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Chapter 4: Elements, Atoms, and Ions

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Conclusions from Dalton's atomic theory. Law of constant composition ... Atoms are indeed breakable, contrary to Dalton's theory ... – PowerPoint PPT presentation

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Title: Chapter 4: Elements, Atoms, and Ions


1
Chapter 4 Elements, Atoms, and Ions
Chemistry 1020 Interpretive chemistry Andy
Aspaas, Instructor
2
Elements
  • All substances on earth are made from a
    combination of 114 or so elements
  • 88 found in nature, others are man-made
  • Abundance found in nature
  • Oxygen most abundant on earth in human body (by
    mass)
  • Abundances vary in different parts of environment
  • Each element has its own symbol
  • One or two letters - first is always capitalized

3
Daltons atomic theory
  • According to Dalton
  • Elements are composed of tiny unbreakable
    particles called atoms
  • All atoms of a given element are identical
  • Atoms of a given element are different from those
    of any other element
  • Atoms of one element combine with atoms of other
    elements to form compounds
  • Atoms are indivisible by chemical processes

4
Conclusions from Daltons atomic theory
  • Law of constant composition
  • All samples of a pure compound contain the same
    proportions of the elements
  • Chemical formulas used to show those proportions
  • Law of conservation of mass
  • Atoms are never created or destroyed, only
    rearranged
  • Atoms of one element cannot change into atoms of
    another element
  • All atoms present at the beginning of a reaction
    are present at the end

5
Compounds and chemical formulas
  • A compound is a pure substance that is composed
    of atoms of two or more elements
  • Compounds are described by giving the number and
    type of each atom in the simples unit of the
    compound
  • Each element represented by letter symbol
  • Quantity of atoms of each element written as
    subscripts (subscript 1 never written)
  • Polyatomic groups in parentheses if more than one

6
Structure of the atom
  • J.J. Thomson investigated beams known as cathode
    rays
  • Made of tiny negatively charged particles called
    electrons
  • Smaller than a hydrogen atom!
  • Atoms of different elements produce the same
    electrons

7
Thomsons plum pudding model
  • Atoms are indeed breakable, contrary to Daltons
    theory
  • Electrons are suspended in a positively charged
    electric field (to balance electrons neg.
    charge)
  • Mass of atom is due mostly to electrons
  • Atom is mostly empty space

8
Rutherfords gold foil experiment
  • Tried to prove Plum Pudding model
  • Shot bullets of alpha particles through thin
    sheet of gold atoms
  • Expected alpha particles to fire straight through
  • Most indeed did
  • But about 2 were deflected by very large angles
  • Disproved Thomsons plum pudding model

9
Rutherfords nuclear model
  • Most of atoms mass is in a tiny dense center
    called the nucleus
  • Positively charged
  • Only 1/10 trillionth the volume of the atom
  • Nucleuss positive charge balances electrons
    negative charge
  • Electrons fly around in the empty space
    surrounding nucleus

10
The modern atom
  • Nucleus composed of two types of particles
  • Protons 1 charge
  • Neutron 0 charge, mass similar to proton
  • Electrons -1 charge, outside of nucleus

11
Isotopes
  • Number of protons defines which element an atom
    is
  • Called atomic number, found on periodic table
  • Atoms of an element with different numbers of
    neutrons are called isotopes
  • All isotopes of an element behave identically in
    chemical reactions
  • But have different masses
  • Identified by mass number
  • Mass number protons neutrons
  • Isotope symbols contain mass number, atomic
    number, and element symbol

12
Elements
  • Arranged in pattern called Periodic Table
  • Properties can be predicted based on position in
    table
  • Metals
  • About 75 of elements
  • Lustrous, malleable, ductile, conduct heat and
    electricity
  • Nonmetals
  • Dull, brittle, insulators
  • Metalloids or semi-metals
  • Properties of both metals and nonmetals

13
Groups and periods
  • Elements with generally similar chemical and
    physical properties are in the same column
  • Columns are called groups or families
  • Different numbering schemes
  • Rows are called periods

14
Regions of the periodic table
  • Main group representative elements
  • A columns in label
  • Transition elements
  • All metals
  • Bottom rows Inner transition elements or Rare
    earth elements
  • All metals
  • Inserted into periodic table after La and Ac

15
A few important groups
  • Group 8A Noble gases
  • Colorless gases at room temperature
  • Non-reactive (inert)
  • Found in nature as single atoms uncombined
  • Group 7A Halogens
  • Very reactive nonmetals
  • Exist as diatomic molecules in nature (Cl2, Br2,
    etc)
  • React with metals to form ionic compounds

16
Allotropes
  • Some solid nonmetallic elements can exist in
    different forms with different physical
    properties
  • Allotropes these different forms
  • Different physical properties are from different
    arrangements of atoms in the solid
  • Allotropes of carbon
  • Diamond
  • Graphite
  • Buckminsterfullerene (C60)

17
Ions
  • Ions are atoms or groups of atoms that carry an
    electrical charge
  • Cations positive charge (loss of electrons)
  • Anions negative charge (gain of electrons)
  • Unlike charges attract, so cations and anions are
    attracted to each other
  • Electrolytes ions that dissolve in water and
    cause it to conduct electricity
  • Ionic compound combination of cations and anions
    that form a pure substance with no total charge

18
Atomic structures of ions
  • Metals always form cations
  • Loss of one electron creates a 1 charge
  • Na 11 e-, Na 10 e-
  • Ca 20 e-, Ca2 18 e-
  • Cations have same name as uncharged metal
  • Charge can be determined from group number

19
Atomic structures of ions
  • Nonmetals form anions
  • Gain of 1 electron -1 charge
  • Anions named by changing ending of element name
    to -ide
  • Fluorine F 1 e- F- (fluoride ion)
  • Oxygen O 2 e- O2- (oxide ion)
  • Charge determined by taking 8 - group number

20
Writing ionic formulas
  • Ionic compound must have no net charge
  • So change number of ions to cancel out positive
    and negative charges
  • Compound made of Mg2 and Cl-
  • Must have formula MgCl2
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