Transcript Bohr`s Model of the Atom

Atoms & Their
Structure
Chapter 2 Section 1
Part 2
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?
Important Vocabulary
Atom
Electron cloud
Atomic theory
Isotope
Law of conservation
of mass/matter
Neutron
Law of definite
proportions
Atomic number
Electron
Proton
Nucleus
Mass number
Atomic mass unit
Atomic mass
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
Discovery of the Proton
Evidence for a positively
charged particle was
found in 1886 by Eugen
Goldstein
He observed a cathoderay tube and found rays
traveling in the direction
opposite to that of the
cathode rays
He called these rays canal
rays
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+
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
Cathode-Ray Tube
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 plumpudding model of an atom
Discovery of the Nucleus
 In 1909, Ernest Rutherford
disproved Thomson’s plumpudding 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
Rutherford’s Experiment
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
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
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
Bohr’s Model
Proton
Nucleus
Electron
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 chargeto-mass ratio of an
electron, he calculated
the mass of an electron
in 1916
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
Quantum Mechanics Model
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
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
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
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
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