 x 2 = +2, 1 x (-2) = -2 Sodium carbonate magnesium hydroxide Calcium bromide Check Table 2.3 for the cations and anions Transition metal ionic compounds indicate charge on metal with Roman numerals FeCl2 +2 for Fe, Fe2+ FeCl3 iron(II) chloride +3 for Fe, Fe3+ iron(III) chloride Acids and Bases An acid can be defined as a substance that yields hydrogen ions (H+) when dissolved in water. HCl •Pure substance, hydrogen chloride •Dissolved in water (H+ Cl-), hydrochloric acid Anions whose names end in “ide" form acids with “hydro-” prefix and “-ic” ending. H2S hydrosulfuric acid HCN hydrocyanic acid (CN-: cyanide) An oxoacid is an acid that contains hydrogen, oxygen, and another element. HNO3 H2SO4 nitric acid sulfuric acid H2CO3 carbonic acid HClO3 chloric acid 2.5 1. 2. 3. Addition of one O atom to “-ic” acid”per…ic”acid HClO4 perchloric acid Removal of one O atom from “-ic” acid”-ous” acid HNO2 nitrous acid Removal of two O atoms from “-ic” acid”hypo…ous” acid HClO hypochlorous acid Acid Anion HClO hypochlorous acid ClOhypochlorite HClO2 chlorous acid ClO2 chlorite HClO3 chloric acid ClO3chlorate HClO4 perchloric acid ClO4 perchlorate A base can be defined as a substance that yields hydroxide ions (OH-) when dissolved in water. KOH potassium hydroxide Ba(OH)2 barium hydroxide Hydrates: compounds that have a specific number of water molecules attached to them. BaCl2•2H2O barium chloride dihydrate CuSO4•5H2O blue CuSO4 white MgSO4•7H2Omagnesium sulfate heptahydrate Name the following compounds: Cu(NO3)2 Copper(II) nitrate KH2PO4 potasium dihydrogen phosphate NH4ClO3 ammonium chlorate Common names of some compounds: H2O Water NH3 ammonia CO2 dry ice solid carbon dioxide Some more listed in Table 2.7 NaCl table salt sodium chloride NaHCO3 baking soda sodium hydrogen carbonate 2.6