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Atoms & Their StructureChapter 2 Section 1
Part 2
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Objectives
How have historic experiments led to the development of the modern model of the atom?
How is the modern model of the atom different from previous models?
What information is available in an element block of the periodic table?
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Important Vocabulary Atom Atomic theory Law of
conservation of mass/matter
Law of definite proportions
Electron Proton
Electron cloud Isotope Neutron Nucleus Atomic number Mass number Atomic mass unit Atomic mass
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Subatomic Particles
In the mid-1880’s, through many experiments, scientists discovered that atoms can be broken down
The smaller parts of atoms are called subatomic particles
The three most important subatomic particles are the electron, proton, & neutron
Other subatomic particles include: quarks, leptons, photons, gravitons, & neutrinos
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Discovery of the Proton
Evidence for a positively charged particle was found in 1886 by Eugen Goldstein
He observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays
He called these rays canal rays
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The Proton Positively charge particle Resides in the nucleus of an atom Has a 1+ charge Its mass is 1.67 x 10-24 g Symbolized with a p or p+
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Discovery of the Electron
Its discovery was by accident and occurred in 1897
J.J. Thomson, an English physicist, was studying current using electrodes, one positive (anode) and the other negative (cathode)
His experiment determined that the cathode ray was negatively charged Cathode-ray tubes, are currently
used in TV sets, computer monitors and radar displays
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Cathode-Ray Tube
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Thomson’s Results
Thomson confirmed his prediction by seeing how electric and magnetic fields affected the cathode ray
His experiments showed that a cathode ray consists of particles that have mass and a negative charge
He also developed the plum-pudding model of an atom
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Discovery of the Nucleus In 1909, Ernest Rutherford
disproved Thomson’s plum-pudding model by shooting a small beam of positively charged particles at a piece of gold foil
Some of the particles in the beam were reflected back
Leading Rutherford to hypothesize that there must be a positively charged mass in the center of the gold atoms
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Rutherford’s Experiment
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Rutherford’s Atomic Model
Is also known as the nuclear atom
In the nuclear atom: The protons and neutrons
are located in the nucleus The electrons are
distributed around the nucleus and occupy almost all the volume of the atom
This model was better than Thomson’s but it still was incomplete
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The Nucleus
It is the dense central portion of the atoms
It contains nearly all the mass of an atom and all of the positive charge Protons and neutrons!
This part of the model of the atom is still considered true today
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Bohr’s Model of the Atom
In 1913, Niels Bohr suggested that electrons in an atom move in set paths around the nucleus
“Like planets in orbit” The path defines the electron’s energy
level
1. Electrons can only be in certain energy levels
2. Electrons must gain energy to move to a higher energy level
3. Electrons must lose energy to move to a lower level
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Bohr’s Model
Nucleus
Electron
Proton
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Robert A Millikan
Was a U.S. physicist He carried out
experiments to find the quantity of charge carried by an electron
Using Thomson’s charge-to-mass ratio of an electron, he calculated the mass of an electron in 1916
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Quantum Mechanics Model In 1926 Erwin Schrödinger, an Austrian
physicist, took the Bohr atom model one step further
He used mathematical equations to describe the likelihood of finding an electron in a certain position
Unlike the Bohr model, the quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of the location of the electron
This model can be portrayed as a nucleus surrounded by an electron cloud
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Quantum Mechanics Model
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Electrons
Are negatively charged particles Charge is 1‒
Symbolized by e or e‒
Mass is 9.11 x 10-28 g or 1/1840 the mass of a hydrogen atom
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Discovery of the Neutron
Was discovered by the English scientist James Chadwick in 1932
Irene Joliot-Curie had discovered that when alpha particles hit a sample of beryllium, a beam that could go through almost anything was produced
Using this experiment done by Irene Joliot-Curie, Chadwick concluded that the particles in the beam had no charge
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Neutrons
Particles that have no charge Reside in the nucleus Have a mass of 1.67 x 10-24 g Symbolized with a n or n0
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Summary of Atoms Are the building blocks of
molecules Smallest part of an element that still
has the element’s properties Unreacted atoms have no overall
charge Atoms have 5 basic parts Atoms are made of protons, neutrons,
& electrons (subatomic particles) The protons & neutrons are housed in
the center of the atom in the nucleus Electrons are moving around outside of
the nucleus within the electron cloud
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Modern Atomic Model By 1925, Bohr’s model didn’t explain
electron behavior The new model proposed that electrons
behave like waves on a vibrating string This is known as the “wave-particle
duality of nature” This model was developed by Louis de
Broglie, Albert Einstein & Max Planck