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Atoms, Molecules, and Ions Chapter 2 1 16 X + 8Y 8 X2Y 2 Dalton’s Atomic Theory (read pages 47-50) 2 3 The Structure of the Atom • Dalton’s atomic theory - atom, the basic unit of element • However, atom has subatomic particles -Electrons, neutrons, protons Electrons -negative charge -negligible mass 4 The Structure of the Atom (cont.) • Atom is neutral • Nucleus-a dense central core of the atom Protons -positive charge - mass is 1840 times the mass of electron Neutrons -electrically neutral - Mass almost the same as proton 5 Rutherford’s Experiment (1908 Nobel Prize in Chemistry) particle velocity ~ 1.4 x 107 m/s (~5% speed of light) 1. atoms positive charge is concentrated in the nucleus 2. proton (p) has opposite (+) charge of electron (-) 3. mass of p is 1840 x mass of e- (1.67 x 10-24 g) 6 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 “If the atom is the Houston Astrodome, then the nucleus is a marble on the 50-yard line.” • Surrounding the nucleus is a diffuse region of negative charge populated by: - electrons - negatively charged particles - mass p ≈ mass n ≈ 1840 x mass e- Selected Properties of the Subatomic Particles Name Charge Mass(amu) Mass (grams) Electrons (e) -1 5.4 x 10-4 9.1095 x 10-28 Protons (p) +1 1.00 1.6725 X 10-24 Neutrons (n) 0 1.00 1.6750 x 10-24 8 mass p = mass n = 1840 x mass e9 Radioactivity is spontaneous emission of particles and /or radiation 10 Atomic Number, Mass Number, and Isotopes • Atomic number (Z) = number of protons in a neutral atom #protons = # of electrons (in a neutral atom) • Mass number (A) = number of protons (Z) + number of neutrons Number of neutrons = A-Z 11 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 Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei Mass Number A ZX Atomic Number 1 1H 2 1H Element Symbol (D) 3 1H (T), one proton in all 0, 1, and 2 neutrons in the isotopes 235 92 U 238 92 U Known as U-235 and U-238 • Isotopes - atoms of the same element having different masses. – contain same number of protons – contain different numbers of neutrons Isotopes of Hydrogen Hydrogen (Hydrogen-1) Deuterium (Hydrogen-2) Tritium (Hydrogen-3) 13 Problems 1. Write nuclear symbols for three isotopes of oxygen (atomic no. = 8) in which there are 8, 9, and 10 neutrons, respectively 2. One of the most harmful species in nuclear fallout is radioactive isotope of strontium, has a mass number of 90 and atomic number of 38. Write the nuclear symbol for Sr. How many protons and neutrons in the nucleus? How many electrons? 3. How many protons, neutrons, and electrons in C-14 4. Write the nuclear symbol for an atom that contains 32 protons and 38 neutrons (Use the Periodic Table to find the symbol of the element) 14 p76 16 Noble Gas Halogen Group Alkali Metal Alkali Earth Metal Period Table 2-2 p58 Molecules • A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical forces (also called chemical bonds) • Atoms of the same element or different elements in a fixed ratio. When it is of the same element, it is a pure element, for e.g.., 2 H atoms make one molecule of H2. Only the six noble gases exist as monoatomic in nature. Others diatomic, polyatomic or ionic in nature. 18 19 Ions 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 11 electrons Na+ 11 protons 10 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 20 A monoatomic ion contains only one atom: Na+, Cl-, Ca2+, O2-, Al3+, N3↑ Atoms with 1,2, and 3 electrons lost or gained. OH-, CN-, NH4+, NO3A polyatomic ion contains more than one atom: (a) What does S8 and 8S signify? (b) Protons and electrons in (a) P3- and (b) Ti4+ 21 Figure 2-21 p65 Table 2-5 p65 Formulas and Models 24 A molecular formula shows the exact number of atoms of each element in the smallest unit of a substance. An empirical formula shows the simplest whole-number ratio of the atoms in a substance. molecular empirical H2O H2O C6H12O6 CH2O O3 O N2H4 NH2 25 Example Write the molecular formula of methanol. There are four H atoms, one C atom, and one O atom. Therefore, the molecular formula is CH4O. However, the standard way of writing the molecular formula for methanol is CH3OH because it shows how the atoms are joined in the molecule. 26 2.3 Write the empirical formulas for the following molecules: (a) acetylene (C2H2), which is used in welding torches (b) glucose (C6H12O6), a substance known as blood sugar (c) nitrous oxide (N2O), a gas that is used as an anesthetic gas (“laughing gas”) and as an aerosol propellant for whipped creams. 27 2.3 Solution (a) There are two carbon atoms and two hydrogen atoms in acetylene. Dividing the subscripts by 2, we obtain the empirical formula CH. (b) In glucose there are 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Dividing the subscripts by 6, we obtain the empirical formula CH2O. Note that if we had divided the subscripts by 3, we would have obtained the formula C2H4O2. Although the ratio of carbon to hydrogen to oxygen atoms in C2H4O2 is the same as that in C6H12O6 (1:2:1), C2H4O2 is not the simplest formula because its subscripts are not in the smallest whole-number ratio. 28 2.3 (c) Because the subscripts in N2O are already the smallest possible whole numbers, the empirical formula for nitrous oxide is the same as its molecular formula. 29 ionic compounds consist of a combination of cations and an anions • the formula is always the same as the empirical formula • the sum of the charges on the cation(s) and anion(s) in each formula unit must equal zero The ionic compound NaCl 30 31 Formulas of Ionic Compounds 2 x +3 = +6 3 x -2 = -6 Al2O3 Al3+ 1 x +2 = +2 Ca2+ 2 x +1 = +2 Na+ O22 x -1 = -2 CaBr2 Br1 x -2 = -2 Na2CO3 CO3232 Chemical Substances; Formulas and Names • Ionic compounds Most ionic compounds contain metal and nonmetal atoms; for example, NaCl. You name an ionic compound by giving the name of the cation followed by the name of the anion. A monatomic ion is an ion formed from a single atom. Chemical Substances; Formulas and Names • Rules for predicting charges on monatomic ions Most of the main group metals form cations with the charge equal to their group number. The charge on a monatomic anion for a nonmetal equals the group number minus 8. Most transition elements form more than one ion, each with a different charge. Chemical Substances; Formulas and Names • Rules for naming monatomic ions Monatomic cations are named after the element – if there is only one such ion. For example, Al3+ is called the aluminum ion. If there is more than one cation of an element, a Roman numeral in parentheses denoting the charge on the ion is used. This often occurs with transition elements. i.e. Fe2+ = Iron(II); Fe3+ = Iron(III) The names of the monatomic anions use the stem name of the element followed by the suffix – ide. For example, Br- is called the bromide ion. Chemical Substances; Formulas and Names • Polyatomic ions A polyatomic ion is an ion consisting of two or more atoms chemically bonded together and carrying a net electric charge. NO3 nitrate NO2 nitrite 2 SO4 sulfate OH- hydroxide 2 SO 3 sulfite Chemical Nomenclature • Ionic Compounds – Often a metal + nonmetal – Anion (nonmetal), add “-ide” to element name BaCl2 barium chloride K2O potassium oxide Mg(OH)2 magnesium hydroxide KNO3 potassium nitrate 37 • Transition metal ionic compounds – indicate charge on metal with Roman numerals FeCl2 2 Cl- -2 so Fe is +2 iron(II) chloride FeCl3 3 Cl- -3 so Fe is +3 iron(III) chloride Cr2S3 3 S-2 -6 so Cr is +3 (6/2) chromium(III) sulfide 38 39 40 • Molecular compounds • nonmetals or nonmetals + metalloids • common names • H2O, NH3, CH4, C60 • element further left in periodic table is 1st • element closest to bottom of group is 1st • if more than one compound can be formed from the same elements, use prefixes to indicate number of each kind of atom • last element ends in ide Molecular Compounds HI hydrogen iodide NF3 nitrogen trifluoride SO2 sulfur dioxide N2Cl4 dinitrogen tetrachloride NO2 nitrogen dioxide N2O dinitrogen monoxide 42 2.7 Name the following molecular compounds: (a) SiCl4 (b) P4O10 43 2.7 Strategy We refer to Table 2.4 for prefixes. In (a) there is only one Si atom so we do not use the prefix “mono.” Solution (a) Because there are four chlorine atoms present, the compound is silicon tetrachloride. (b) There are four phosphorus atoms and ten oxygen atoms present, so the compound is tetraphosphorus decoxide. Note that the “a” is omitted in “deca.” 44 An acid can be defined as a substance that yields hydrogen ions (H+) when dissolved in water. For example: HCl gas and HCl in water •Pure substance, hydrogen chloride •Dissolved in water (H3O+ and Cl−), hydrochloric acid 45 An oxoacid is an acid that contains hydrogen, oxygen, and another element. HNO3 nitric acid H2CO3 carbonic acid H3PO4 phosphoric acid 46 Table 2-7 p70 Table 2-8 p70 Figure 2-24 p70 50 51 A base can be defined as a substance that yields hydroxide ions (OH-) when dissolved in water. NaOH sodium hydroxide KOH potassium hydroxide Ba(OH)2 barium hydroxide 52 Hydrates are compounds that have a specific number of water molecules attached to them. BaCl2•2H2O barium chloride dihydrate LiCl•H2O lithium chloride monohydrate MgSO4•7H2O magnesium sulfate heptahydrate Sr(NO3)2 •4H2O strontium nitrate tetrahydrate CuSO4•5H2O CuSO4 53 54 Homework Problems, Ch.2 11,12,13,14,15,21, 27, 31, 49,50,51,52,53,55,56,57,58,64,65,71,72, 73,74,82,2.84,85,86 55