Atomic history

1
Atomic history
2
Dalton's Atomic Theory

• (1) Matter is made of atoms atoms are
  indivisible and indestructible.
• (2) All atoms of one element are exactly
  identical to one another atoms of different
  elements are different from one another.
• (3) Atoms are neither created nor
• destroyed in chemical reactions.
• (4)Atoms combine to form compounds. A given
  compound always has the same relative number and
  kind of atoms.
• (5) Chemical reactions are needed to rearrange
  compounds.

3
INSTANT RESPONSE
Which one of the following is not one of
the postulates of Dalton's atomic theory? A.
Atoms are made of protons, neutrons, and
electrons. B . All atoms of a given element are
identical C . Atoms are neither created nor
destroyed in chemical reactions. D . Compounds
are formed when atoms of more than one element
combine E . Each element is composed of extremely
small particles called atoms.
4
Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to
deduce the presence of a negatively charged
particle the electron http//www.youtube.com/watc
h?vO9Goyscbazkfeaturerelated

• Cathode ray tubes pass electricity through a gas
  that is contained at a very low pressure.

5
INSTANT RESPONSE
Which one of the following is not true concerning
cathode rays? A. They originate from the
negative electrode. B. They travel in straight
lines in the absence of electric or magnetic
fields. C. They impart a negative charge to
metals exposed to them. D. They are made up of
electrons. E. The characteristics of cathode
rays depend on the material from which they are
emitted.
6
JJ Thompsons Idea

• He knew that the atom contained and charges.
• The atom was like Plum Pudding Electrons
  scattered in a cloud of positively charged matter

7
Mass of the Electron
Mass of the electron is 9.11 x 10-28 g
The oil drop apparatus
1916 Robert Millikan determines the mass of the
electron 1/1840 the mass of a hydrogen atom has
one unit of negative charge
8
Discovery and Properties of the Electron
q was always an integer multiple of the same
number, which was given the symbol "e"
The currently accepted value of e is 1.602x10-19
C Knowing e allows the electron mass to be
calculated 9.11 X 10 -31 kg electrons x
integers charge
9
INSTANT RESPONSE
Which of these could be the charge of an
object? A. 0.80 x 10-19 C B. 2.0 x 10-19 C C.
3.2 x 10-19 C D. 4.0 x 10-19 C
10
INSTANT RESPONSE
The magnitude of the charge on an electron was
determined in the __________. A. cathode ray
tube, by J. J. Thomson B. Millikan oil drop
experiment C. Dalton atomic theory D. atomic
theory of matter
11
Conclusions from the Study of the Electron

1. Cathode rays have identical properties regardless
   of the element used to produce them. All elements
   must contain identically charged electrons.
2. Atoms are neutral, so there must be positive
   particles in the atom to balance the negative
   charge of the electrons
3. Electrons have so little mass that atoms must
   contain other particles that account for most of
   the mass

12
Ernest RutherfordsGold Foil Experiment - 1911

• Alpha particles are helium nuclei - The alpha
  particles were fired at a thin sheet of gold foil
• Particles that hit on the detecting screen
  (film) are recorded

13
Rutherfords Experiment

• Gold Foil
• Shot a particles at gold.
• Most pass through.
• Very few deflected
• Giegor and Marsden were scientists who worked
  under Rutherford to carry out the experiment

14
Rutherfords Findings

• Most of the particles passed right through
• A few particles were deflected
• VERY FEW were greatly deflected

Conclusions

1. The nucleus is small
2. The nucleus is dense
3. The nucleus is positively charged
4. The atom is mostly empty space

15
INSTANT RESPONSE
The gold foil experiment performed
in Rutherford's lab __________. A. confirmed the
plum-pudding model of the atom B . led to the
discovery of the atomic nucleus C. was the basis
for Thomson's model of the atom D. utilized the
deflection of beta particles by gold foil E.
proved the law of multiple proportions
16
INSTANT RESPONSE
In the Rutherford nuclear-atom model A. the
heavy subatomic particles reside in the
nucleus B. the principal subatomic particles all
have essentially the same mass C. the light
subatomic particles reside in the nucleus D.
mass is spread essentially uniformly throughout
the atom
17
James Chadwick

• Studied the mass of the nucleus of the atom and
  realized there is more mass than number of
  protons.
• The neutron accounted for the extra mass.
• It was so hard to find because it is neutral and
  in the nucleus.

18
The Bohr Model

• Studied the light emitted from atoms when excited
  by electricity - Spectroscopy
• Add Energy electrons jump up to a higher level.
• Remove Energy electrons fall back to lower
  level.

19
(No Transcript)
20
Niels Bohr
1913

• Electrons exist on energy levels.
• Like planets orbiting the sun
• Each orbit only contains a set number of
  electrons.
• Each orbit possessed a given quantum of energy.

21
Bohrs Atom
electrons in orbits
nucleus
22
The electron cloud

• The area in space surrounding the nucleus that
  has a high probability of holding an electron.
• Schrodinger developed the cloud model.

23
Schrodingers Modern Model of the Atom

• Atom still has a positive nucleus.
• Electron location based on probability.
• Created cloud shapes to show the most probable
  location of an electron.

24
HELIUM ATOM
Shell
proton
N

-

N
-
neutron
electron
25
ATOMIC STRUCTURE
Particle
Charge
Mass
proton
charge
1
neutron
No charge
1
electron
- charge
nil