Transcript Models of the Atom

Models of the Atom
Objectives
• Recognize that science is a progressive endeavor that
reevaluates and extends what is already known.
(SPI 3221. Inq.1)
• Identify the contributions of major atomic theorists:
Dalton, Thomson, Rutherford, and Bohr.
(SPI 3221.1.1)
• Compare and contrast historical models of the atom.
(SPI 3221.1.2)
Class Opener:
Create a drawing that illustrates Dalton’s view of an
atom.
Based on what you learned about atoms in other
science classes, create a diagram that represents
the structure of an atom.
• JJ Thomson – 1897
– discovered that within atoms there are negatively
charged particles
– called these particles electrons.
• Thomson performed experiments that involved passing
electric current through gases at low pressure.
• The result was a glowing beam called a cathode ray.
• A cathode ray is deflected by electrically charged plates.
• Thomson concluded that a cathode ray is a stream of
electrons. Electrons are parts of the atoms of all elements.
These observations changed the way atoms
were portrayed since it was now determined
that atoms could be further broken down.
– from his findings Thomson developed the “Plum
pudding” model.
– the atom was filled with positively charged
material and the electrons were evenly distributed
throughout.
Images of Thomson’s Model
Thomson had correctly concluded that the atom
had electrons, was spherically shaped, and
electrically neutral.
• Ernest Rutherford (1911)– Gold foil experiment
– directed a narrow beam of alpha particles at a very thin
sheet of gold foil.
– Alpha particles are positively charged particles that are
released by certain radioactive elements as they decay.
Rutherford’s Gold Foil Experiment
Rutherford’s gold-foil experiment yielded evidence of the
atomic nucleus.
a) Rutherford and his coworkers aimed a beam of alpha
particles at a sheet of gold foil surrounded by a
fluorescent screen. Most of the particles passed
through the foil with no deflection at all. A few
particles were greatly deflected.
b) Rutherford concluded that most of the alpha particles
pass through the gold foil because the atom is mostly
empty space. The mass and positive charge are
concentrated in a small region of the atom.
Rutherford called this region the nucleus. Particles
that approach the nucleus closely are greatly
deflected.
Alpha particles scatter from the gold foil
The Rutherford Atomic Model
• Rutherford concluded that the atom is mostly
empty space.
• All the positive charge and almost all of the
mass are concentrated in a small region in the
center of the atom.
• He called this tiny core the nucleus.
As Rutherford’s model of the atom was being
analyzed, it predicted that, according to the
theory of electricity and magnetism, opposite
charges attract each other and the electrons
would lose energy and spiral inward toward the
nucleus. Niels Bohr came up with a theory that
explained why electrons do not spiral inward.
Bohr’s Atomic Model
He stated that electrons follow two rules:
1. Electrons orbit at specific distances from the
nucleus. ( All electrons can be found at a certain
distance form the nucleus in an orbital.)
2. Atoms give off energy when an electron jumps
from a higher energy orbit to a lower energy
orbit and atoms absorb energy when the
electron gets blasted from a low energy orbit to
a high energy orbit.
His model was known as the planetary model
since the electrons are orbiting around the
nucleus much like the planets orbit around the
Sun.
Bohr’s Model of the Atom
• Niels Bohr (1913) – Planetary Model
– electron is found only in specific circular paths, or orbits,
around the nucleus.
Summary of the Atomic Models
• Dalton’s model- shows that an atom is made
up of solid particles and cannot be further
broken down
• Thomson’s model- the “plum pudding” model;
the atom is made of protons and electrons
that are scattered throughout
• Rutherford’s model- protons are found in the
center of the atom in the nucleus and
electrons are scattered in the empty space
that surrounds it
• Bohr’s model- the “planetary model”; protons
and neutrons are found in a central nucleus
and electrons are placed in orbits that circle
around the nucleus at a specific distance
What is still unclear about today’s lesson?
After today’s lesson, on your guided notes, draw
the Thomson, Rutherford, and Bohr models of
the atom.