Defining the Atom 4.1 The lab technician shown here is using a magnifying lens to examine a bacterial culture in a petri dish. When scientists cannot see the details of what they study, they try to obtain experimental data that help fill in the picture.

Early Models of the Atom 4.1 Early Models of the Atom Early Models of the Atom An atom is the smallest particle of an element that retains its identity in a chemical reaction. Philosophers and scientists have proposed many ideas on the structure of atoms.

Early Models of the Atom 4.1 Early Models of the Atom Democritus’s Atomic Philosophy How did Democritus describe atoms? Democritus

Early Models of the Atom 4.1 Early Models of the Atom Democritus believed that atoms were indivisible and indestructible. Democritus’s ideas were limited because they didn’t explain chemical behavior and they lacked experimental support.

Early Models of the Atom 4.1 Early Models of the Atom Dalton’s Atomic Theory How did John Dalton further Democritus’s ideas on atoms?

Early Models of the Atom 4.1 Early Models of the Atom By using experimental methods, Dalton transformed Democritus’s ideas on atoms into a scientific theory. The result was Dalton’s atomic theory.

Early Models of the Atom 4.1 Early Models of the Atom What are the theories of Dalton?

Early Models of the Atom 4.1 Early Models of the Atom 1. All elements are composed of tiny indivisible particles called atoms. According to Dalton’s atomic theory, an element is composed of only one kind of atom, and a compound is composed of particles that are chemical combinations of different kinds of atoms. a) Atoms of element A are identical. b) Atoms of element B are identical, but differ from those of element A. c) Atoms of elements A and B can physically mix together. d) Atoms of elements A and B can chemically combine to form a compound. Interpreting Diagrams How does a mixture of atoms of different elements differ from a compound?

Early Models of the Atom 4.1 Early Models of the Atom 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. According to Dalton’s atomic theory, an element is composed of only one kind of atom, and a compound is composed of particles that are chemical combinations of different kinds of atoms. a) Atoms of element A are identical. b) Atoms of element B are identical, but differ from those of element A. c) Atoms of elements A and B can physically mix together. d) Atoms of elements A and B can chemically combine to form a compound. Interpreting Diagrams How does a mixture of atoms of different elements differ from a compound?

Early Models of the Atom 4.1 Early Models of the Atom 3. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. According to Dalton’s atomic theory, an element is composed of only one kind of atom, and a compound is composed of particles that are chemical combinations of different kinds of atoms. a) Atoms of element A are identical. b) Atoms of element B are identical, but differ from those of element A. c) Atoms of elements A and B can physically mix together. d) Atoms of elements A and B can chemically combine to form a compound. Interpreting Diagrams How does a mixture of atoms of different elements differ from a compound?

Early Models of the Atom 4.1 Early Models of the Atom 4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element are never changed into atoms of another element in a chemical reaction. According to Dalton’s atomic theory, an element is composed of only one kind of atom, and a compound is composed of particles that are chemical combinations of different kinds of atoms. a) Atoms of element A are identical. b) Atoms of element B are identical, but differ from those of element A. c) Atoms of elements A and B can physically mix together. d) Atoms of elements A and B can chemically combine to form a compound. Interpreting Diagrams How does a mixture of atoms of different elements differ from a compound? Scientists used a scanning tunneling microscope to generate this image of iron atoms, shown in blue. The radius of this circle of atoms is just 7.13 × 10-9 m.

4.1 Sizing up the Atom Sizing up the Atom What instruments are used to observe individual atoms?

4.1 Sizing up the Atom Despite their small size, individual atoms are observable with instruments such as scanning tunneling microscopes.

Iron Atoms Seen Through a Scanning Tunneling Microscope 4.1 Sizing up the Atom Iron Atoms Seen Through a Scanning Tunneling Microscope

Subatomic Particles What are three kinds of subatomic particles? 4.2

4.2 Subatomic Particles Three kinds of subatomic particles are electrons, protons, and neutrons.

4.2 Subatomic Particles Electrons In 1897, the English physicist J. J. Thomson (1856–1940) discovered the electron. Electrons are negatively charged subatomic particles.

4.2 Subatomic Particles Thomson performed experiments that involved passing electric current through gases at low pressure. The result was a glowing beam, or cathode ray, that traveled from the cathode to the anode.

Cathode Ray Tube 4.2 Subatomic Particles In a cathode-ray tube, electrons travel as a ray from the cathode (-) to the anode (+). A television tube is a specialized type of cathode-ray tube.

A cathode ray is deflected by a magnet. 4.2 Subatomic Particles A cathode ray is deflected by a magnet. Thomson examined two ways that a cathode ray can be deflected: a) by using a magnet, and b) by using electrically charged plates. Inferring If a cathode ray is attracted to a positively charged plate, what can you infer about the charge of the particles that make up the cathode ray?

A cathode ray is deflected by electrically charged plates. 4.2 Subatomic Particles A cathode ray is deflected by electrically charged plates. In a cathode-ray tube, electrons travel as a ray from the cathode (-) to the anode (+). A television tube is a specialized type of cathode-ray tube.

4.2 Subatomic Particles Thomson concluded that a cathode ray is a stream of electrons. Electrons are parts of the atoms of all elements.

Such positively charged subatomic particles are called protons. 4.2 Subatomic Particles Protons and Neutrons In 1886, Eugen Goldstein (1850–1930) observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays. He concluded that they were composed of positive particles. Such positively charged subatomic particles are called protons.

4.2 Subatomic Particles In 1932, the English physicist James Chadwick (1891–1974) confirmed the existence of yet another subatomic particle: the neutron. Neutrons are subatomic particles with no charge but with a mass nearly equal to that of a proton.

4.2 Subatomic Particles Table 4.1 summarizes the properties of electrons, protons, and neutrons.

4.2 The Atomic Nucleus The Atomic Nucleus How can you describe the structure of the nuclear atom?

4.2 The Atomic Nucleus J.J. Thompson and others supposed the atom was filled with positively charged material and the electrons were evenly distributed throughout. This model of the atom turned out to be short- lived, however, due to the work of Ernest Rutherford (1871–1937).

Rutherford’s Gold-Foil Experiment 4.2 The Atomic Nucleus Rutherford’s Gold-Foil Experiment In 1911, Rutherford and his coworkers at the University of Manchester, England, directed a narrow beam of alpha particles at a very thin sheet of gold foil.

Rutherford’s Gold-Foil Experiment 4.2 The Atomic Nucleus 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. 4.2 The Atomic Nucleus Alpha particles scatter from the gold foil. 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.

The Rutherford Atomic Model 4.2 The Atomic Nucleus 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 called the nucleus. The nucleus is the tiny central core of an atom and is composed of protons and neutrons.

4.1 Section Quiz 4.1.

4.1 Section Quiz 1. The ancient Greek philosopher credited with suggesting all matter is made of indivisible atoms is Plato. Aristotle. Democritus. Socrates. ANS: C PTS: 1 REF: p. 101 OBJ: 4.1.1

2. Dalton's atomic theory improved earlier atomic theory by 4.1 Section Quiz 2. Dalton's atomic theory improved earlier atomic theory by teaching that all matter is composed of tiny particles called atoms. theorizing that all atoms of the same element are identical. using experimental methods to establish a scientific theory. not relating atoms to chemical change. ANS: C PTS: 1 REF: p. 102 OBJ: 4.1.2

3. Individual atoms are observable with the naked eye. 4.1 Section Quiz 3. Individual atoms are observable with the naked eye. a magnifying glass. a light microscope. a scanning tunneling microscope. ANS: D PTS: 1 REF: p. 103 OBJ: 4.1.3

1. Which of the following is NOT an example of a subatomic particle? 4.2 Section Quiz 1. Which of the following is NOT an example of a subatomic particle? proton molecule electron neutron ANS: B PTS: 1 REF: p. 104 OBJ: 4.2.1

2. The nucleus of an atom consists of electrons only. protons only. 4.2 Section Quiz 2. The nucleus of an atom consists of electrons only. protons only. protons and neutrons. electrons and neutrons. ANS: C PTS: 1 REF: p. 107 OBJ: 4.2.2

3. Most of the volume of the atom is occupied by the 4.2 Section Quiz 3. Most of the volume of the atom is occupied by the electrons. neutrons. protons and neutrons. protons. ANS: A PTS: 1 REF: p. 108 OBJ: 4.2.2

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