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Chapter 2
Atoms, Molecules, and Ions
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Atoms
Atomic theory : on the basis of Dalton’s theory
Components : proton, neutron, electron
Atomic Number : number of protons
Mass Number : number of protons and neutrons
Nuclear Stability : radioactivity
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Atomic Theory
Postulates
Elements consist of tiny particles called atoms which retain their identity in reactions
In a compound, atoms of two or more elements are combined in a fixed ratio of small whole numbers; e. g. 1:1, 2:1, etc.
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Some features of Dalton’s theory
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Some features of Dalton’s theory
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Cathode ray tube
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Rutherford’s scattering experiment
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Components
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Atomic Number
Atomic number
equals number of protons in nucleus
equals number of electrons in neutral atom
is a characteristic of a particular elementall H atoms have 1 proton, all He atoms have 2 protons, etc.
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Mass Number
Mass numberequals number of protons + number of neutronsatoms of the same element can differ in mass number
atomic nuclear massprotons neutrons number symbol number
carbon-12 6 6 6 12
carbon-14 6 8 6 14
126
C
146
C
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Nuclear Stability
Neutron-to-proton ratio increases with atomic number.
Unstable isotopes decompose by radioactivity. Ordinarily the result is the transmutation of elements
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Nuclear Stability - Belt of stability
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Periodic table
Structure : period (horizontal rows) & group (vertical columns)
Metals, Nonmetals, & Metalloids
metal : high electrical conductivity
metalloid : intermediate electrical conductivity between
metal and nonmetal
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Structure
Periods : horizontal rows
Groups : vertical columnsnumbering system for groups
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Metals, Nonmetals, Metalloids
MetalsAbout 80 elementslocated at lower left of periodic table
NonmetalsAbout 18 elementslocated at upper right of periodic table
Metalloids
B, Si, Ge, As, Sb, Te
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Molecules
Composition : isolated atoms and combined atomes
Types of Formulasmolecular : number of atoms of each element is indicated by a
subscript written after the symbol of the elementstructural : bonding pattern within the moleculecondensed structural : bonding pattern in the molecule and
highlights the presence of a reactive group
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Composition
Two or more atoms may combine with one another to form an uncharged molecule. Usually nonmetal atoms made up molecules.
Within the molecule, atoms are held together by covalent bonds
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Types of Formulas
Consider the molecule ethaneMolecular formula: C2H6
Structural formula:
Condensed structural formula: CH3CH3 ( -CH3 is alkyl)
H
H
H
H
HH
CC
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Molecular elements and their physical states
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Ions
Formation of Monatomic Ions
cation (positively charged), anion (negatively charged)
Charges of Monatomic Ions with noble-gas structure
atoms form ions that contain the same number of
electrons as the neighboring noble-gas atom
Polyatomic Ions
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Ions
Formulas of compoundsthe total positive charge of the cations in the formula must equal the total negative charge of the anions
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Formation of Monatomic Ions
Formation
Cation : Na atom (11p+, 11e -) → Na+ ion (11p+, 11e -) + e -
Anion : F atom (9p+, 9e -) + e - → F - ion (9p+, 10e -)
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Charges of Monatomic Ions with noble-gas structures
Cation: Group 1 (+1); Group 2 (+2); Al3+
Anion: Group 16 (-2); Group 17 (-1); N3-
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Formulas of compounds
Apply principle of electroneutralitycalcium fluoride: Ca2+, F- ions: CaF2
aluminum nitrate: Al3+, NO3- : Al(NO3)3
sodium dihydrogen phosphate: Na+, H2PO4- : NaH2PO4
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Names of Compounds
Ionic : consist of two words – first names the cation and the
second names the anion
Binary Molecular Compounds : when two nonmetals combine
with each other, the product is most often a binary
molecular compounds
Acids : form H+ ions
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Ionic
Name cation followed by anion
Note that with transition metal cations, charge is indicated by Roman numeralNa2SO4 sodium sulfateFe(NO3)3 iron (III) nitrate
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Ionic (cont.)
Systematic names of oxoanions-ate, -ite, per-, hypo--per (largest number of oxygen atoms) > -ate > -ite > -hypeCalcium hypochlorite: Ca(ClO)2
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Binary Molecular Compounds
The first word gives the name of the element that appears first in the formula and the second. Greek prefixes are used to show the number of atoms of that element in the formula
SF6 sulfur hexafluorideN2O3 dinitrogen trioxide
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Acids
Binary Acids: containing H atoms ionize in water to form H+ ionshydrochloric acid (HCl)
Oxoacids: simply related to those of the corresponding oxoanionsate salt → ic acidite salt → ous acid
Examples:HClO4 perchloric acidHClO hypochlorous acid