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Section 3.3
Atomic Structure
Dalton described the
atomic nature of matter –
is that the end of the
story?
Section 3.3
Atomic Structure
JOHN DALTON
• Thought that atoms were solid sphereskinda like a billiard (pool) ball. This theory
had to later be revised. WHY?
Section 3.3
Atomic Structure
A. The Structure of the Atom
• Experiments by J.J. Thomson in the 1890’s showed that
atoms contain electrons.
• Cathode ray tube
The Discovery
of the Electron
Electric Potential =
Voltage
Section 3.3
Atomic Structure
A. The Structure of the Atom by JJ Thomson
The Plum Pudding (Chocolate Chip Cookie) Model
The Discovery
of the Electron
Section 3.3
Atomic Structure
A. The Gold Foil Experiment
Ernest Rutherford’s Experiment (1911)
He discovered the positively charged nucleus
Section 3.3
Atomic Structure
A. The Structure of the Atom
• Results of the Rutherford experiment
(a) The results that the metal foil
experiment would have yielded if the
plum pudding model had been correct
(b) Actual results
Section 3.3
Atomic Structure
B. Introduction to the Modern Concept of Atomic
Structure
• Ernest Rutherford showed that
atoms have internal structure.
– The nucleus, which is at the
center of the atom, contains
protons (positively charged)
and neutrons (uncharged).
– Electrons move around the
nucleus.
The Discovery of the Atomic Nucleus
Section 3.3
Atomic Structure
B. Introduction to the Modern Concept of Atomic
Structure
Comparing the Parts of an Atom
Section 3.3
Atomic Structure
Determining the number of subatomic particles
• Protons are equal to the atomic number above the
symbol on the periodic table.
• Electrons are equal to the number of protons only in
NEUTRAL atoms. Neutral atoms have no charge so the
positively charged protons must cancel the negatively
charge electrons
• Neutrons: subtract the number of protons from the mass
number
Section 3.3
Atomic Structure
C. Isotopes
• Isotopes are atoms with the same number of protons but
different numbers of neutrons.
Section 3.3
Atomic Structure
C. Isotopes
• A particular isotope is represented by the symbol
e.g. Carbon: What is Z for 12C, 13C and 14C?
.
Section 3.3
Atomic Structure
Isotopes
• True or False? Atoms that have the same number of
neutrons but different number of protons are called
isotopes
• True or False? The mass number of a nucleus
represents the number of protons in the nucleus
• Are all atoms of the same element identical? If not, how
do they differ? Is this consistent with Dalton’s atomic
theory?
Section 3.3
Atomic Structure
Isotope Math
• What is the symbol for the isotopes below?
 Z = 8, number of neutrons = 9
 Isotope of chlorine with A = 37
 Z = 27, A = 60
 Number of protons = 26, number of neutrons = 31
 The isotope of iodine with a mass number of 131
 Z = 3, number of neutrons = 4