Chapter 2
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Transcript of Chapter 2
Chapter 2Atoms, Molecules and Ions
The Early History of ChemistryBefore 16th Century Greeks were the first to attempt to explain why
chemical changes occur. Alchemy: Attempts to change cheap metals into
gold. They invented the idea of atoms, that matter is not continuous. They discovered several elements and learned to prepare mineral acids.
The Early History of Chemistry
16th Century German develop the systematic metallurgy
(extraction of metal from ores) Swiss develop the medicinal application of
minerals17th Century
Robert Boyle: First chemist to perform quantitative experiments
Fundamental Chemical LawsLaw of Conservation of Mass (Antoine Lavoisier, 18th Century) – Mass is neither created nor destroyed Law of Definite Proportion (Joseph Proust, 19th Century) – A given compound always contains exactly the same proportion of elements by mass. This principle of constant composition of compounds is a law of definite proportion. Carbon tetra chloride is always 1 atom carbon per 4 atoms chlorine
Fundamental Chemical Laws
Law of Multiple Proportions (John Dalton, 19th Century) – When two elements form a series of compounds, the ratio of the masses of the second element that combine with 1g of the first element can always be reduced to small whole numbers. The ratio of the masses of oxygen in H2O and H2O2 will be a small whole number (“2”).
Dalton’s Atomic Theory (1808)1. Each element is made up of tiny particles called atoms2. The atoms of a given element are identical; the atoms of different elements are different in some fundamental
way or ways3. Chemical compounds are formed when atoms of
different elements combine with each other. A given compound always has the same relative numbers and types of atoms
4. Chemical reactions involve reorganization of the atoms – changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction.
Avogadro’s Hypothesis (1811)
At the same temperature and pressure, equal volumes of different gases contain the same number of particles
5 liters of oxygen and 5 liters of nitrogen contain the same number of particle
Thomson Atomic Model (1903) An atom consists of a diffuse cloud of positive
charge with the negative electrons embedded randomly in it. This model is often called plum (or raisin) pudding model.
Observed cathode ray (produced at the negative electrode and repelled by the negative pole of an applied electric field.
Cathode ray was a stream of negatively charged particles now called electrons
Deflection of Cathode Rays by an Applied Electric Field
The Plum Pudding Model of the Atom
Rutherford Atomic Model (1911)
An atom with a dense center of positive charge (the nucleus) with electrons moving around the nucleus at a distance that is large relative to the nuclear radius. Nucleus is very small compared with the overall size of the atom. Nucleus is extremely dense, accounts for almost all of the atom’s mass.
RadioactivitySpontaneous emission of radiation Gamma () rays: high energy light Beta () particles: high speed electron Alpha () particles (He2+): 2+ charge,
charge twice that of electron and with opposite sign. The mass of an -particle is 7300 times that of the electron
Rutherford’s Experiment on particle Bombardment of Metal Foil
Expected and Actual Results of Rutherford’s Experiment
The Modern View of Atomic Structure
The atom contains: Electrons: move around the nucleus (mass:
9.11 X 10-31 kg, Charge 1-) Protons: found in the nucleus, they have a
positive charge equal in magnitude to the electron’s negative charge (mass: 1.67 X 10-
27 kg, charge 1+) Neutrons: found in the nucleus, virtually
same mass as a proton but no charge. (mass: 1.67 X 10-27 kg, charge: 0)
Nuclear Atom Viewed in Cross Section
The Chemists’ Shorthand Atomic Symbols
Atomic number (Z): number of protons, gives the symbol of the element (X)Mass number (A): Total number of protons and neutronsElemental form = Zero net chargeTherefore, # electrons = # of protons
K Element Symbol39
19
Mass number
Atomic number
IsotopesAtoms with the same number of protons but different number of neutrons. In nature mostelements contain mixtures of isotopes
2311Na : 11 protons, 11 electrons, and 12 neutrons
2411Na : 11 protons, 11 electrons, and 13 neutrons
Two Isotopes of Sodium
Molecules and Ions Chemical Bonds: The forces that hold atoms
together in compounds. H2O, NO, CO2
Covalent bonds: Covalent bonds result from atoms sharing electrons. Cl2
Ionic bonds: Force of attraction between oppositely charged ions.
Molecule: A collection of covalently-bonded atoms. H2, O2
Ions Ions: An ion is an atom or group of atoms
that has a net positive charge or negative charge particle (an unequal number of protons and electrons) is obtained by removing or adding electrons. Na+, Cl-
Cation: A positive ion (Na+, Mg2+, NH4+)
Anion: A negative ion (Cl-, SO42-)
Formulas Chemical Formula: In which the symbols for
the elements are used to indicate the types of atoms present and subscripts are used to indicate the relative numbers of atoms.CO2 indicates each molecule contains 1 atom of carbon and 2 atoms of oxygen.
Structural Formula: In which the individual bonds are shown by lines. It may or may not indicates the actual shape of the molecules.O=C=O
Periodic Table Periodic table is organized based on the
properties that elements have in common with one another.
Groups: Elements in the same vertical columns are in the same group have similar chemical properties.
Group 1A: Alkali metals: Li, Na, K, Rb, Cs, Fr Group 2A: Alkaline earth metals: Be, Mg, Ca, Sr, Ba,
Ra Group 7A: Halogens: F, Cl, Br, I, At (astatine) Group 8A: Noble gases: He, Ne, Ar, Kr, Xe, Rn (radon)
The Periodic Table
Periodic Table Periods: The horizontal rows of elements in the
periodic table are called periods. First period: horizontal row one contains H and He Second period: row two contains Li through Ne Letters in the boxes are the symbols for the
elements Abbreviations are based on the current element
names or the original names. The number above each symbol is the atomic
number (number of protons)
Periodic Table Most of the elements are metals in the periodic
table. Metals: Conduction of heat and electricity,
malleability, ductility, lustrous, form positive ions
Nonmetals: appear in the upper right hand corner of the periodic table except hydrogen. Nonmetals lack the physical properties that characterize the metal, gain electrons in chemical reaction and form negative ions, form covalent bond to each other.
Naming Compounds Binary Compounds: Compounds composed of
two elements Binary Ionic Compounds (Type 1): contains a
positive ion (cation) always written first in the formula and a negative ion (anion)
Rules: 1. The cation is always named first and the anion
second2. A monatomic (meaning one atom) cation takes its
name from the name of the element3. A monatomic anion is named by taking the root of
the element name and adding –ide
Binary Ionic CompoundsCompound Ions Present Name
NaCl Na+, Cl- Sodium chloride
KI K+, I- Potassium iodide
CaS Ca2+, S2- Calcium sulfide
Li3N Li+, N3- Lithium nitride
CsBr Cs+, Br- Cesium bromide
MgO Mg2+, O2- Magnesium oxide
Naming compounds Binary Ionic Compounds (Type II): Metals that
form more than one type of positive ion. Fe2+ and Fe3+
Transition metals form several positive oxidation states
Charge on the metal ion must be specified Roman numeral indicates the charge of the cation.
Iron (II) chloride and iron (III) chloride The ion with the higher charge has a name ending
in –ic and the one with the lower charge has a name ending in –ous; ferrous chloride and ferric chloride
Naming CompoundsIonic Compounds with Polyatomic Ions: Need to know the names of the polyatomic ions (Table
2.5). NH4
+ ammonium, SO42- sulfate
Na2SO4 Sodium sulfate KH2PO4 Potassium dihydrogen phosphate Fe(NO3)3 Iron(III) nitrate CsClO4 Cesium perchlorate NaOCl sodium hypochlorite Al2(Cr2O7)3 Aluminum dichromate Sr(CN)2 Strontium cyanide
Naming compoundsBinary Covalent Compounds (Type III):Formed between two nonmetalsRules: The first element in the formula is named first,
using the full element’s name Second element is name as if it were an anion Use prefixes to denote the number of atoms
present Never use mono – prefix for naming the first
element
CO ==> carbon monoxide, not monocarbon monoxideP2O5 ==> diphosphorus pentoxide
S2Cl4 ==> disulfur tetrachloride
NO2 ==> nitrogen dioxide
N2O5 ==> dinitrogen penoxide
Common Cations and Anions
Formulas from Names
Name Chemical FormulaDiphosphorus pentasulfide P2S5 (two non metals)
Cesium peroxide Cs2O2 (Cs1+, O22-)
Aluminum fluoride AlF3 (Al3+, F-1)
Vanadium (v) fluoride VF5 (V5+, F-1)
Dioxygen difluoride O2F2 (two non metals)
Gallium oxide Ga2O3 (Ga3+, O2-)
Ammonium dichromate (NH4)2Cr2O7 (NH4+, Cr2O7
2-)
Cupric phosphate Cu3(PO4)2 (Cu2+, PO43-)
Flowchart for Naming Binary Compounds
Acids When dissolved in water produce a solution
containing free H+ ions (protons) An acid is a molecule with one or more H+
ions attached to an anion If the anion does not contain oxygen, the
acid is named with the prefix hydro – and the suffix –icHCl Hydrochloric acidHCN Hydrocyanic acid
Acids If the anion contains oxygen, the acidic name is
formed from the root name of the anion with the suffix of –ic or –ous depending on the anionHNO3 Nitric acid (Nitrate anion)H2SO4 Sulfuric acid (Sulfate anion)H3PO4 Phosphoric acid (Phosphate anion)HC2H3O2 Acetic acid (Acetate anion)H2SO3 Sulfurous acid (sulfite anion)HNO2 Nitrous acid (nitrite anion)
Flowchart for Naming Acids
Summary Laws Dalton’s atomic Theory Avogadros’ Hypothesis Various Atomic Models Radio Activity (, , rays) Atomic Symbol (atomic #, mass#, isotopes) Chemical Bonds (covalent bonds, Ionic bonds) Periodic Table Naming various types of compounds Formulas Acids