The Structure of the Atom

1
The Structure of the Atom

• Chapter 4

2
The Beginning

• Democritus- Greek Philosopher (460-370 BC)
• Came up with the term atomos- now called
  atoms
• Believed that atoms could not be created,
  destroyed, or further divided
• Matter is composed of empty space through which
  atoms move
• Atoms are solid, homogenous, indestructible, and
  indivisible
• Different kinds of atoms have different sizes and
  shapes
• The different properties of matter are due to
  size, shape and the movement of atoms

3

• Aristotle- Greek Philosopher (384-322 B.C.)
• One of the most influential philosophers
• Wrote extensively on many subjects
• Rejected Democritus idea that the nothingness
  of empty space could exist-
• His denial went largely unchallenged for two
  thousand years!

4

• Dalton (Schoolteacher- England) (1766-1844)
• Atomic Theory
• All matter is made up of atoms.
• Atoms are indestructible and cannot be divided
  into smaller particles.
• All atoms of one element are exactly alike, but
  they are different from atoms of other elements.
• Different atoms combine in simple whole number
  ratios to form compounds
• In a chemical reaction, atoms are separated,
  combined, or rearranged.

5

• Daltons atomic theory explains the conservation
  of mass when a compound forms from its component
  elements.

6
Subatomic Particles and the Nuclear Atom
7
Discovering the Electron

• Sir William Crookes- English Physicist
• Saw a flash of light within one of the vacuum
  tubes with metal electrodes at opposite ends
• Accidental discovery of the cathode ray (ray of
  radiation originating from the cathode end of the
  tube)

8
ELECTRONS!!

• Scientists continued their research using cathode
  ray tubes, and by the end of the 1800s were
  fairly convinced of the following
• Cathode rays were actually a stream of charged
  particles
• The particles carried a negative charge. (exact
  value of the negative charge was not known)

9
JJ Thompson

• Series of experiments to determine the ratio of
  its charge to mass
• He concluded that the mass of the charged
  particles (electrons) was much less than that of
  a hydrogen atom (lightest known atom)
• SHOCKING!- this meant Dalton was WRONG and atoms
  were divisible into smaller subatomic particles
• Proposed a model of the atom that came known as
  the plum pudding model.

10
JJ Thomson
11
Robert Millikan

• Determined the charge of an electron
• His experimental set up and technique was
  accurate to within 1 of the currently accepted
  value.
• Oil Drop Experiment

12
Ernest Rutherford

• Studied how positively charged alpha particles
  interacted with solid matter

13
Thompsons Plum Pudding Model
Rutherfords results when alpha particles where
beamed through foil
14
Rutherford

• He expected most of the fast moving and
  relatively massive particles to pass straight
  through the gold atoms
• He concluded
• an atom consisted mostly of empty space through
  which atoms move
• There was a tiny, dense region centrally located
  within the atom that contained all of an atoms
  positive charge and virtually all of its mass
  (aka NUCLEUS)

15
Eight years later. (1920)

• Rutherford refined his concept of the nucleus
• It contained positively charged particles called
  PROTONS

16
Few years later

• James Chadwick (Rutherfords coworker)
• Showed that the nucleus also contained another
  subatomic particle- a neutral particle called a
  NEUTRON
• The neutron has a mass equal to that of a proton

17
(No Transcript)
18
How Atoms Differ

• Section 4.3

19
Atomic Number and Masses

• Atomic Number the number of PROTONS in the
  nucleus of an atom of an element
• Also tells you the number of ELECTRONS since
  elements have no over all charge.
• The number of PROTONS determines the IDENTITY of
  the element

20
Isotopes

• Have different mass numbers because they have a
  different amount of NEUTRONS
• If they had a different amount of PROTONS, they
  would be a different element completely
• Because isotopes have a different number of
  NEUTRONS, they also have a different mass number
• Mass Number Protons Neutrons

21
Sodium Isotopes
22
Example NEON

• Atomic Number10
• Mass Number 20.180
• of Protons 10
• of Electrons 10
• of Neutrons 10

23
Mass of Individual Atoms

• Atomic Mass Unit (amu) is defined as 1/12 the
  mass of a carbon-12 atom.
• The atomic mass of an element is the weighted
  average mass of the isotopes of that element.

75
25
24
Average Atomic Mass

• To calculate the atomic mass of an element
• Multiply the mass of each isotope by its natural
  abundance (expressed as a decimal)
• Add the products

25
Unstable Nuclei and Radioactive Decay
26
Radioactivity

• Nuclear Reactions- involve a change in an atoms
  nucleus (unlike most chemical reactions)
• Radioactivity- a process in which substances
  spontaneously emit radiation
• Radiation- the rays and particles emitted by the
  radioactive material

27

• Radioactive atoms undergo significant changes
  that can alter their identities
• One element can change into atoms of another
  element
• Unstable nuclei lose energy by emitting radiation
  in a spontaneous process (aka radioactive decay)
• They continue to do this until they form a stable
  nonradioactive atoms

28
Types of Radiation

• Alpha Radiation
• Deflects towards a negatively charged plate
• Alpha Particles- 2 protons and 2 neutrons (2
  charge)

29

• Beta Radiation
• Radiation that is deflected towards a positively
  charged plate
• Consists of fast moving electrons called beta
  particles

30

• Gamma Radiation
• Gamma Rays are high-energy radiation that possess
  no mass
• Because gamma rays are massless, the emission of
  gamma rays by themselves cannot result in the
  formation of a new atom

31
(No Transcript)
32
(No Transcript)
33
(No Transcript)