Atoms, Molecules, and Ions

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Chapter 2

• Atoms, Molecules, and Ions

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Daltons Atomic Theory

• Elements are composed of ATOMS
• Atoms of each element are identical atoms of
  different elements are different
• Atoms cannot be created nor destroyed
  (conservation of mass)
• Compounds are comprised of more than one atom
  the relative number of atoms in any compound is
  the same

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A Brief History of the Atom

• Mid-19th Century Electric potential (voltage)
  applied to a vacuum tube produced radiation
• 1897 J.J. Thompson publishes conclusion naming
  the electron
• Thomson also calculated the charge/mass ratio of
  electron
• 1909 R. Millikan measures charge per electron,
  leading to accurate determination of its mass
• (q 1.60 x 10-19 C mass 9.10 x 10-28g)
• Small mass suggests more matter in atoms

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Millikans Oil Drop Experiment
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A Brief History of the Atom, continued

• Late 19th Century Becquerel, Curies investigate
  radioactive emissions discover and describe
  alpha, beta, and gamma rays
• Adds to the evidence that the atom as smallest
  particle may not be the whole story
• 1910 Ernest Rutherford et al. conduct
  gold-foil experiment show most of mass, all
  positive charge to reside in a small region of
  the atom

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Rutherfords Gold Foil Experiment
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Modern View of the Atom

• Proton has charge equal in magnitude, opposite in
  sign, of electron (For convenience, 1 and -1 are
  used to express the charges on a proton and
  electron, respectively)
• Neutron has a mass similar to that of a proton
  charge zero
• Mass (proton) ? mass (neutron) gtgt mass (electron)
• Atoms are comprised of a nucleus, which contains
  protons and neutrons, and electrons most of the
  atom is thus empty space
• 1 amu (u) 1.66054 x 10-24 g 1 g 6.02214 x
  1023amu

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Atomic Structure

• Atomic number number of protons
• Mass number number of protons number of
  neutrons
• Number of protons number of electrons (all
  atoms are neutral in charge!)

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The atomic mass unit (amu) defined

• The definition of the unit arises from assigning
  12 amu (exactly) as the mass of 1 12C atom
• All other masses are relative to 12C
• Isotopes Most atoms exist as mixtures of
  different isotopes (same atomic number, different
  mass number)
• Atomic masses (in amu) reported on the periodic
  table are weighted averages of all naturally
  occurring isotopes

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Example

• Consider the following data for silicon (Si)

If the atomic mass of Si is reported on the
periodic table as 28.0855 amu. What is the atomic
mass of 30Si?
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The Worlds Largest Cheat Sheet

• Elements behaviors follow predictable trends
• Arrangement of elements in order of increasing
  atomic number displays periodicity
• Rows PERIODS
• Numbered 1-7
• Columns GROUPS

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Chemical Formulas

• Subscripts indicate number of atoms in molecule
  (molecular formula)
• Always in definite proportions of whole numbers
• Simplest ratio of atoms is called empirical
  formula
• Subscripts are not arithmetically reducible
• e.g.,
• C6H12O6 is molecular formula of glucose
• CH2O is empirical formula
• NOTE Subscripts in molecular formula are always
  integral multiple of those in empirical formula
• Structural formula shows connectivity of atoms
• Lines describe bonds between atoms (more to come
  on this)

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Ions and Ionic Compounds

• Electrons are reactive particles in atoms
• Atoms can lose/gain electrons resulting in
  formation of IONS (charged particle)
• Result of LOSS of e-
• more protons than e- ? positive charge (CATION)
• Result of GAIN of e-
• More e- than protons ? negative charge (ANION)
• Charges are written as superscripts
• More than one e- can be lost/gained, resulting in
  charges greater than 1 (in magnitude)

Examples How many protons, neutrons and
electrons in Na? Cl-? Mg2 ? Al3 ?
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Charges on ions can be predicted

• Atoms easily gain/lose electrons such that they
  have the same number as the Inert (noble) gases
• (more to come on this, too)

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Ionic Compounds

• Usually composed of metal (cation) nonmetal
  (anion)
• Affinity of oppositely charged ions is GREAT
• Transfer of electrons from one atom to another
  (such that both achieve a noble gas number of e-)
  results in ionic compound.
• e.g., Na Cl NaCl
• Mg Cl MgCl2
• Sum of charges in a neutral compound must be zero
• e.g., Mg has 2 Cl has 1-. Thus there must be 2
  Cl- for every Mg (2 x (-1)) (2) 0

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Nomenclature INaming Inorganic Compounds

• Some ions contain more than 1 atom Polyatomic
  ions (Uh, ohstuff to memorize!)

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Rules for naming inorganic compoundsCations

• Cation has same name as parent atom
• Na (sodium ion) Mg2 (magnesium ion) etc.
• For metals that can form more than one cation
  (e.g., Fe2 or Fe3 Cu or Cu2)
• Roman numerals are used to distinguish them
• (e.g., iron (II) ion or iron (III) ion copper
  (I) ion or copper (II) ion)
• Suffix ous is used for lower charge ic is
  used for greater
• (e.g., ferrous or ferric cuprous or cupric)
• Non-metal cations end in ium
• (e.g., ammonium (NH4)

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Rules for naming inorganic compoundsAnions

• Monatomic anions end in ide
• Cl- (chloride) Br- (bromide) S2- (sulfide)
• Exceptions OH- (hydroxide) CN- (cyanide) O22-
  (peroxide)
• Oxyanions (containing O)
• end in ate or ite (same charge as on ate,
  but one less Oconfused yet?)
• NO3- (nitrate) and NO2- (nitrite)
• SO42- (sulfate) and SO32- (sulfite)
• Prefixed by per (as in hyper, meaning MORE)
  or hypo, meaning LESS, both referring to the
  number of oxygen (how about now?)
• ClO4- (perchlorate) ClO3- (chlorate) ClO2-
  (chlorite) ClO- (hypochlorite)

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Rules for naming inorganic compoundsAnions(cont
inued)

• Oxyanions containing H
• Hydrogen________ or dihydrogen________
• HCO3- (hydrogen carbonate) H2PO4- (dihydrogen
  phosphate) (also called bicarbonate

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Rules for naming inorganic compounds(continued)

• Ionic Compounds
• Cation name first, then anion
• NaCl (sodium chloride) KNO3 (potassium nitrate)
  Li2SO4 (lithium sulfate)
• Acids (Hn anionn-)
• Prefix with hydrogen change ide ending of
  anion to ic
• HCl (hydrochloric acid) HNO3 (nitric acid)
• Molecular compounds
• Greek numerical prefixes used to show number of
  atoms
• Left of PT goes first
• EXCEPTION Oxygen always last, except when with F