Docstoc

Atoms_ Elements_ and Ions

Document Sample
Atoms_ Elements_ and Ions Powered By Docstoc
					           Ch. 4
     Atomic Structure
•   Atomic Theory
•   Atomic Structure
•   Isotopes
•   Average atomic mass
 Atoms are so small that, even today, direct visual
 inspection is all but impossible.




This picture, taken with an electron microscope, shows
the best possible direct observation of iron oxide
molecules. As you can see, details of the atoms
cannot be seen.
           The Atom
An atom consists of a
• nucleus
   –(of protons and neutrons)
• electrons in space about the nucleus.


                          Electron cloud

                            Nucleus
• An _____ is the smallest particle of
  an element that has the chemical
  properties of the element.



                                      Copper
                                      atoms on
                                      silica
                                      surface.




 Distance across = 1.8 nanometer (1.8 x 10-9 m)
                 Sheep to Shawl
•   Democritus 400 B.C.
•   John Dalton 1766-1844
•   J.J. Thomson 1856-1940
•   Ernest Rutherford 1871-1937
•   Robert Millikan 1868 - 1953
•   Niels Bohr 1885-1962
•   James Chadwick 1891–1974
    Scientists credited with the development
    of current atomic structure…..
              Democritus – first believed atoms were
              indivisible & indestructible

              Dalton- atomic theory

Cathode tube Thomson - discovered electrons

  Oil drop    Millikan - discovered the charge of electrons

  Gold Foil   Rutherford - discovered the nucleus; first
              proposed atoms as mostly empty space

              Chadwick - discovered neutrons

              Bohr Model - proposed solar system model
              The Greek Model:


Democritus: Greek
philosopher around the
year 400 BC.


Democritus concluded that matter could
not be divided into smaller and smaller
pieces forever. Eventually, the smallest
piece of matter would be found. He
used the word atomos to describe the
smallest possible piece of matter.
        Dalton’s Atomic Theory
John Dalton (1766-1844) proposed an atomic
   theory

While this theory was not completely correct, it
  revolutionized how chemists looked at
  matter and brought about chemistry as we
  know it today instead of alchemy

Thus, it’s an important landmark in the history
  of science.
Fast Forward to Dalton
Sept. 6, 1766, England-July 27, 1844

           Dalton's Atomic Theory

          1) All matter is made of atoms. Atoms are indivisible and
          indestructible.
          2) All atoms of a given element are identical in mass and
          properties
          3) Compounds are formed by a combination of two or more
          different kinds of atoms.
          4) A chemical reaction is a rearrangement of atoms.
Dalton’s Model of the Atom




 Solid spheres with different masses.
                   The Dalton Model:

John Dalton English chemist that proposed first atomic
theory in 1803.
Points of Dalton's Theory:
1.   All elements are composed of
     indivisible particles.
2.   Atoms of the same element are
     exactly alike.
3.   Atoms of different elements are
     different.
4.   Compounds are formed by joining
     atoms of two or more elements.
   Dalton’s Atomic Theory - Summary

1. matter is composed, indivisible particles
   (atoms)
2. all atoms of a particular element are
   identical
3. different elements have different atoms
4. atoms combine in certain whole-number
   ratios
5. In a chemical reaction, atoms are merely
   rearranged to form new compounds; they
   are not created, destroyed, or changed into
   atoms of any other elements.
  Problems with Dalton’s Atomic Theory?
1. matter is composed, indivisible particles
     Atoms Can Be Divided, but only in a nuclear
     reaction
2. all atoms of a particular element are identical
     Does Not Account for Isotopes (atoms of the same
     element but a different mass due to a different
     number of neutrons)!
3. different elements have different atoms
     YES!
4. atoms combine in certain whole-number ratios
     YES! Called the Law of Definite Proportions
5. In a chemical reaction, atoms are merely rearranged
     to form new compounds; they are not created,
     destroyed, or changed into atoms of any other
     elements.
     Yes, except for nuclear reactions that can change
     atoms of one element to a different element
            The Thomson Model:

J.J. Thomson: English
scientist who discovered
    electrons in 1897.
  Sometimes called the
 "plum pudding" model,
 Thomson thought of an
      atom as being
      composed of a
   positively charged
     material with the
   negatively charged
   electrons scattered
        through it.
                J. J. Thompson
Dec. 18, 1856, England
   - Aug. 30, 1940




JJ Thompson discovered
  the electron using his
    cathode ray tube
              The Rutherford Model:

Ernest Rutherford British physicist who discovered
the nucleus in 1908.

Rutherford's model
proposed that an atom is
mostly empty space. There
is a small, positive nucleus
with the negative electrons
scattered around the
outside edge.
      Ernest Rutherford
Aug. 30, 1871 New Zealand – Oct. 19, 1937
The modern view of the atom was
 developed by Ernest Rutherford (1871-
 1937).
Rutherford’s experiment.
   Results of
        foil
   experiment
     if Plum
    Pudding
   model had
       been
     correct.




http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf
What Actually Happened
Rutherford’s Model of the Atom
 Robert A. Millikan
March 22, 1868, U.S.A. – Dec. 19 1953

                •Established the charge of an
                electron
                •Robert Millikan defined a more
                accurate mass of the electron using
                the Oil Drop Experiment. He
                timed the rate at which the charge
                oil drop would rise and fall
                depending upon the strength of
                the electromagnetic field vs the
                strength of gravity.
                •Mass is .0055amu=9.13x10-28g.
           The Structure of the Atom
             Subatomic Particles
Particle      Symbol   Relative   Relative
                       Charge      Mass
Electron        e-       1-       1/1840

Proton          p+       1+          1

Neutron         n0        0          1
  ATOMIC COMPOSITION
• Protons (p+)
  – + electrical charge
  – mass = 1.672623 x 10-24 g
  – relative mass = 1.007 atomic
    mass units (amu) but we can round to 1
• Electrons (e-)
  – negative electrical charge
  – relative mass = 0.0005 amu
                       but we can round to 0
• Neutrons (no)
  –  no electrical charge
  – relative mass = 1.009 amu but we can
   round to 1
ATOM
COMPOSITION
 The atom is mostly
 empty space
•protons and neutrons in
      the nucleus.
•the number of electrons is equal to the
number of protons.
•electrons in space around the nucleus.
•extremely small. One teaspoon of water has
3 times as many atoms as the Atlantic Ocean
has teaspoons of water.
If this were the proton in
a hydrogen atom, it
would take a screen 1
mile across to display
the electron's orbit.




                             If an electron weighed
                             the same as a dime, a
                             proton would weigh the
                             same as a gallon of
                             milk.
     What’s in an Atom?

                   eutrons
          rotons

Nucleus
                        lectrons
                       in surrounding cloud
     From the Periodic Table …     6


             tomic Number
                                           C
                                        Carbon
                                          12
= No. of
           rotons          ass number

= No. of   lectrons   -    tomic number

               = No. of    eutrons


                                 http://us.bestgraph.com/gifs/singes-5.html
 Atomic Number and Mass
• Atomic number = Z = # of protons.
• Atomic mass = M = # of protons
  and neutrons.
• An individual atom is electrically
  neutral.
• Protons = Electrons.
• p = Z, Charge = p - e, and
          n = M – Z.
                Ions

• Ions are atoms that have lost or
  gained electrons

• Ca  loses 2 electrons: Ca2+
  20 p+                   20 p+
  20 e-                   18 e-
                 What’s in the Box

                   Name: Oxygen
                                             8
                   Symbol: O
                                             O
                   Atomic Number: 8
                                          Oxygen
                   Atomic Mass: 15.9994
                                          15.9994



# of e- = z-charge(oxidation #)
                                  8
# of p+ = z                       8
                                  8
# of n0 = m-z (m=mass)
     Let’s Give it a WhirL

e- = 7+3=10
p+= 7                   7
                       N
n0= 14-7=7          Nitrogen
                    14.0067


    N -3
  Atomic Number, Z
All atoms of the same element
 have the same number of
 protons in the nucleus, Z

    13       Atomic number
     Al     Atom symbol
   26.981    AVERAGE Atomic Mass
      Mass Number, A
• C atom with 6 protons and 6 neutrons
  is the mass standard
• = 12 atomic mass units
• Mass Number (A)
     = # protons + # neutrons
• NOT on the periodic table…(it is the
  AVERAGE atomic mass on the table)
• A boron atom can have
     A = 5 p + 5 n = 10 amu
             A     10
                        B
            Z       5
             Isotopes
• Atoms of the                11B
  same element
  (same Z) but
  different mass        10B
  number (A).
• Boron-10 (105B)
  has 5 p and 5 n

• Boron-11 (115B)
  has 5 p and 6 n
 Ways to write Isotopes
• Two systems exist to write Isotopes.
• Let’s take Chlorine for Example. There are
  two major isotopes of chlorine. One where
  the atom has 17 protons and 18 neutrons and
  one where it has 17 protons and 20 neutrons.
• The easy way to write it is Cl-35 or Cl-37.
• Or 3517Cl, and 3717Cl.
• Top number = Mass # (A)
• Bottom = Atomic Number (# of protons) (Z)
Figure 3.10: Two isotopes of
          sodium.
What are isotopes?

    It’s just a change in the
     neutrizzle fo shizzle!!!

            • They are atoms
              of the same
              element that
              have different
              numbers of
              neutrons.
Isotopes &
Their Uses
Bone scans with
radioactive
technetium-99.
  Isotopes & Their Uses




The tritium content of ground water is
used to discover the source of the water,
for example, in municipal water or the
source of the steam from a volcano.
         Atomic Symbols
 Show the name of the element, a hyphen, and
 the mass number in hyphen notation

               sodium-23

 Show the mass number and atomic number
 in nuclear symbol form
 mass number
                     23 Na

 atomic number        11
              Isotopes?
Which of the following represent
 isotopes of the same element?
 Which element?

234         234         235         238
      92X         93X         92X         92X
       Counting Protons, Neutrons,
              and Electrons
• Protons: Atomic Number (from periodic table)
• Neutrons: Mass Number minus the number of protons
  (mass number is protons and neutrons because the
  mass of electrons is negligible)
• Electrons:
   – If it’s an atom, the protons and electrons must be
     the SAME so that it is has a net charge of zero
     (equal numbers of + and -)
   – If it does NOT have an equal number of electrons, it
     is not an atom, it is an ION. For each negative
     charge, add an extra electron. For each positive
     charge, subtract an electron (Don’t add a proton!!!
     That changes the element!)
If you change   It will affect   And the result
the number      the…             will be…
of…

Protons         Identity of the Completely
                atom            different atom

Electrons       Charge           Ion


Neutrons        Mass             Isotope
          Learning Check – Counting

 Naturally occurring carbon consists of three
 isotopes, 126C, 136C, and 146C. State the number
 of protons, neutrons, and electrons in each of
 these carbon atoms.
    12              13               14
         6C              6C               6C



#p+ _______        _______          _______
#no _______        _______          _______
#e- _______        _______          _______
              Answers

    12         13        14
         6C         6C        6C



#p+ 6          6         6

#no 6          7         8

#e- 6          6         6
                Learning Check

An atom has 14 protons and 20 neutrons.
  A. Its atomic number is
     1) 14           2) 16            3) 34
  B. Its mass number is
     1) 14            2) 16             3) 34

  C. The element is
     1) Si          2) Ca               3) Se

  D. Another isotope of this element is
     1) 3416X         2) 3414X          3) 3614X
                    Answers

An atom has 14 protons and 20 neutrons.
  A. Its atomic number is
     1) 14           2) 16            3) 34
  B. Its mass number is
     1) 14            2) 16             3) 34

  C. The element is
     1) Si          2) Ca               3) Se

  D. Another isotope of this element is
     1) 3416X         2) 3414X          3) 3614X
 AVERAGE                           11B

 ATOMIC
 MASS                        10B


• Because of the existence of isotopes, the
  mass of a collection of atoms has an average
  value.
• Boron is 20% 105B and 80% 115B. That is, 115B
  is 80 percent abundant on earth.
• For boron atomic weight
  = 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu
 Average Atomic Mass

• Atomic Mass is actually pretty
  complex.
It’s an average mass of all the
  different isotopes.
(%abundance * mass) +
  (%abundance*mass) etc… =
  average atomic mass
              Neon
• Neon is made up to 3 isotopes
• Ne-20 = 90.38%
• Ne-21 =0.27%
• Ne-22 = 9.25%
• Atomic mass =
(20.00*.9038) + (21.00*.0027) +
  (22.00*.0925) = 20.1677 = 20.17 amu =
  average atomic mass
 So the masses on the periodic tables are average masses of all
 the isotopes.
Isotopes & Average Atomic
Mass
• Because of the existence of isotopes, the
  mass of a collection of atoms has an average
  value.
•   6
        3 Li = 7.5% abundant and 73Li = 92.5%
                                     6.925 amu
        – Avg. Atomic mass of Li = ______________
•   28 Si
      14     = 92.23%, 2914Si = 4.67%, 3014Si = 3.10%
                                     28.1087 amu
        – Avg. Atomic mass of Si = ______________

				
DOCUMENT INFO
Categories:
Tags:
Stats:
views:28
posted:12/6/2011
language:English
pages:53