Atomic Models

1
Atomic Models
2
Atoms are so small that, even today, direct
visual inspection is all but impossible. Our
model of the atom is based on indirect
experimental data. Because of this, our model of
the atom changes as our experimental ability
improves
3

• Democritus
• (460 B.C. 370 B.C)
• Greek Philosopher
• 1st to suggest the existence of atoms
• Believed they were indivisible and indestructible
• Based solely on philosophy

4

• Daltons Atomic Theory
• John Dalton (1766-1844)
• Elements are composed of tiny indivisible
  particles called atoms
• Atoms of the same element are identical
• Atoms of different elements combine in whole
  number ratios to create compounds
• Chemical reactions - elements are combined,
  separated, or rearranged
• does not change the element

5
Sizing up the Atom Elements can be divided into
smaller and smaller particles these particles
retain the elements properties Elements can be
viewed using special instruments scanning
tunneling (electron) microscopes
6
Structure of the Atom Atoms are divisible into
subatomic particles Electrons, Protons, and
Neutrons
7
Discovering the Electron JJ Thomson (1897) Used
a Cathode Ray tube to deduce the presence of a
negatively charged particle electron
8

• Thomsons Atomic Model
• Believed that the electrons were like plums
  embedded in a positively charged pudding Plum
  Pudding Model

9
Mass of the Electron Robert Milikan
(1916) Determined the mass to be 1/1840 the mass
of hydrogen atom has one unit of a negative
charge from his oil drop experiment Mass of the
electron is 9.11 x 10-28 g
10

• Conclusion based on the Electron
• Cathode rays have identical properties regardless
  of the elements used therefore, elements must
  contain identically charged particles
• Atoms are neutral therefore, there must be
  positive particles to balance the atom
• Atoms have so little mass that there must be
  other particles that make up the difference

11

• Proton
• Eugene Goldstein (1886)
• in addition to the electrons that travel from the
  negatively charged cathode toward the positively
  charged anode, there is another ray that travels
  in the opposite direction, from the anode toward
  the cathode He termed these particles Protons
• Protons positive charge, relative mass 1 (1840
  times that of an electron)

12

• Neutron
• James Chadwick (1932)
• Confirmed the presence of the neutron, a particle
  with no charge, but a mass nearly equal to that
  of a proton

13
Properties of Atoms
14
ef
What is the Nucleus? Ernest Rutherfords Gold
Foil Experiment 1911 Alpha helium
particles were shot at a thin piece of gold foil
and the area they hit was recorded to determine
the shape
15

• Rutherfords Findings
• Most particles passed right through
• A few were deflected
• A very few were greatly deflected
• Conclusion
• The nucleus is small
• The nucleus is dense
• The nucleus is positively charged

16

• Rutherfords Atomic Model
• Based on Experimental Data
• The atom is mostly empty space
• All the positive charge most of the mass is
  located in the nucleus
• Nucleus protons and neutrons
• Electron distributed around nucleus and occupy
  most of the volume
• Called nuclear model

17

• Bohr Model Planetary Model
• Niels Bohr (1913)
• Electrons move in a definite orbit around the
  nucleus
• Each electron occupies its own energy level

18

• Wave Model
• Wave Mechanics
• electrons have NO definite path in an atom
• The probable location of an electron is based on
  how much energy it has.
• The more energy an electron has, the farther from
  the nucleus.
• The small, positively charged nucleus is
  surrounded by a large space in which there are
  enough electrons to make the atom neutral.