Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chapter 2 Atoms, Molecules, and Ions 2.1 The Atomic Theory of Matter Is there such thing like a indivisible particle of matter (atom)? There was no evidence, so people could not decide one way or another. Based on the following observations: 1. Law of conservation of mass/matter (Antoine Lavoisier). (Total mass before rxn. = total mass after rxn.) 2. Law of constant composition/definite proportions. (Joseph Proust) A given compound always contains the same elements in the same ratio. Proportions in Sodium Chloride A 100.0 g sample of sodium chloride contains 39.3 g of sodium and 60.7 g of chlorine A 200.0 g sample of sodium chloride contains 78.6 g of sodium and 121.4 g of chlorine A 58.44 g sample of sodium chloride contains 22.99 g of sodium and 35.44 g of chlorine Dalton’s Atomic Theory (1803) 1. Elements are composed of atoms: indivisible, indestructible. 2. The atoms of a given element are similar to each other and different from the atoms of all other elements. 3. Chemical compounds consist of atoms combined in specific, whole number, ratios. (Constant Composition) 4. Chemical reactions change only the way the atoms are combined in compounds; the atoms themselves are unchanged. (Conservation of Mass) What Makes a Theory (Model) a Good Theory? A good theory (model): a. explains know facts. b. predicts new ones… a. Dalton’s theory explained the laws of conservation of matter and constant composition. (How?) b. Dalton predicted the law of multiple proportions - two elements can form more than one compound by combining in different ratios. (This was later confirmed.) H2O and H2O2; CO and CO2 2.2 The Discovery of Atomic Structure Thomson’s Experiment J. J. Thompson,1897. (Cathode rays = beam of electrons) Describe the experiment. In the presence of electric and magnetic forces, when the two forces are balanced, the charge to mass ratio can be calculated (see the handout to look at the math) and Thompson found it to be: charge/mass ratio = 1.76 108 coulombs/g. Robert Millikan determined the charge on the electron in 1909. (So much patience!) 1. He watched drops of oil fall due to gravity (the mass was determined). 2. The oil drops were irradiated with X-rays and as a result became charged. 3. When the electrical force on the charged drops balanced the gravity force, then oil drops were suspended on air. Different droplets had different charges, but all were a multiple of: 1.602 10-19 coulombs. The conclusion was: • Charge of electron: 1.602 10-19 coulombs • Mass of electron: 9.10 10-28 g (from Thompson’s ratio.) 1 Fossum-Reyes Radioactivity (1896): the spontaneous emission of radiation by an atom. Three types of radiation were discovered by Ernest Rutherford: – particles – particles – rays Atomic Structure By the early 1900’s scientists already new atoms contain electrons (negatively charged), but they are neutral overall… (i.e. they must contain something that neutralizes the charge). Thomson’s Model J.J. Thomson proposed the Plum Pudding Model (didn’t last long…): He thought of the atom as a sphere of uniformly distributed positive charge with electrons (negatively charged) embedded in it (like a chocolate chip cookie) to yield a neutral atom. Ernest Rutherford’s Experiment (1911) Shot α particles (high energy, positively charged and very massive) through a thin metal foil. If the plum pudding model was correct, the α particles should pass straight through the foil with a few minor deflections at most (why?). What he found instead, was that some α particles suffered large deflections and some even bounced backwards, although most did go straight through. (He described the phenomenon as “shooting a cannonball through tissue paper and having the ball bouncing back”.) Conclusion: atoms contain a very tiny central core (the nucleus) that contains all of the positive charge and most of the mass of the atom. The atom is mostly empty space. 2.3 Modern view of Atomic Structure • Protons were discovered by Rutherford in 1919. • Neutrons were discovered by James Chadwick in 1932. The Nuclear Atom Particle Charge Charge (Atomic Mass Mass (Atomic Relative Mass Scale) Scale) Proton (p+) + 1.602 x 10-19 C +1 1.673 x 10-27 kg 1.0073 ≈1 0 Neutron (n ) 0 0 1.675 x 10-27 kg 1.0087 ≈1 -19 -31 -4 Electron (e ) – 1.602 x 10 C –1 9.1 x 10 kg 5.486 x 10 ≈0 If the atom is compared to a baseball stadium, the nucleus would be the size of a pea; nevertheless, the mass of that peasized nucleus would be… 250 million tons! The nucleus is extremely dense. Atoms have no net electrical charge. Atomic Number, Mass Number and Isotopes Atomic number (Z) = # p+ Mass number (A) = # p+ + # n0 *In a neutral atom, the number of e- and p+ must be equal.* Isotopes • Isotopes are atoms of the same element with different masses (i.e. they have different numbers of neutrons). Isotope symbol For Carbon -12 → 2 Fossum-Reyes Problem 1 Q. Give the number of each type of particle (p+, n0 and e-) for the following isotopes. a. 32 15 b. 18 8 P O Q. Write the isotope symbol for an atom of nickel (Z = 28) containing 32 neutrons. 2.4 Atomic Weights Atomic Mass Units (amu) are a more convenient unit: 1 amu = 1.66054 x 10-24 g An atom of C-12 weighs exactly 12 amu. All other masses are measured relative to C-12. The Periodic Table Shows a Weighted Average 𝐴 𝑐 𝑊 𝑔ℎ = ∑ 𝐴 𝐴 𝑐 𝑊 𝑔ℎ = 𝑐 𝑊 𝑔ℎ = 𝑎 𝑢× 𝑎 𝑢 𝑝 𝑎 × 𝑓 𝑎𝑐 𝑎 𝑝 𝑎 𝑢 𝑎 𝑐 𝑎 𝑢× Problem 2 Calculate the weighed-average atomic mass of Magnesium. Mg 24 23.9850 amu 78.99% 25 24.9858 amu 10.00% 26 25.9826 amu 11.01% 100.00% Mass spectrometer (p.49) Separates samples in fractions of different mass; the fractional abundances are indicated by the relative signal intensities. 2.5 The Periodic Table When one looks at the chemical properties of elements, one notices a repeating pattern of reactivities. (Periodicity) Metals are on the left side of the chart. Solids at room temperature, except Hg.. 1. Reflective surface (shiny). 2. Conduct heat and electricity. 3. Malleable (can be shaped) and ductile (pulled into wires). 4. Form cations (lose electrons to form positive ions). Nonmetals are on the right side of the periodic table (with the exception of H). Found in all three states. 1. Poor conductors of heat and electricity. 2. Their solids are brittle. 3. Form anions (gain electrons to form negative ions). 4. Found in all three states. 3 Fossum-Reyes Metalloids border the stair-step line (with the exception of Al, Po, and At) – intermediate properties. 1. Show some properties of metals and some of nonmetals. 2. Also known as semiconductors. 2.6 Molecules and Molecular Compounds Molecules – two or more atoms joined together. Molecular compounds (composed of molecules) almost always contain only nonmetals. Chemical Formulas indicate the type and number of atoms found (subscripts). • • Certain elements occur as 2 atom molecules rule of 7’s (Memorize them!) Other elements occur as polyatomic molecules P4, S8, Se8 Types of Formulas • Empirical formulas - lowest whole-number ratio of atoms. • Molecular formulas - exact number of atoms. Empirical Formula Molecular Formula CH C6H6 (Benzene) CH C2H2 (Ethene) • • Structural formulas – how atoms are bonded. 3D formulas (Perspective drawings) - show the three-dimensional array of atoms. Molecular and Structural Formulas Molecular Formula Structural Formula O H2O H O CO2 C2H6O H H C H H C C O H H H C H CH3CH2OH H H C2H6O O H O C H CH3OCH3 H 2.7 Ions and Ionic Compounds By gaining or losing one or more electrons neutral atoms become ions. • Cations (metals) are positive • Anions (nonmetals) are negative Ion Charge and the Periodic Table • The charge on an ion can often be determined from an element’s position on the Periodic Table • Metals always form positively charged cations • For many main group metals, the charge = the group number • Nonmetals form negatively charged anions • For nonmetals, the charge = the group number − 8 4 Fossum-Reyes Elements try to lose or gain electrons to end up with the same number of electrons as the noble gas closest to them (octet rule). Practice – What is the charge on each of the following ions? • potassium cation • sulfide anion • calcium cation • bromide anion • aluminum cation Ionic Compounds Ionic compounds (such as NaCl) are generally formed between metals and nonmetals (ions). Formulas of ionic compounds The rule is: Compounds are electrically neutral. Ions Present Formula If the subscripts are not the lowest whole-number ratio, divide them by the greatest common factor. Formulas of Ionic Compounds Ions Li+ and N3Ca2+ and ClNa+ and S2Al3+ and Br- Formula Li3N CaCl2 Na2S AlBr3 Problem 3 Use the ions of the pairs of elements below to give the formula for their ionic compound. a. Ba and O b. Al and S c. K and P Names of Common Ions Memorize: Their Structure, Charge and Name. (See the handout on common ions.) Look for patterns. Polyatomic Ions 2.8 Naming Compounds 1. Classify the compound: a) Ionic – metal and nonmetal (contains ions). b) Acid – formula begins with H. c) Binary molecular – two nonmetals. d) Organic – contains C, H, possibly O, N (carbon based). 2. Use appropriate rules to name the compound. 5 Fossum-Reyes Naming Ionic Compounds a. Write the name of the cation. (Name of the element). b. If the anion is an element, change its ending to -ide; if the anion is a polyatomic ion, simply write the name of the polyatomic ion. c. If the cation can have more than one possible charge, write the charge as a Roman numeral in parentheses. d. Name the cation, then the anion. (We do not need to specify how many of each there are. Why?) Naming Ionic Compounds Compound Contains BaBr2 Ba2+, Br– KOH K+,OH– SnCl2 Sn2+, Cl– SnCl4 Sn4+, Cl– PbO2 Pb4+, O2– MgSO4 Mg2+, SO42– Fe3(PO4)2 Fe2+, PO43– FePO4 Fe3+, PO43– (NH4)2CO3 NH4+, CO32– Name Barium bromide Potassium hydroxide Tin (II) chloride Tin (IV) chloride Lead (IV) oxide Magnesium sulfate Iron (II) phosphate Iron (III) phosphate Ammonium carbonate Problem 4 Name: CaI2 Problem 5 Write the formulas: Calcium hypochlorite K2SO4 Sodium bicarbonate (NH4)2S Cobalt (III) oxide CuF Lithium phosphate CuF2 Nickel (II) cyanide Ag2CO3 Aluminum nitrite Cr(NO3)3 Naming Acids – Contain H+ and an anion. a. Binary acids (its anion ends in “_ide”) Hydro _____ic acid HCl Hydrochloric acid HBr Hydrobromic acid H2S Hydrosulfuric acid b. Oxyacids (anion ends in “_ite” or “_ate”.) The name is related to the name of the anion: If the anion ends in: acid ends in: “_ate” “_ic acid” “_ite” “_ous acid” HNO3 HNO2 H2SO4 H2SO3 H3PO4 NO3– NO2– SO4 2 – SO3 2 – PO4 3 – nitrate nitrite sulfate sulfite phosphate nitric acid nitrous acid sulfuric acid sulfurous acid phosphoric acid 6 Fossum-Reyes Problem 6 Name: HC2H3O2 ; HI H2CO3 ; HClO4 Problem 7 Give the Formula: Hydrofluoric acid ; Chlorous acid Hypochlorous acid ; Chromic acid Naming Binary Molecular Compounds Made of two nonmetals, these compounds DO NOT contain ions. Common names: H2O Water NH3 Ammonia PH3 Phosphine Naming Binary Molecular Compounds Systematic names use numeral prefixes (since they do not have charges, we can’t predict how many atoms of each element are present): - Name the first element. - Second element ends in “_ide”. - Don’t use “mono” for the first element. Naming Binary Molecular Compounds CO2 carbon dioxide CO carbon monoxide (just one “o”) N2O dinitrogen monoxide PCl5 phosphorus pentachloride N2O4 dinitrogen tetroxide (“a” omitted) Naming Binary Molecular Compounds Note: Pure Substance Aqueous Solution HCl(g) hydrogen chloride hydrochloric acid H2S(g) hydrogen sulfide hydrosulfuric acid Problem 8 Name: ClF3 ; Formulas: Phosphorus triiodide P4O10 ; Sulfur dichloride SO3 ; Xenon trioxide Problem 9 Name: Co(NO2)2 ; PCl3 HBrO3 ; CaSO3 N2F4 ; Zn(C2H3O4) 2 HF 7 Fossum-Reyes Problem 10 Formulas: Tin (IV) permanganate ; Sulfurous acid Lithium sulfite ; Dioxygen difluoride Silver oxide ; Periodic acid Nickel (II) carbonate 2.9 Organic Compounds Organic chemistry is the study of carbon; it has its own system of nomenclature. There are literally millions of organic compounds, which are divided in families (posses certain structural features that identify them). Alkanes contain C and H only and no multiple bonds (the simplest hydrocarbons). H H C H H H CH4 Methane H H C C H H H H C2H6 Ethane H H H C C C H H H H C3H8 Propane The family of alcohols: molecules that contain the –OH group. We use the –ol ending. H H C O H CH3OH Methanol H H H H C C H H C2H5OH Ethanol O H H H H H C C C H H H O H C3H7OH 1-Propanol 8 Fossum-Reyes