Download ATOM

Chapter 3: Atoms – The
Building Blocks of
Matter
Section 2: The Structure of the Atom
Lesson Starter
• Even though the two shapes look different, the characteristics
of the various parts that compose them are the same.
• The same is true with the atom.
• Though atoms of different elements display different
properties, isolated subatomic particles have the same
properties.
Objectives
• Summarize the observed properties of cathode rays that led
to the discovery of the electron.
• Summarize the experiment carried out by Rutherford and his
co-workers the led to the discovery of the nucleus.
• List the properties of protons, neutrons, and electrons.
• Define atom.
The Structure of the Atom
• ATOM – the smallest particle of an element that retains the
chemical properties of that element.
• NUCLEUS – small region located near the center of the atom
• PROTON – positively charged particle found in the nucleus
The Structure of the Atom
• NEUTRON – electrically neutral particle found in the nucleus
• ELECTRON – negatively charged particle surrounding the
nucleus
• SUBATOMIC PARTICLES – protons, neutrons, and electrons
Cathode Rays and Electrons
• JJ THOMSON -- Experiments in the late
1800s showed that cathode rays were
composed of negatively charged particles.
• When a CURRENT was passed through a
cathode ray tube, the cathode glowed
• caused by a stream of particles called a CATHODE RAY
• traveled from the CATHODE to the ANODE
Cathode Rays and Electrons
Cathode Rays and Electrons
• An object placed between the cathode and the
opposite end of the tube cast a SHADOW on
the glass.
Cathode Rays and Electrons
• A PADDLE WHEEL placed on rails between the
electrodes, rolled along the rails from the
CATHODE toward the ANODE.
• CRT Paddle Wheel Video
Cathode Rays and Electrons
• Cathode rays were deflected by a MAGNETIC
FIELD in the same manner as a wire carrying
ELECTRIC CURRENT (which has a NEGATIVE
CHARGE)
Cathode Rays and Electrons
• The rays were deflected AWAY FROM a
NEGATIVELY charged object.
• These particles were named ELECTRONS.
Charge and Mass of the Electron
• Millikan’s experiment measured the CHARGE-TOMASS RATIO of an electron.
• Robert A. Millikan’s oil drop experiment measured
the CHARGE of an ELECTRON.
• the mass of an electron is ABOUT 1/2000 the mass of
the simplest hydrogen atom
• electrons carry a NEGATIVE charge
• electrons are present in ATOMS OF ALL ELEMENTS
Charge and Mass of the Electron
• inferences were made about atomic structure
• Because atoms are electrically neutral, they must contain a POSITIVE
CHARGE to BALANCE the negative electrons
• Because electrons have so much less mass than atoms, atoms must
contain other particles that ACCOUNT FOR MOST OF THEIR MASS.
Charge and Mass of the Electron
• Oil Drop Experiment Video
Discovery of the Atomic Nucleus
• Gold Foil Experiment Video
• GOLD FOIL EXPERIMENT – 1911 – ERNEST
RUTHERFORD
• a thin piece of gold foil was bombarded with
ALPHA PARTICLES
• most of the particles were expected to GO
STRAIGHT THROUGH the foil and they did
Discovery of the Atomic Nucleus
• About 1 in 8000 particles that struck the foil were deflected
back towards the source
• conclusions were drawn about the structure of
an atom
• particles must have been deflected by a DENSELY PACKED BUNDLE
of POSITIVELY charged matter
Discovery of the Atomic Nucleus
• the VOLUME of a nucleus is SMALL compared
to that of the atom itself
• like a MARBLE in a FOOTBALL FIELD
• Rutherford suggested that the ELECTRONS
SURROUND THE NUCLEUS like planets around
the sun
Composition of the Nucleus
• Except for the nucleus of the simplest type of hydrogen atom,
ALL ATOMIC NUCLEI are made of PROTONS and NEUTRONS.
• A PROTON has a POSITIVE CHARGE EQUAL in magnitude to
the NEGATIVE CHARGE of an ELECTRON.
Composition of the Nucleus
• Atoms are ELECTRICALLY NEUTRAL because they contain
EQUAL NUMBERS of protons and electrons.
• A neutron is electrically NEUTRAL.
Composition of the Nucleus
• The nuclei of atoms of DIFFERENT ELEMENTS differ in their
number of PROTONS and therefore in the amount of POSITIVE
CHARGE they possess.
• Thus, the NUMBER OF PROTONS determines that atom’s
IDENTITY.
Forces in the Nucleus
• When TWO PROTONS are EXTREMELY CLOSE to each other,
there is a STRONG ATTRACTION between them.
• A similar attraction exists when NEUTRONS are very close to
each other or when PROTONS AND NEUTRONS are very close
together.
• The SHORT-RANGE proton-neutron, proton-proton, and
neutron-neutron FORCES that hold the nuclear particles
together are referred to as NUCLEAR FORCES.
The Sizes of Atoms
• The RADIUS of an atom is the distance from the CENTER OF
THE NUCLEUS to the outer portion of its ELECTRON CLOUD.
• Because atomic radii are so small, they are expressed using a
unit that is more convenient for the sizes of atoms.
• The unit is the PICOMETER, pm.