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Spring 2016 Chapter 2 Atoms, molecules and ions Atoms, Molecules, and Ions Atoms: The Greek Idea ~384 B.C.E., Aristotle: All matter is composed of four elements and all matter is continuous, not atomistic. Aristotle declared matter to be infinitely divisible. Atoms, Molecules, and Ions 1 Spring 2016 Atoms: The Greek Idea ~ 450 B.C.E., Leucippus and Democritus Atomos: The point at which matter can no longer be subdivided. Atoms, Molecules, and Ions Atomic Theory of Matter The theory that atoms are the fundamental building blocks of matter reemerged in the early 19th century, championed by John Dalton. Atoms, Molecules, and Ions 2 Spring 2016 Dalton’s Postulates 1. Each element is composed of extremely small particles called atoms. Atoms, Molecules, and Ions Dalton’s Postulates 2. Atoms of an element cannot be created, destroyed, broken into smaller parts or transformed into atoms of another element • The discovery of nuclear processes showed that it was possible to transform atoms from one element into atoms of another. But we don't consider processes that affect the nucleus to be chemical processes. The postulate is still useful. A slightly more restrictive wording is "Atoms cannot be created, destroyed, or transformed into other atoms in a chemical change". Atoms, Molecules, and Ions 3 Spring 2016 Dalton’s Postulates 2. Atoms of an element cannot be created, destroyed, broken into smaller parts or transformed into atoms of another element • The discovery of nuclear processes showed that it was possible to transform atoms from one element into atoms of another. But we don't consider processes that affect the nucleus to be chemical processes. The postulate is still useful. A slightly more restrictive wording is "Atoms cannot be created, destroyed, or transformed into other atoms in a chemical change". Atoms, Molecules, and Ions Dalton’s Postulates 3. All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements. Actually most elements occur in nature as mixtures of two or more kind of atoms called isotopes that have slightly different masses but same nuclear charges. In modern atomic theory, the postulate has been amended to read: "Elements are characterized by the nuclear charge of their atoms". Atoms, Molecules, and Ions 4 Spring 2016 Dalton’s Postulates 3. All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements. Actually most elements occur in nature as mixtures of two or more kinds of atoms called isotopes that have slightly different masses but same nuclear charges. In modern atomic theory, the postulate has been amended to read: "Elements are characterized by the nuclear charge of their atoms". Atoms, Molecules, and Ions Dalton’s Postulates 4. When elements react, their atoms combine in simple, whole-number ratios. Atoms, Molecules, and Ions 5 Spring 2016 Dalton’s Postulates 5.When elements react, their atoms sometimes combine in more than one simple, whole-number ratio. Nitrogen and oxygen combine to produce more than one product Atoms, Molecules, and Ions Atoms, Molecules, and Ions 6 Spring 2016 Law of Constant Composition Joseph Proust (1754–1826) • Also known as the law of definite proportions. • The elemental composition of a pure substance never varies. Atoms, Molecules, and Ions Law of Conservation of Mass The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place. Atoms, Molecules, and Ions 7 Spring 2016 Law of multiple proportions “If two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.” Or… ”If two elements form 2 different compounds, the mass ratio of the elements in one compound is related to the mass ratio in the other by a small whole number”. Atoms, Molecules, and Ions Discovery of Electron 1897, Joseph John Thomson: Determined the charge:mass ratio of cathode rays (discovered electrons). Atoms, Molecules, and Ions Charge/mass = 1.76 × 108 C/g 8 Spring 2016 The Electron Thomson used a magnetic field to bend the cathode rays. The amount the cathode ray bent from the straight line using the magnetic field allowed Thomson to calculate the e/m ratio= 1.76 108 coulombs/g. Atoms, Molecules, and Ions CRT TV (Cathode Ray Tube) Atoms, Molecules, and Ions 9 Spring 2016 CRT TV (Cathode Ray Tube) Atoms, Molecules, and Ions Sir William Thomson (1824-1907) J. J. Thomson (1856-1940) British mathematical physicist British physicist 1892: Knighted by Queen Victoria. Titled « Baron Kelvin of Largs » (Lord Kelvin) Atoms, Molecules, and Ions 10 Spring 2016 Millikan Discovery Atoms, Molecules, and Ions Millikan Discovery Robert Millikan determined the charge on the electron to be -1.60 10- 19 coulombs. For convenience, electronic charge is expressed as a multiple of that charge rather than coulombs. Thus the charge of the electronic charge is -1. Atoms, Molecules, and Ions 11 Spring 2016 Millikan Discovery 1) The drop is allowed to fall and its terminal velocity v1 in the absence of an electric field is calculated. The drag force (ie friction force/air resistance) acting on the drop, Fd, is terminal velocity of the falling drop radius of the drop viscosity of the air (eta) Atoms, Molecules, and Ions Millikan Discovery W m g d V g 2) For a perfectly spherical droplet the apparent weight, W, can be written as density of air radius of the drop gravity acceleration constant density of the Atoms, oil Molecules, and Ions 12 Spring 2016 Discovery of Electron • Streams of negatively charged particles were found to exit from cathode tubes. • J. J. Thompson is credited with their Atoms, discovery (1897). Molecules, and Ions Millikan Discovery 3) At terminal velocity the oil drop is not accelerating. This implies W F d 4 3 r g air 6rv1 3 r 9v1 2 g air Atoms, Molecules, and Ions 13 Spring 2016 Millikan Discovery 4) Now the electrical field is turned on, and the electric force on the drop, FE, is FE qE electrical field charge on the oil drop E For parallel plates, So: FE q V d V d potential difference distance between the plates Atoms, Molecules, and Ions Millikan Discovery 5) By adjusting V until the oil drop remains steady, we get FE W q V 4 3 r g air d 3 4 d q r 3 g air 3 V charge on the oil drop q was found to be a multiple of -1.60 10- 19 coulombs. Atoms, Molecules, and Ions 14 Spring 2016 Electron Mass The electron mass was determined indirectly to be m= 9.10938 x 10-28g. Atoms, Molecules, and Ions Radioactivity: • The spontaneous emission of radiation by a radioactive atom. • First observed by Henri Becquerel (1896) • Also studied by Marie and Pierre Curie. Atoms, Molecules, and Ions 15 Spring 2016 Radioactivity • Three types of radiation were discovered by Ernest Rutherford: particles particles rays Atoms, Molecules, and Ions Radioactivity Atoms, Molecules, and Ions 16 Spring 2016 Model of the atom J. J. Thomson who discovered the electron, proposed the “plum pudding model” of the atom Atoms, Molecules, and Ions Rutherford Gold Foil Experiment Using an apparatus similar to that shown below, Ernest Rutherford discovered the atomic nucleus. Atoms, Molecules, and Ions 17 Spring 2016 Rutherford Gold Foil Experiment Atoms, Molecules, and Ions The Nuclear Atom • Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom. • Most of the volume of the atom is empty space. Atoms, Molecules, and Ions 18 Spring 2016 Atoms, Molecules, and Ions Other Subatomic Particles • Protons were discovered by Rutherford (1919). • Neutrons were discovered by James Chadwick in (1932). Atoms, Molecules, and Ions 19 Spring 2016 Subatomic Particles • Protons and electrons are the only particles that have a charge. • Protons and neutrons have essentially the same mass. • The mass of an electron is so small we ignore it. • 1 amu = 1.66054 x 10–24 g. Atoms, Molecules, and Ions Symbols of Elements Elements are symbolized by one or two letters. Atoms, Molecules, and Ions 20 Spring 2016 Atomic Number All atoms of the same element have the same number of protons: The atomic number (Z) Atoms, Molecules, and Ions Atomic Mass The mass of an atom in atomic mass units (amu) is the total number of protons and neutrons in the atom. Atoms, Molecules, and Ions 21 Spring 2016 Isotopes: • Atoms of the same element with different masses. • Isotopes have different numbers of neutrons. 11 6C 12 6C 13 6C 14 6C Atoms, Molecules, and Ions Isotopes of Hydrogen Atoms, Molecules, and Ions 22 Spring 2016 Average Mass • Average mass is calculated from the isotopes of an element weighted by their relative abundances. Atoms, Molecules, and Ions Example • Naturally occurring C consists of 98.93% 12C (12 amu) and 1.07% 13C (13.00335 amu). The average mass of C is: • (0.9893)(12 amu) + (0.0107)(13.00335 amu) = 12.01 amu. • 1 amu= 1.66054 x 10- 24 g • 1 g = 6.02214 x 1023 amu Atoms, Molecules, and Ions 23 Spring 2016 Atomic weight • Atomic weight (AW) is also known as average atomic mass. • Atomic weights are listed on the periodic table. Atoms, Molecules, and Ions The Mass Spectrometer • A mass spectrometer is an instrument that allows for direct measure of atomic mass of elements • A graph of signal intensity vs. mass of the ion is called a mass spectrum. Nobel prize in 1921 Atoms, Molecules, and Ions 24 Spring 2016 Atoms, Molecules, and Ions Atoms, Molecules, and Ions 25 Spring 2016 Atoms, Molecules, and Ions Atoms, Molecules, and Ions 26 Spring 2016 Periodic Table • The periodic table is used to organize the elements in a meaningful way. Rows in the periodic table are called periods. Columns in the periodic table are called groups. Atoms, Molecules, and Ions Periodic Table • Several numbering conventions are used (i.e., groups may be numbered from 1 to 18, or from 1A to 8A and 1B to 8B). • Some of the groups in the periodic table are given special names. • These names indicate the similarities between group members. • Examples: • Group 1A: alkali metals • Group 2A: alkaline earth metals • Group 7A: halogens • Group 8A: noble gases Atoms, Molecules, and Ions 27 Spring 2016 Periodicity When one looks at the chemical properties of elements, one notices a repeating pattern of reactivities. Atoms, Molecules, and Ions Groups These five groups are known by their names. Atoms, Molecules, and Ions 28 Spring 2016 Periodic Table Metals are on the left side of the chart. Metals tend to be malleable and are good thermal and electrical conductors. Atoms, Molecules, and Ions Periodic Table Nonmetals are on the right side of the periodic table (with the exception of H). Nonmetals tend to be gases, liquids or solids (brittle) Atoms, and do not conduct heat or electricity well. Molecules, and Ions 29 Spring 2016 Periodic Table Metalloids border the stair-step line (with the exception of Al and Po). Elements with properties similar to both metals and nonmetals are called metalloids Atoms, Molecules, and Ions Molecules and Molecular Compounds • A molecule consists of two or more atoms bound tightly together. • Each molecule has a chemical formula. Atoms, Molecules, and Ions 30 Spring 2016 Chemical Formulas The chemical formula indicates : 1. which atoms are found in the formula unit 2. in what proportion they are found. Atoms, Molecules, and Ions Molecular Compounds Molecular compounds are composed of molecules and almost always contain only nonmetals. • A molecule made up of two atoms is called a diatomic molecule. Atoms, Molecules, and Ions 31 Spring 2016 Diatomic Molecules These seven elements occur naturally as molecules containing two atoms. Atoms, Molecules, and Ions Allotropes • Different forms of an element, which may have different chemical formulas, are known as allotropes. Allotropes differ in their chemical and physical properties. Atoms, Molecules, and Ions 32 Spring 2016 Molecular Formulas • These formulas give the actual numbers and types of atoms in a molecule. • Examples: H2O, CO2, CO, CH4, H2O2, O2, O3, and C2H4. Atoms, Molecules, and Ions Empirical formulas • Empirical formulas give the lowest whole-number ratio of atoms of each element in a compound. Atoms, Molecules, and Ions 33 Spring 2016 Picturing Molecules The structural formula gives the connectivity between individual atoms in the molecule • Molecules occupy threedimensional space. • However, we often represent them in two dimensions. Atoms, Molecules, and Ions Ions • When atoms lose or gain electrons, they become ions. – Cations are positive and are formed by elements on the left side of the periodic chart. – Anions are negative and are formed by Atoms, elements on the right side of the periodic Molecules, chart. and Ions 34 Spring 2016 Predicting Ionic Charges • An atom or molecule can lose more than one electron. • Many atoms gain or lose enough electrons to have the same number of electrons as the nearest noble gas (group 8A). • The number of electrons that an atom loses is related to its position on the periodic table. Atoms, Molecules, and Ions Ionic Bonds Transfer of electrons between species. They are generally formed between metals and nonmetals • Example:NaCl can be formed by the transfer of an electron from Na to Cl. By losing an electron Na and Cl become Na+ and Cl– respectively. • The Na+ and Cl– ions are attracted to form an ionic NaCl lattice, which crystallizes. Atoms, Molecules, and Ions 35 Spring 2016 Writing Formulas For Ionic Compounds Writing formulas for ionic compounds: • You need to know the ions of which it is composed. • The formula must reflect the electrical neutrality of the compound. You must combine cations and anions in a ratio so that the total positive charge is equal to the total negative charge. Atoms, Molecules, and Ions Example • Because compounds are electrically neutral, one can determine the formula of a compound this way: – The charge on the cation becomes the subscript on the anion. – The charge on the anion becomes the subscript on the cation. Atoms, Molecules, and Ions 36 Spring 2016 Common Cations Atoms, Molecules, and Ions Common Anions Atoms, Molecules, and Ions 37 Spring 2016 Chemical nomenclature • Chemical nomenclature is the naming of substances. • Chemical compounds are divided into organic compounds (those containing C, usually in combination with H, O, N, or S) and inorganic compounds (all other compounds). IUPAC nomenclature Atoms, Molecules, and Ions Names and Formulas of Ionic Compounds Positive Ions (Cations) • Cations formed from a metal have the same name as the metal. • Example: Na+ = sodium ion. • If the metal can form more than one cation, then the charge is indicated in parentheses in the name. • Examples: Cu+ = copper(I) ion; Cu 2+ = copper(II) ion. Atoms, Molecules, and Ions 38 Spring 2016 Names and Formulas of Ionic Compounds Positive ions: • An alternative nomenclature method uses the endings -ous and -ic to represent the lower and higher charged ions, respectively. • Examples: Cu+ = cuprous ion; Cu2+ = cupric ion. • Cations formed from nonmetals end in ium. • Examples: NH4+ = ammonium ion; Atoms, H3O+ = hydronium ion. Molecules, and Ions Names and Formulas of Ionic Compounds Negative Ions (Anions) • Monatomic anions (with only one atom) use the ending -ide. • Example: Cl– is the chloride ion. • Some polyatomic anions also use the -ide ending: • Examples: hydroxide, cyanide ions, bromide, nitride,.. Atoms, Molecules, and Ions 39 Spring 2016 Names and Formulas of Ionic Compounds • Polyatomic anions (with many atoms) containing oxygen are called oxyanions. • Their names end in -ate or -ite. (The one with more oxygen is called -ate.) • Examples: NO3– is nitrate; NO2– is nitrite. Atoms, Molecules, and Ions Patterns in Oxyanion Nomenclature • When there are two oxyanions involving the same elements: – The one with fewer oxygens ends in -ite • NO2− : nitrite; SO32− : sulfite – The one with more oxygens ends in -ate • NO3− : nitrate; SO42− : sulfate Atoms, Molecules, and Ions 40 Spring 2016 Oxyanion Nomenclature • When there are more than two oxyanions involving the same elements: • The one with the fewest oxygens has the prefix hypoand ends in -ite ClO− : hypochlorite • The one with the most oxygens has the prefix per- and ends in ate ClO4− : perchlorate The one with the second fewest oxygens ends in -ite ClO2− : chlorite The one with the second most oxygens ends in -ate ClO3− : chlorate Atoms, Molecules, and Ions Inorganic Nomenclature Atoms, Molecules, and Ions 41 Spring 2016 Inorganic Nomenclature Atoms, Molecules, and Ions Acid Nomenclature • If the anion ends in –ide,, change the ending to -ic acid and add the prefix hydro – HCl: hydrochloric acid HBr hydrobromic acid Atoms, Molecules, and Ions 42 Spring 2016 Acid Nomenclature • For oxyanions ending in ite change the ending to -ous acid • For oxyanions ending in ate change the ending to ic acid - HClO4: perchloric acid HClO3: chloric acid HClO2: chlorous acid HClO: hypochlorous acid Atoms, Molecules, and Ions Nomenclature of Binary Compounds • The less electronegative atom is usually listed first. • A prefix is used to denote the number of atoms of each element in the compound. Atoms, Molecules, and Ions 43 Spring 2016 Nomenclature of Molecular Compounds • The ending on the more electronegative element is changed to -ide. • CO2: carbon dioxide • CCl4: carbon tetrachloride • P2S5: diphosphorous pentasulfide Atoms, Molecules, and Ions Nomenclature of Binary Compounds If the prefix ends with a or o and the name of the element begins with a vowel, the two successive vowels are often elided into one: N2O5: dinitrogen pentoxide Atoms, Molecules, and Ions 44 Spring 2016 Simple Organic Compounds Organic chemistry is the study of carbon-containing compounds. • Organic compounds are those that contain carbon and hydrogen, often in combination with other elements. Atoms, Molecules, and Ions Alkanes • Compounds containing only carbon and hydrogen are called hydrocarbons. • Alkanes are hydrocarbons in which carbon is linked up to 4 hydrogen atoms by single bonds (i.e. they are saturated compounds) • Saturated hydrocarbons have the general formula CnH2n+2. They can be: linear (n≤3), or branched (n>3) Atoms, Molecules, and Ions 45 Spring 2016 Example • C5H12 pentane Isopentane Neopentane Atoms, Molecules, and Ions Derivatives of Alkanes • Functional groups are specific groups of atoms that replace hydrogen atoms on alkanes. New class of organic compounds are obtained: • Alcohols are obtained by replacing a hydrogen atom of an alkane with an –OH group (R-OH). • Alcohol names derive from the name of the alkane and have an -ol ending. • Examples: methane becomes methanol (CH3OH); ethane becomes ethanol (CH3CH2OH). Atoms, Molecules, and Ions 46