End Show Slide 1 of 18 © Copyright Pearson Prentice Hall Defining 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. 4.1

End Show © Copyright Pearson Prentice Hall Defining the Atom > Slide 2 of 18 Early Models of the Atom Democritus’s Atomic Philosophy How did Democritus describe atoms? Democritus 4.1

End Show © Copyright Pearson Prentice Hall Slide 3 of 18 Defining the Atom > 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 4.1

End Show Slide 4 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Early Models of the Atom Dalton’s Atomic Theory How did John Dalton further Democritus’s ideas on atoms? 4.1

End Show Slide 5 of 18 © Copyright Pearson Prentice Hall Defining the Atom > 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. 4.1

End Show © Copyright Pearson Prentice Hall Slide 6 of 18 Defining the Atom > Early Models of the Atom All elements are composed of tiny indivisible particles called atoms. 4.1

End Show © Copyright Pearson Prentice Hall Slide 7 of 18 Defining the Atom > Early Models of the Atom Atoms of the same element are identical. The atoms of any one element are different from those of any other element. 4.1

End Show © Copyright Pearson Prentice Hall Slide 8 of 18 Defining the Atom > Early Models of the Atom Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. 4.1

End Show © Copyright Pearson Prentice Hall Slide 9 of 18 Defining the Atom > Early Models of the Atom 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. 4.1

End Show © Copyright Pearson Prentice Hall Defining the Atom > Slide 10 of 18 Sizing up the Atom What instruments are used to observe individual atoms? 4.1

End Show © Copyright Pearson Prentice Hall Slide 11 of 18 Defining the Atom > Sizing up the Atom Despite their small size, individual atoms are observable with instruments such as scanning tunneling microscopes. 4.1

End Show Slide 12 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Sizing up the Atom Iron Atoms Seen Through a Scanning Tunneling Microscope 4.1

End Show Slide 13 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Subatomic Particles What are three kinds of subatomic particles? 4.2

End Show Slide 14 of 18 © Copyright Pearson Prentice Hall Defining the Atom > 4.2 Subatomic Particles Three kinds of subatomic particles are electrons, protons, and neutrons.

End Show Slide 15 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Subatomic Particles Electrons In 1897, the English physicist J. J. Thomson (1856–1940) discovered the electron. Electrons are negatively charged subatomic particles. 4.2

End Show Slide 16 of 18 © Copyright Pearson Prentice Hall Defining the Atom > 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. 4.2

End Show Slide 17 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Subatomic Particles Cathode Ray Tube 4.2

End Show Slide 18 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Subatomic Particles A cathode ray is deflected by a magnet. 4.2

End Show Slide 19 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Subatomic Particles A cathode ray is deflected by electrically charged plates. 4.2

End Show Slide 20 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Subatomic Particles Thomson concluded that a cathode ray is a stream of electrons. Electrons are parts of the atoms of all elements. 4.2

End Show Slide 21 of 18 © Copyright Pearson Prentice Hall Defining the Atom > 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

End Show Slide 22 of 18 © Copyright Pearson Prentice Hall Defining the Atom > 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

End Show Slide 23 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Subatomic Particles Table 4.1 summarizes the properties of electrons, protons, and neutrons. 4.2

End Show Slide 24 of 18 © Copyright Pearson Prentice Hall Defining the Atom > The Atomic Nucleus How can you describe the structure of the nuclear atom? 4.2

End Show Slide 25 of 18 © Copyright Pearson Prentice Hall Defining the Atom > 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). 4.2

End Show Slide 26 of 18 © Copyright Pearson Prentice Hall Defining the Atom > The Atomic Nucleus Ernest Rutherford’s Portrait 4.2

End Show Slide 27 of 18 © Copyright Pearson Prentice Hall Defining the Atom > 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. 4.2

End Show Slide 28 of 18 © Copyright Pearson Prentice Hall Defining the Atom > The Atomic Nucleus Rutherford’s Gold-Foil Experiment 4.2

End Show Slide 29 of 18 © Copyright Pearson Prentice Hall Defining the Atom > The Atomic Nucleus Alpha particles scatter from the gold foil. 4.2

End Show Slide 30 of 18 © Copyright Pearson Prentice Hall Defining the Atom > 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. 4.2

End Show Slide 31 of 18 © Copyright Pearson Prentice Hall Defining the Atom > The Atomic Nucleus In the nuclear atom, the protons and neutrons are located in the nucleus. The electrons are distributed around the nucleus and occupy almost all the volume of the atom. 4.2

End Show Slide 32 of 18 © Copyright Pearson Prentice Hall Defining the Atom > 4.3 Atomic Number What makes one element different from another?

End Show Slide 33 of 18 © Copyright Pearson Prentice Hall Defining the Atom > 4.3 Atomic Number Elements are different because they contain different numbers of protons. The atomic number of an element is the number of protons in the nucleus of an atom of that element.

End Show Slide 34 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Atomic Number 4.3

End Show Slide 35 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Mass Number How do you find the number of neutrons in an atom? 4.3

End Show Slide 36 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Mass Number The total number of protons and neutrons in an atom is called the mass number. The number of neutrons in an atom is the difference between the mass number and atomic number. 4.3

End Show Slide 37 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Mass Number Au is the chemical symbol for gold. 4.3

End Show Slide 38 of 18 © Copyright Pearson Prentice Hall Defining the Atom > for Sample Problem 4.1 Problem Solving 4.17 Solve Problem 17 with the help of an interactive guided tutorial.

End Show Slide 39 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Isotopes How do isotopes of an element differ? 4.3

End Show Slide 40 of 18 © Copyright Pearson Prentice Hall Defining the Atom > 4.3 Isotopes Isotopes are atoms that have the same number of protons but different numbers of neutrons. Because isotopes of an element have different numbers of neutrons, they also have different mass numbers.

End Show Slide 41 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Isotopes Despite these differences, isotopes are chemically alike because they have identical numbers of protons and electrons. 4.3

End Show Slide 42 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Atomic Mass How do you calculate the atomic mass of an element? 4.3

End Show Slide 43 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Atomic Mass It is useful to to compare the relative masses of atoms to a standard reference isotope. Carbon- 12 is the standard reference isotope. Cabon-12 has a mass of exactly 12 atomic mass units. An atomic mass unit (amu) is defined as one twelfth of the mass of a carbon-12 atom. 4.3

End Show Slide 44 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Atomic Mass The atomic mass of an element is a weighted average mass of the atoms in a naturally occurring sample of the element. A weighted average mass reflects both the mass and the relative abundance of the isotopes as they occur in nature. 4.3

End Show Slide 45 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Atomic Mass To calculate the atomic mass of an element, multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products. 4.3

End Show Slide 46 of 18 © Copyright Pearson Prentice Hall Defining the Atom > Atomic Mass For example, carbon has two stable isotopes: Carbon-12, which has a natural abundance of 98.89%, and Carbon-13, which has a natural abundance of 1.11%. 4.3

End Show Slide 47 of 18 © Copyright Pearson Prentice Hall Defining the Atom > 4.2

End Show Slide 48 of 18 © Copyright Pearson Prentice Hall Defining the Atom > 4.2