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Chapter Introduction
Lesson 1 Discovering Parts of an Atom
Lesson 2 Protons, Neutrons, and Electrons—How Atoms Differ
Chapter Wrap-Up
What do you think?
Before you begin, decide if you agree or disagree with each of these statements. As you view this presentation, see if you change your mind about any of the statements.
1. The earliest model of an atom contained only protons and electrons.
2. Air fills most of an atom.
3. In the present-day model of the atom, the nucleus of the atom is at the center of an electron cloud.
Do you agree or disagree?
4. All atoms of the same element have the same number of protons.
5. Atoms of one element cannot be changed into atoms of another element.
6. Ions form when atoms lose or gain electrons.
Do you agree or disagree?
• What is an atom?
• How would you describe the size of an atom?
• How has the atomic model changed over time?
Discovering Part of an Atom
• atom
• electron
• nucleus
• proton
Discovering Part of an Atom
• neutron
• electron cloud
Democritus (460–370 BC) believed that matter is made of small, solid objects called atomos, from which the English word atom is derived.
Early Ideas About Matter
Early Ideas About Matter (cont.)
• Aristotle (384–322 BC) did not believe that empty space exists, but instead believed that all matter is made of fire, water, air, and earth.
• Because Aristotle was so influential, his ideas were accepted and Democritus’s ideas about atoms were not studied again for more than 2,000 years.
John Dalton combined data from his own scientific research with data from the research of other scientists to propose a new atomic theory.
Dalton’s Atomic Model
An atom is the smallest piece of an element that still represents that element.
The Atom
What is a copper atom?
• Atoms of different elements are different sizes, but all are very, very small.
• You cannot see atoms with just your eyes or even with most microscopes.
The Atom (cont.)
How would you describe the size of an atom?
• The 1981 invention of a high-powered microscope, called a scanning tunneling microscope (STM), enabled scientists to see individual atoms for the first time.
• Scientists have learned that atoms are not the smallest particles of matter.
The Atom (cont.)
Following his experiments with cathode ray tubes, scientist J.J. Thomson concluded that cathode rays were made of small, negatively charged particles which he called electrons.
An electron is a particle with one negative charge (1–).
Thomson—Discovering Electrons
electronfrom Greek electron, means “amber,” the physical force so called because it first was generated by rubbing amber. Amber is a fossilized substance produced by trees.
• Because atoms are neutral, or not electrically charged, Thomson proposed that atoms also must contain a positive charge that balances the negatively charged electrons.
• Thomson’s proposed atom was a sphere with a positive charge evenly spread throughout and negatively charged electrons within it.
Thomson—Discovering Electrons (cont.)
Thomson’s model of the atom contained a sphere of positive charge with negatively charged electrons within it.
Scientist Ernest Rutherford set up experiments to test Thomson’s atomic model and to learn more about what atoms contain.
Rutherford—Discovering the Nucleus
Rutherford expected the positive alpha particles to travel straight through the foil without changing direction.
Some alpha particles traveled in a straight path, as expected. But some changed direction, and some bounced straight back.
Rutherford—Discovering the Nucleus (cont.)
Given the results of the gold foil experiment, how do you think an actual atom differs from Thomson’s model?
• Rutherford concluded that most of an atom’s mass and positive charge is concentrated in a small area in the center of the atom called the nucleus.
• Additional research showed that the positive charge in the nucleus was made of positively charged particles called protons.
Rutherford—Discovering the Nucleus (cont.)
• A proton is an atomic particle that has one positive charge (1+).
• Negatively charged electrons move in the empty space surrounding the nucleus.
Rutherford—Discovering the Nucleus (cont.)
Rutherford’s model contains a small, dense, positive nucleus. Tiny, negatively charged electrons travel in empty space around the nucleus.
• James Chadwick discovered that, in addition to protons, the nucleus also contained neutrons.
• A neutron is a neutral particle that exists in the nucleus of an atom.
Discovering Neutrons
• Niels Bohr proposed that electrons move in circular orbits, or energy levels, around the nucleus.
• Electrons closer to the nucleus have less energy than electrons farther away from the nucleus.
Bohr’s Atomic Model
• More research showed that, although electrons have specific amounts of energy, energy levels are not arranged in circular orbits.
• When an electron moves from a higher energy level to a lower energy level, energy is released—sometimes as visible light.
Bohr’s Atomic Model (cont.)
In Bohr’s model of the atom, electrons move in circular orbits around the atom.
Bohr’s Atomic Model (cont.)
How did Bohr’s model of the atom differ from Rutherford’s?
• In the modern atomic model, electrons form an electron cloud.
• An electron cloud is an area around an atomic nucleus where an electron is most likely to be.
The Modern Atomic Model
In this atom, electrons are more likely to be found closer to the nucleus than farther away.
The Modern Atomic Model (cont.)
How has the model of the atom changed over time?
• Protons and neutrons are made of smaller particles called quarks.
• Scientists theorize that there are six types of quarks: up, down, charm, strange, top, and bottom.
• Protons are made of two up quarks and one down quark.
Quarks
• Neutrons are made of two down quarks and one up quark.
• The current atomic model might change with the invention of new technology that aids the discovery of new information.
Quarks (cont.)
• If you were to divide an element into smaller and smaller pieces, the smallest piece would be an atom.
• Atoms are so small that they can be seen only by using very powerful microscopes.
• Scientists now know that atoms contain a dense, positive nucleus surrounded by an electron cloud.
A. atom
B. electron
C. nucleus
D. proton
Which term describes a particle with one negative charge?
Whose model of the atom contained a sphere of positive charge with negatively charged electrons within it?
A. Dalton
B. Democritus
C. Rutherford
D. Thomson
A. electron cloud
B. neutron
C. nucleus
D. proton
Which term refers to an area around an atomic nucleus where an electron is most likely to be?
1. The earliest model of an atom contained only protons and electrons.
2. Air fills most of an atom.
3. In the present-day model of the atom, the nucleus of the atom is at the center of an electron cloud.
Do you agree or disagree?
• What happens during nuclear decay?
• How does a neutral atom change when its number of protons, electrons, or neutrons changes?
Protons, Neutrons, and Electrons—How Atoms Differ
• atomic number
• isotope
• mass number
• average atomic mass
Protons, Neutrons, and Electrons—How Atoms Differ
• radioactive
• nuclear decay
• ion
• The mass of electrons is much smaller than the mass of protons or neutrons.
• Most of the mass of an atom is found in the nucleus.
The Parts of the Atom
• The number of protons in an atom of an element is the element’s atomic number.
• The atomic number is the whole number listed with each element on the periodic table.
• Atoms of different elements contain different numbers of protons.
Different Elements—Different Numbers of Protons
• Neutral atoms of different elements also have different numbers of electrons.
• In a neutral atom, the number of electrons equals the number of protons; therefore, the number of positive charges equals the number of negative charges.
Different Elements—Different Numbers of Protons (cont.)
• Atoms of the same element can have different numbers of neutrons.
• Isotopes are atoms of the same element that have different numbers of neutrons.
• Most elements have several isotopes.
Neutrons and Isotopes
Neutrons and Isotopes (cont.)
isotope
from Greek isos, means “equal”; and topos, means “place”
• The mass number of an atom is the sum of the number of protons and neutrons in an atom.
Mass number = number of protons + number of neutrons
• An isotope is often written with the element name followed by the mass number.
Neutrons and Isotopes (cont.)
The average atomic mass of an element is the average mass of the element’s isotopes, weighted according to the abundance of each isotope.
Neutrons and Isotopes (cont.)
• Marie Curie called elements that spontaneously emit radiation radioactive.
• Henri Becquerel and Pierre and Marie Curie discovered that the radiation released by uranium was made of energy and particles.
Radioactivity
• This radiation came from the nuclei of the uranium atoms.
• When uranium releases radiation, it changes to a different element.
Radioactivity (cont.)
• Nuclear decay is a process that occurs when an unstable atomic nucleus changes into another more stable nucleus by emitting radiation.
• Nuclear decay can produce three different types of radiation—alpha particles, beta particles, and gamma rays.
Radioactivity (cont.)
An alpha particle is made of two protons and two neutrons. When an atom releases an alpha particle, its atomic number decreases by two.
When beta decay occurs, a neutron changes into a proton and a high-energy electron called a beta particle. The atomic number of an atom increases by one because it has gained a proton.
Because gamma rays do not contain particles, the release of gamma rays does not change one element into another element.
• The energy released by radioactive decay can be both harmful and beneficial to humans.
• Radiation therapy can be beneficial to humans by destroying harmful cells such as cancer cells.
Radioactivity (cont.)
• An ion is an atom that is no longer neutral because it has gained or lost electrons.
• An ion can be positively or negatively charged depending on whether it has lost or gained electrons.
Ions—Gaining or Losing Electrons
• When a neutral atom loses one or more electrons, it has more protons than electrons and as a result, has a positive charge.
• An atom with a positive charge is called a positive ion.
• When a neutral atom gains one or more electrons, it now has more electrons than protons and as a result, has a negative charge.
• An atom with a negative charge is called a negative ion.
Ions—Gaining or Losing Electrons(cont.)
How does a neutral atom change when its number of protons or electrons changes?
• Two isotopes of a given element contain different numbers of neutrons.
• When a neutral atom gains or loses an electron, it becomes an ion.
A. electrons
B. neutrons
C. nucleus
D. protons
Where is most of the mass of an atom found?
A. atomic number
B. average atomic mass
C. isotope
D. mass number
Which term refers to the sum of the number of protons and neutrons in an atom?
A. ion
B. isotopes
C. nuclear decay
D. radioactive
What term did Marie Curie use to describe elements that spontaneously emit radiation?
4. All atoms of the same element have the same number of protons.
5. Atoms of one element cannot be changed into atoms of another element.
6. Ions form when atoms lose or gain electrons.
Do you agree or disagree?
Key Concept Summary
Interactive Concept Map
Chapter Review
Standardized Test Practice
An atom is the smallest unit of an element and is made mostly of empty space. It contains a tiny nucleus surrounded by an electron cloud.
• If you were to divide an element into smaller and smaller pieces, the smallest piece would be an atom.
• Atoms are so small that they can be seen only by powerful scanning microscopes.
• The first model of the atom was a solid sphere. Now, scientists know that an atom contains a dense positive nucleus surrounded by an electron cloud.
Lesson 1: Discovering Parts of the Atom
Lesson 2: Protons, Neutrons, and Electrons—How Atoms Differ
• Nuclear decay occurs when an unstable atomic nucleus changes into another more stable nucleus by emitting radiation.
• Different elements contain different numbers of protons. Two isotopes of the same element contain different numbers of neutrons. When a neutral atom gains or loses an electron, it becomes an ion.
A. atom
B. electron
C. neutron
D. proton
Which term describes a neutral particle that exists in the nucleus of an atom?
A. Thomson
B. Rutherford
C. Chadwick
D. Bohr
Who discovered that, in addition to protons, the nucleus also contained neutrons?
A. electron cloud
B. isotope
C. nucleus
D. quarks
Protons and neutrons are made of smaller particles called what?
A. radiation
B. radioactivity
C. nuclear decay
D. radiation therapy
Which term refers to the process that occurs when an unstable atomic nucleus changes into another more stable nucleus by emitting radiation?
A. positive ion
B. negative ion
C. isotope
D. quarks
Which describes an atom with a positive charge?
A. quark
B. nucleus
C. electron
D. atom
Which term refers to the smallest piece of an element that still represents that element?
A. Aristotle
B. Bohr
C. Chadwick
D. Rutherford
Who concluded that most of an atom’s mass and positive charge is concentrated in the nucleus?
A. proton
B. nucleus
C. neutron
D. electron
Which is an atomic particle that has one positive charge?
A. ion
B. isotope
C. neutron
D. proton
Which term refers to an atom that is no longer neutral because it has gained or lost electrons?
A. atomic number
B. mass number
C. relative mass
D. average atomic mass
Which refers to the average mass of an element’s isotopes, weighted according to the abundance of each isotope?