PowerPoint Presentation by HC121003051458

VIEWS: 0 PAGES: 41

									Practice

How many significant figures in each of the following?
                          138.20
                          0.6040
                       1.300 x 10-4
                           1050
                        1.06 x 103
How many significant figures are allowed in each of the
answers to the following arithmetic problems:
126.23 x 0.10
0.6831 + 125.26
3.632 / 0.0061
28 + 0.75
                                                          1
Last Time

   Use of significant figures
   Units and Measurement

Today
  Dimensional analysis
      using units to guide calculations

  Atoms and Subatomic Particles, Chapter 5




                                             2
Units / Dimensions

 Equivalence Statement
     -relates one unit to another
     1.00 metre = 100.0 cm        1 inch = 2.54 cm

 Conversion Factors
      -relates one unit to another as a ratio for use in
      mathematical operation
How many cms are in 3.75 metres?
    100.0 cm        or     1.00 metres
   1.0 metres                100.0cm


    3.75 metres x 100.0 cm  375cm
                   1.00 m                                  3
Dimensional Analysis

    A problem solving technique using the units as a guide
     -use the units in calculations
     -identify the given information, number and units
     -identify the units required for the correct answer
     -set up equivalence statements and or conversion factors
     -do the math, cancel out units where possible


      If the solution has been set up correctly, the correct units
      should be generated in the process.


                                                                4
Physical Properties as Conversion Factors

 Density       mass             g     g      kg
             unit volume        mL    cm3     L

   Density is specific for temperature, that is density
   changes with temperature. Standard is 25 oC


 Specific Gravity

       density of a substance     g/mL
                                            unitless
         density of water         g/mL



                                                          5
Density

 A copper (Cu) coin has a mass of 3.14 g. The density of copper is
   8.96 g/cm3. What is the volume of the coin?
given quantities and units:
                 mass 3.14 g   density 8.96 g/cm3

quantity and units required:     volume: cm3

equivalence statement:    1 cm3 = 8.96 g

set up conversion factor: 1 cm3 / 8.96 g     or     8.96 g / 1 cm3

calculation and cancel units:                 3

                                3.14 g x 1 cm  0.350 cm
                                                         3

                                         8.96 g

significant figures: 3 sig figs allowed in answer
                                                                      6
Example:
Percent as a Conversion Factor

 How many grams of water are contained in 65.3 g of a mixture of water and
  alcohol that is 34.2% H2O by mass?

   Given:         65.3 g of mixture , 34.2 % water ; 100 g mix = 34.2 g water

   Required:      grams of water

   Conversion factor:      34.2 g water         100g mixture
                          100 g mixture         34.2 g water
   Calculation:       34.2 g water
                                     x 65.3 g of mixture
                     100 g mixture

   Significant figures: 100 comes from 100%, considered an exact
                         number. Answer allowed 3 sig figs
                                                                          7
Atoms and Subatomic Particles
Chapter 5

     Element: a pure substance that cannot be broken down
      into simpler pure substances


     Compound: a pure substance that can be broken down to
      two or more simpler substances



     Atom: the smallest particle of an element that still has the
      properties of the element



                                                               8
John Dalton         19th century

 Experimental observations:
    -most natural materials are mixtures of pure substances


    -pure substances are either elements or compounds

    -a given compound always contains the same proportion,
    by mass of the elements
    (The law of definite proportions)

   e.g.:
   water always contains 8 grams of oxygen for every 1 gram
   of hydrogen

                                                              9
Dalton’s Atomic Theory               (original early 1800’s) (Atomic
Theory of Matter) pg. 123

                           Modern Version
 All matter is made up of small, neutral particles called atoms. There
  are 112 different types of atoms. Each type corresponds to a different
  element.
 All atoms of a given element are similar to one another.
 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.
 A compound always has the same relative numbers and types of atoms.
 During a chemical reaction, changes occur only in the way atoms are
  grouped together
   ie: atoms are neither created nor destroyed in chemical reactions


                                                                           10
Prediction:

Atoms of a given pair of elements could combine in different
proportions and produce different compounds.

e.g:   N nitrogen and O oxygen
       1:1     NO    nitric oxide, colourless gas

       1:2     NO2 nitrogen dioxide, brown gas

       2:1     N2O nitrous oxide, colourless gas, (laughing gas)

        3 different pure substances (compounds) each with its own
distinct set of chemical and physical properties but made up from the
same two elements.
                                                               11
Charge

 A piece of amber rubbed with cloth will attract some
  objects to it and repel others.
 A glass rod rubbed with silk will attract some objects and
  repel others.
 The glass rod will repel other glass rods that have been
  rubbed with silk; the amber will repel other pieces of
  amber that have been rubbed with cloth. The glass rod and
  the amber will attract each other.
 The amber and the glass rod have become “charged”
 By convention the amber is assigned a NEGATIVE
  charge.
 The glass rod has a POSITIVE charge
 Like charges repel, opposite charges attract
                                                               12
Structure of the Atom -Subatomic Particles

   Electrons:       1890’s J. J. Thomson

      observations:
      -atoms of any element could be made to emit tiny negative
      particles
      -same (-)ve particle no matter which element

      conclusion:
      -all atoms contain negative particles (called electrons),
      that have almost negligible mass




                                                                  13
J.J. Thomson (1856-1940):


» experimented with Cathode-ray tubes
» beam could be deflected with an applied electrical field or
applied magnetic field
» deflected in manner expected for negatively charged particles




                                                            14
Last Time

 Dalton’s Atomic Theory
 Subatomic Particles
   – JJ Thompson, cathode rays, electrons



Today
 other subatomic particles
 modern view of the atom
 describing an atom




                                            15
J.J. Thompson

 Conclusions:
   – cathode rays are negatively charged fundamental
     particles of matter found in all atoms
   – all atoms contain these negative particles (electrons)
   – measured charge-to-mass ratio of an electron

   – http://www.howstuffworks.com/tv2.htm




                                                              16
J.J’s Atomic Model

   -electrons distributed randomly in a diffuse positive
   cloud.
   -“plum pudding” model: raisins dispersed in pudding.




           Plum Pudding model
                                                           17
Subatomic Particles

   Nucleus:      1911 Ernest Rutherford
     -atoms are neutral therefore there must be a positive
     component

     experiment:
     -directed positive alpha particles (heavy particles with a
     +2 charge) toward a thin metal foil

     prediction:
     -large, fast alpha particles would pass straight through with
     no deflection



                                                              18
Rutherford’s gold foil experiment
  02_24




                                                   Some alpha            Most particles
                                                   particles are         pass straight
    Uranium source of                              scattered             through foil
    alpha particles (embedded
    in a lead block to absorb
    most of the radiation)




                                                                        Thin
                                Beam of
                                                                        metal foil
                                alpha particles   Luminescent screen
                                                  to detect scattered
                                                  alpha particles
                                                       particles




                                                                                     19
Subatomic Particles

  observations:
   -most alpha particles passed straight through
   -some were deflected with large angles
   -some were reflected straight back



  conclusion:
   -there must be a large centre of concentrated positive
   charge in the atom



                                                            20
Subatomic Particles: The Nucleus

    Nucleus:
     -dense centre of positive charge surrounded by moving
     (-)ve electrons.
     -positive charge must balance the negative charge of the
     total number of electrons

    Protons:
     -particle of (+)ve charge, contained in the nucleus
     -same magnitude of charge as the electron (e-)
     -much greater mass than an electron (~1800 x mass of e-)
     -number of protons must be equal to the number of
     electrons
                                                            21
Subatomic Particles

 Neutrons: 1932 Chadwick
    -most nuclei also contain neutral particles called neutrons

    -slightly larger mass than a proton but no charge

    -neutrons and protons collectively called nucleons



    - some elements can have atoms with different numbers
       of neutrons


                                                             22
Revise Dalton’s Atomic Theory


  All atoms of the same element contain the same number of
  protons and electrons.

                            but

   Atoms of a given element may have different numbers of
                         neutrons.

  All atoms of a given element are similar to one another



                                                             23
Modern View of the Atom

The Nuclear Atom
 nucleus: -dense centre of positive charge
             -contains (+)ve charged particles, protons,
             and neutral particles, neutrons
 electrons: -particles of (-)ve charge, fill the space around
             the nucleus

 mass: - protons and neutrons make up most of the mass of
        an atom

 neutral: - same number of protons and electrons


     The chemistry of an atom arises from its electrons          24
Description of an Atom

  Atomic Number
      -characteristic of the element
      -equal to the number of protons in the nucleus
       (therefore also equal to the number of electrons in the
       neutral atom


  Mass Number
      -equal to the number of protons + the number of
       neutrons in the nucleus of the atom



                                                            25
Representation of an Element


                         A
                         Z   X

  Where X is the symbol of the element
  A is the mass number (always a whole number)
  Z is the atomic number

          12        14           16
           6   C     7   N        8   O


  so this carbon has 6 protons, 6 electrons and 6 neutrons
                                                             26
Isotopes
      -atoms of the same elements with the same number of
      protons, but with different number of neutrons
     12        14        1                 2     3
      6   C,    6   C    1   H,         H, 1     1H
                                  D, deuturium   T, tritium


Isobars
       -atoms that have the same mass number but different
       atomic number (so they are different elements)
                        14        14
                         6   C,    7   N

                                                              27
Mass of an Atom

    Atomic Mass (weight)
     eg: one atom of 12C weighs 1.99 x 10-23 grams

    Atomic Mass Unit – AMU
        a unit of mass, set relative to a standard mass

   definition: mass of 1 atom of   12C   = 12.00000..amu

     12.0000 amu = 1.99 x 10-23 grams
            1 amu = 1.66 x 10-24 grams



                                                           28
Atomic Masses of the Elements

  atomic masses of the individual elements are generally
   shown on the periodic table

  represents the average mass, in amu, of an atom of the
   element, considering the relative abundance of the isotopes

 Atomic mass of C given as 12.01 amu
   12C 98.98%          12.000 amu            11.866
   13C 1.11%           13.003 amu             0.144
   14C <0.01 %         14.003 amu             0.001
    Weighted average, atomic mass of C: 12.011 amu


                                                            29
Other Atomic Species

 Ions
   -obtained when electrons are added to, or removed from a
   neutral species

 Cation
   -a positively charged ion
   -the result of removing 1 or more electrons from a neutral
   species
                      Li  Li+ + e-
 Anion
   -a negatively charged ion
   -the result of a neutral species gaining one or more electron
                        O + 2e-  O2-                          30
Ions

  Simple ions:
   – charged species containing only one atom:
      eg:    Cl- , Na+ , Ca2+, O2-


  Polyatomic ions:
   – charged species containing a group of atoms held
     together by strong bonds:

       NO2-, CN- , NH4+, SO42-

       see Table 8.4 Chapter 8 page 263

                                                        31
 The formation of ions is only ever due to the addition or
  loss of electrons.

 Protons cannot be removed or added to atomic or other
  species under normal chemical circumstances




                                                              32
Structural Units of Pure Substances


 the smallest units:

          atoms      Add or remove electrons    ions
         (neutral)                        (charged species)




These small units combine in various ways to produce pure
substances



                                                              33
Structural Units of Pure Substances

1) Combinations of Neutral Atoms
       atoms are the building blocks of matter
   a) Elements
       -contain only one kind of atom, grouped together
       -an extended array of individual atoms
       Atomic solids
             eg: gold, carbon , iron

       Atomic gases
             eg: argon , neon

       Atomic liquid
             eg: mercury
                                                          34
Structural Units of Pure Substances

b) Molecules:
   -group of two or more atoms that function as a unit
   (molecule)
   -atoms are tightly bound together in the unit
   -each molecule behaves as an individual unit or particle
      Elements
         -all atoms are the same, atoms are grouped in
         molecules
         eg: O2 oxygen, N2 nitrogen, Cl2 chlorine
      Compounds
        -more than one type of atom in the molecule
        H2O, CO2 ,
                                                              35
Structural Units of Pure Substances

2) Combination of ions:

     Ionic Compounds
        -from combinations of anions and cations to
        form neutral species

     eg: Na+ and     Cl- give NaCl

          Mg2+ and Cl- give MgCl2




                                                      36
Natural States of Elements

 Most elements are very reactive and so tend to exist as
  compounds
 Some are found in pure form in nature

 Noble Metals
    -inert, that is unreactive, very stable
    - eg: gold, silver, platinum, palladium

   Noble (Inert)Gases
    -eg: helium, neon, argon, krypton, xenon

   Air
     -oxygen, nitrogen, hydrogen
                                                            37
Writing Chemical Formula

Chemical Formula
  -chemical representation of a substance
  -use chemical symbols of the atoms
  -gives some indication of the relative numbers and types of
  atoms or ions in that substance




                                                            38
Chemical Formula
 Elements (extended array of atoms)
     -represented by the element symbol
     Ag, Au, Ne, Fe

 Molecular compounds
    -element symbols plus numerical subscript giving the
    number of atoms of each element present in one
    molecular unit
   Elements: molecular units: H2 O2 N2 F2 Cl2 Br2 I2
                              (P4) (S8)
   Compounds: eg: H2O, CCl4,, P2O4
     -formula does not give any information about how the
     atoms are arranged or joined together, or anything
     about the shape of the molecule                        39
Chemical Formula

  Ionic Compounds
    -represent the simplest whole number ratio of anions to
       cations
    charge must balance to give a neutral species
     Combination of simple ions:
       NaCl , MgCl2 ,
    Combination of simple ions and polyatomic ions:
      Na+ and SO42- Na2SO4 ,
      Ca2+ and PO43- Ca3(PO4)2
    Combination of polyatomic anions and cations
      NH4+ and NO3-       NH4NO3

                                                              40
 Summary

                              Matter
                                    physical techniques


                         pure substances
         elements                                    compounds
                            chemical means


Aggregate of   Homoatomic                     Molecular        Ionic
 like atoms     molecules                    compounds      compounds
  Cu                O2                         H2O           NaCl
  Au                F2                         CO2           CaF2
  Ag                N2                         NO2           (NH4)2CO3

                                                                  41

								
To top