Chapter 4
Atomic Structure
4.1 Early ideas about matter
4.2 Defining the atom
4.3 How atoms differ
4.4 Unstable nuclei and radioactive decay
Crosby High School
4.1 Defining the Atom
• Who discovered the atom?
• How has the concept of the atom evolved
over time?
Earliest record of the atom
• Greek philosopher Democritus (~400 B.C) is
credited with defining the atom.
• Democritus also coined the word ‘atom’
• ‘Atom’ comes from the Greek word ‘atomos,’
which means something that cannot be cut or
divided further.
The Atomic Theory
• Established 2000 years after Democritus
• Developed by John Dalton, an English
chemist, in 1803
• Look on page 104 of your text book; copy the
parts of Dalton’s Atomic Theory into your notes.
• Which parts of the atomic theory have been
revised?
Review
• Page 105, answer questions 1, 2, 4
4.2 Structure of the Nuclear Atom
• Which scientists were involved in discovering
the structure of an atom?
• What are three kinds of subatomic particles?
• How can you describe the structure of the
nuclear atom?
Electron
•
•
Discovered by J.J. Thomson in 1897; he was
an English physicist.
Thomson was working with cathode ray tubes
when he discovered electrons
How it Works: Cathode Ray Tube (CRT)
• Gas is sealed in the tube
• Electrodes (pos and neg.) were placed at each
end.
• When electricity was turned on, activating the
electrodes, a beam of light was seen.
• The beam was attracted to positive plates, and
repelled by negative plates; therefore the beam
(or ray) was negatively charged.
Video clip
• http://www.youtube.com/watch?v=O9Goysc
bazk
Protons
• In 1886, Eugene Goldstein discovered canal
rays: rays that traveled in a direction opposite
of cathode rays.
• Concluded that canal rays were composed of
positively charged particles; protons
Neutrons
• Discovered by James Chadwick in 1932
• Used atomic disintegration experiments to
discover the neutron
The Structure of the Atom
• 1904: J.J. Thomson: Plum-pudding model
Ernest Rutherford
• In 1911, tested Thomson’s Model and found it
to be inaccurate.
• Rutherford’s Model places the protons in a
dense center (nucleus) with the electrons
surrounding the nucleus. Most of the atom,
according to Rutherford, was empty space.
• Gold Foil experiment
Gold Foil Experiment
Gold Foil Experiment
Discoveries
Scientist
Democritis
Dalton
Thomson
Goldstein
Chadwick
Year of
What did he discover?
Discovery
The Structure of the Atom
Scientist
Thomson
Rutherford
Year
Draw his atom model
Create a timeline
- Use the information from the previous tables and create a class timeline of
the discoveries
- Who will do which job?
- Create the actual timeline
- Make a title for the timeline
- Decorations – drawings
- Staple everything on the timeline
- Democritis
- Dalton
- Thomson (electron)
- Goldstein
- Chadwick
- Rutherford
- Thomson (plum pudding)
4.2 Review (worksheet)
1. Which scientists were involved in discovering the
subatomic particles of an atom?
2. Which scientist discovered the basic structure of the
atom that we still use today?
3. What was the name of Thomson’s atomic model?
4. Of the 3 subatomic particles, which 2 are relatively
the same mass?
5. What are the charges and locations of the 3 main
subatomic particles?
4.3 Distinguishing Among Atoms
• What makes one element different from
another?
• How do isotopes of an element differ?
• How do you calculate the atomic mass of an
element?
Atomic Number
• Identifies an element
• Equal to the number of protons
• Also equal to the number of electrons, if
the atom is neutral
Mass Number
• The number of particles in the nucleus of an atom
• Round the Atomic Mass to the nearest whole number
to get the Mass Number
• Equal to the sum of protons and neutrons in an atom
How many neutrons are in Helium?
Summary
• Atomic Number = # of protons = # electrons
• Mass Number = # protons + # neutrons
• # Neutrons = Mass Number – Atomic Number
Practice - Kr
• Pb has an atomic number of 82. How many
protons in Pb? How many electrons?
• Find Pb on the Periodic Table. What is the
Mass Number?
• How many neutrons in Pb?
Practice - Kr
•
•
•
•
•
What is Kr’s atomic number?
How many protons in Kr?
How many electrons in Kr?
What is Kr’s Mass Number?
How many neutrons in Kr?
Individual Review
• Page 116, #12-14
Stop Here
Mass of Atoms
• Depends on the mass of protons and neutrons
(proton has a mass of 1.67x10-24g)
• Numbers are too small to work with, so
scientists developed the atomic mass unit
(amu). Mass of 1 proton is 1.007276amu 
much easier to work with
• Atomic mass unit = standard unit for indicating
mass on the atomic scale
Atomic Mass
• Is equal to the weighted average mass of the isotopes
that make up an element
• For example: Carbon
– Exists as 2 isotopes: 98.89% is C-12 (mass 12.000)
1.11% is C-13 (mass 13.003)
- Atomic Mass of Carbon is
(12.000 x 0.9889) + (13.003 x 0.0111)
= 12.011 (atomic mass on the periodic table)
Isotopes
• Atoms of the same element that have different
numbers of neutrons, and therefore different
mass numbers
Practice: Calculate the atomic mass of Element X from the
following isotopes of Element X:
•
6X,
with an atomic mass of 6.015 and 7.59% abundant
7X, with an atomic mass of 7.016 and 92.41% abundant
Atomic Mass = (mass isotope x % abund.)+(mass x % abund)…..
Review
1. On page 121: # 18 & 19
2. On page 121, #20-24
Concept Check
• The atomic mass of copper is 63.546. Which of
Copper’s two isotopes, Copper-63 or Copper65, is more abundant?
Concept Check
• Boron has two isotopes (Boron-10 and Boron11). Which is more abundant given that the
atomic mass of Boron is 10.81?
Concept Check
• There are three isotopes of Silicon: (Si-28, Si29, Si-30). The atomic mass of Silicon is
28.086. Which isotope is most abundant in a
sample of Silicon?
Summary of Terms
• Mass number = Total number of protons and
neutrons
• Atomic mass unit (amu) = standard unit for
indicating mass on an atomic scale
• Atomic mass = Average masses of isotopes of an
element in a sample