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Chapter 3
Atoms: The Building Blocks of Matter
3.2 The Structure of the Atom
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 that
led to the discovery of the nucleus.
• List the properties of protons, neutrons, and electrons.
• Define atom.
3.2 The Structure of the Atom
The Structure of the Atom
• An atom is the smallest particle of an element that retains the chemical properties
of that element.
• The nucleus is a very small region located at the center of an atom.
• The nucleus is made up of at least one positively charged particle called a proton
and usually one or more neutral particles called neutrons.
3.2 The Structure of the Atom
The Structure of the Atom, continued
• Surrounding the nucleus is a region occupied by negatively charged particles
called electrons.
• Protons, neutrons, and electrons are often referred to as subatomic particles.
3.2 The Structure of the Atom
Properties of Subatomic Particles
3.2 The Structure of the Atom
Democritus
“The universe is composed of two elements: the atoms and
the void in which they exist and move.”
Everything is composed of "atoms", which are physically, but
not geometrically, indivisible; that between atoms, there lies
empty space; that atoms are indestructible, and have always
been and always will be in motion; that there is an infinite
number of atoms and of kinds of atoms, which differ in shape
and size. Of the mass of atoms, Democritus said, "The more
any indivisible exceeds, the heavier it is".
3.2 The Structure of the Atom
John Dalton
Dalton’s Atomic Theory
1. All elements are composed of tiny indivisible particles
called ATOMS. (Agreed with Democritus theory from 2000
years earlier)
2. Atoms of the same element are identical. Atoms of any one
element are different from atoms of other elements.
3. Atoms of different elements may mix physically or combine
chemically in simple whole number ratios to form
compounds.
4. Chemical reactions occur when atoms are separated, joined
or rearranged. Atoms of one element are never changed into
elements of another element as result of a chemical reaction.
3.2 The Structure of the Atom
Discovery of the Electron
• Cathode Rays and Electrons
• Experiments in the late 1800s showed that cathode rays were composed of negatively charged
particles.
• These particles were named electrons.
3.2 The Structure of the Atom
Discovery of the Electron, continued
• Charge and Mass of the Electron
• Joseph John Thomson’s cathode-ray tube experiments measured the charge-to-mass ratio of an
electron.
• Robert A. Millikan’s oil drop experiment measured the charge of an electron.
• With this information, scientists were able to determine the mass of an electron.
3.2 The Structure of the Atom
JJ Thomson
Thomson discovered the electron by using
the cathode ray tube.
Cathode tube. Ray travels from – to +.
3.2 The Structure of the Atom
Discovery of the Electron, continued
3.2 The Structure of the Atom
JJ Thomson cont
• Plum pudding model of the atom. Electrons were set in + charged matter like
pudding.
3.2 The Structure of the Atom
Robert Millikan
Oil Drop Experiment
Experiment involved measuring the force on oil droplets in a
glass chamber sandwiched between two electrodes, one above
and one below. With the electrical field calculated, he could
measure the droplet's charge, the charge on a single electron
being (1.592×10−19 C).
3.2 The Structure of the Atom
Oil Drop Experiment
3.2 The Structure of the Atom
Discovery of the Atomic Nucleus
• More detail of the atom’s structure was provided in 1911 by Ernest Rutherford
and his associates Hans Geiger and Ernest Marsden.
• The results of their gold foil experiment led to the discovery of a very densely
packed bundle of matter with a positive electric charge.
• Rutherford called this positive bundle of matter the nucleus.
3.2 The Structure of the Atom
Ernest Rutherford
Rutherford determines the nucleus is positively charged.
Proves “Plum Pudding Theory” wrong. He came up with a
model suggesting that the atom was mostly empty space
except in the center where a positively charged mass was
concentrated (the cause for the great deflection angles).
3.2 The Structure of the Atom
Gold Foil Experiment
3.2 The Structure of the Atom
Gold Foil Experiment on the Atomic Level
3.2 The Structure of the Atom
Composition of the Atomic 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.
• Atoms are electrically neutral because they contain equal numbers of protons and
electrons.
• A neutron is electrically neutral.
3.2 The Structure of the Atom
Composition of the Atomic Nucleus, continued
• 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.
3.2 The Structure of the Atom
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.
3.2 The Structure of the Atom
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.
• This unit is the picometer, pm.