Download Chapter 3 – Atomic Structure - Mercer Island School District

Chapter 3 – Atomic Structure
Chem 2H
Early Models of the Atom
• Greek philosopher Democritus, 450 BC
– All matter is composed of tiny, invisible particles
called atomos.
• French accountant Antoine Lavoiser, 1782
– Law of Conservation of Matter – matter is neither
created nor destroyed.
• French chemist Joseph Proust, 1799
– Law of Constant Composition – compounds contain
the same elements always in the same proportions.
Early Models of the Atom
• English schoolteacher John Dalton, 1803
– Dalton’s Atomic Theory
• Each element is composed of tiny atoms
• Atoms of an element are identical but differ from
those of other elements.
• Atoms are neither created nor destroyed.
• A given compound always has the same relative
numbers and kinds of atoms.
• Still no protons, neutrons or electrons…
Discovering Atomic Structure
• Michael Faraday, 1839
– What makes atoms
different? It may be due
to electricity.
• J.J. Thomson, 1896
– Cathode ray tube.
– Discovered and named
the electron.
– Determined charge of
electron: -1.76 x 108 C/g
Discovering Atomic Structure
• Robert Millikan, 1909
– Oil drop experiment
– Electron charge = 1.60
x 10-19 coulombs
– Electron mass = 9.11 x
10-28 g
• 1890s – 19teensish
– Henri Becquerel
– Pierre and Marie Curie
– Discovery of
radioactive elements
Discovering Atomic Structure
• Ernest Rutherford, 1909
– Gold foil experiment
– Discovery of the positively
charged nucleus
– Empty electron cloud
– Suspected protons and
neutrons
Subatomic Particles
Particle
Location
Charge
(C)
Mass
(g)
Mass
(amu)
Proton
Nucleus
+1.602 E-19 1.673 E-24 1.0073
Neutron
Nucleus
0
1.675 E-24 1.0087
Electron Electron -1.602 E-19 9.109E-28 0.0006
cloud
Terminology
• Atomic number = number of protons
– Hydrogen = 1
– Oxygen = 8
• Ion: atom that has taken on a charge because of the loss or gain of
electrons.
• Isotope: differing forms of the same element, caused by differing
amounts of neutrons.
• Atomic mass: average mass of atoms of an element based on
percent abundance of existing isotopes.
• Mass number = protons + neutrons
– Chlorine-35: 17 protons + 18 neutrons
Atomic Mass
• Calculate the atomic mass of chlorine
using the following data:
• Cl-35: 34.969amu, 75.53%
• Cl-37: 36.966amu, 24.47%
Changes in the Nucleus
• Strong nuclear force: an attractive force that
overcomes the electrostatic repulsion between
protons in the nucleus.
• Attractions between P-N and N-N exist.
• Presence of neutrons adds a net attractive force
to the inside of the nucleus.
• Elements 1-20 have nearly 1:1 ratio of P to N.
• Elements >20 have more neutrons than protons.
• Elements >83 have unstable nuclei = radioactive
Types of Radioactive Decay
Name
Identity
Charge
Penetrating
Ability
Alpha (α)
Helium-4 nuclei
2+
Low, stopped
by paper
Beta (β)
Electrons from
the nucleus
1-
Medium,
stopped by
heavy cloth
Gamma (γ) Electromagnetic None
radiation
High,
stopped by
lead
Radioactive Decay
• Nuclear equations
help radioactive
decay make sense
• Alpha decay: loss of
an alpha particle
• Beta decay: decay of
a neutron
accompanied by
gamma release
Radiometric Dating
• All organic matter contains a small
percentage of C-14
• Since Carbon-14 undergoes beta decay
with a ½-life of 5730 years, scientists can
estimate the age of a fossil, antique, etc.
by evaluating the ratios of other carbon
isotopes to the C-14 currently in the
sample.
• This process is called radiometric dating.