Download Atomic Structure Scientists

History of the Atom
Chemistry
Mrs. Herrmann
Democritus 460-370 BC
• Atomos: The point
at which matter can
no longer be
subdivided.
John Dalton 1766-1844
Dalton’s Atomic Theory 1803
• Elements consisted of tiny particles called atoms.
• All atoms of an element were identical .
• Atoms of each element were different from one
another. (they had different masses, sizes,
properties, etc…)
• Compounds consist of atoms of different elements
combined together.
• Compounds have constant composition because
they contain a fixed ratio of atoms.
• Chemical reactions involve the rearrangement of
combinations of those atoms.
Dalton’s Atomic Model
• Atoms were tiny,
indivisible,
indestructive particles
and each one had a
certain mass, size and
chemical behavior.
• The picture to the right
illustrates Dalton’s
idea of an atom.
J.J. Thomson 1856-1940
• April 30, 1897 announced that
cathode rays were made of
negatively charged particles
which he called “corpuscles”.
These particles would later be
called electrons.
• Determined charge to mass
ratio of the electron using the
cathode ray tube, thus known
for the negative one charge
associated with the electron and
the idea that atoms could be
divided into smaller particles.
• Won Nobel Prize in Physics in
1906 for discharge of electricity
Thomson Experiment
1897, Joseph John Thomson:
• discovered negatively charged particles--Electrons
• calculated mass-to-charge ratio of electrons
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Mass of Electron
• Mass of electron calculated to be
•
•
•
9.1 x 10–28 g
1837 times smaller than a proton
0.0005 amu (0 amu)
• Mass of proton
• 1.7 x 10 –24 g
0.0000000000000000000000017
• 1 amu
g
0.00000000000000000000000000091 g
0.00000000000000000000000170091 g
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Thomson’s Plum Pudding
Atomic Model
• Believed the model of
an atom was much like
plum pudding. The
plums would be the
electrons and the
pudding would be the
the sphere of positive
charge
•
Ernest Rutherford 1871-1937
• Born in New Zealand
• There were 12 children in his
family: 7 boys and 5 girls
• Age 16 first attended college
• Most known for the
discovery of the nucleus
through the Gold Foil
Experiment
• Coined the term proton for
the positive particles of the
nucleus
Model Explaining Rutherford’s Experiment
Atoms
• have positively charged nucleus
• have a tiny, very dense nucleus
• are made mostly of empty space
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Structure of the Atom
• Rutherford suggested:
– Most mass in nucleus
– positive charge in nucleus
• James Chadwick, English physicist
– discovered neutrons in 1932
• Nucleus: protons and neutrons
– Neutrons have mass but no charge
– Very small compared
to rest of atom
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James Chadwick 1891-1974
Subatomic Particles
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Chapter 3
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Atomic Structure
Atomic number = number protons in atom
• whole number on periodic table
• atoms of same element have same atomic #
• For a neutral atom,
#protons = #electrons
16
Chapter 3
Isotopes
mass number = number protons + neutrons
Mass # is NOT on periodic table!
Chapter 3
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Isotopes
Isotopes have the same atomic number, but
have different mass numbers (same number of
protons, but different number of neutrons).
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Chapter 3
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Symbols for Isotopes
A
A = mass number
Z
Z = atomic number
X = symbol of element
Isotopes of hydrogen:
X
1H
1
Chapter 3
mass #
atomic #
2H
1
X
3H
1
19
Names for Isotopes
• Name element followed by mass number
1H
2H
Isotopes of hydrogen:
1
1
Hydrogen-1
Hydrogen-2
Mg isotopes:
magnesium-24
magnesium-25
magnesium-26
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Practice
atomic number for cobalt? 27
How many protons in phosphorus atom? 15
How many electrons in tungsten atom? 74
mass # of sodium isotope with 12 neutrons?
11 + 12 = 23
Write isotope symbol for tin with mass number 119.
119
50Sn
Chapter 3
50 p
50 e69 n
21
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