ChemistryToombs County High School

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▪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.

Democritus’s Atomic Philosophy How did Democritus describe atoms?

Democritus

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.

Dalton’s Atomic Theory▪How did John Dalton further Democritus’s ideas on atoms?

By using experimental methods, Dalton transformed Democritus’s ideas on atoms into a scientific theory. The result was Dalton’s atomic theory.

All elements are composed of tiny indivisible particles called atoms.

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Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

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Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.

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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.

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Sizing up the AtomWhat instruments are used to observe individual atoms?

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Despite their small size, individual atoms are observable with instruments such as scanning tunneling microscopes.

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▪ Iron Atoms Seen Through a Scanning Tunneling Microscope

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Cathode-ray tubes are found in TVs, computer monitors, and many other devices with electronic displays.

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Subatomic Particles What are three kinds of subatomic

particles?

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

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

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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.

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▪ Cathode Ray Tube

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▪ A cathode ray is deflected by a magnet.

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A cathode ray is deflected by electrically charged plates.

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

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Protons and NeutronsIn 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.

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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.

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▪ Table 4.1 summarizes the properties of electrons, protons, and neutrons.

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The Atomic Nucleus How can you describe the structure of

the nuclear atom?

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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).

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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.

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▪ Rutherford’s Gold-Foil Experiment

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▪ Alpha particles scatter from the gold foil.

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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.

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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.

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Just as apples come in different varieties, a chemical element can come in different “varieties” called isotopes.

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Atomic Number What makes one element different from

another?

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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.

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Mass Number How do you find the number of neutrons

in an atom?

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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.

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▪ Au is the chemical symbol for gold.

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Isotopes How do isotopes of an element differ?

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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.

▪ Despite these differences, isotopes are chemically alike because they have identical numbers of protons and electrons.

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Atomic Mass How do you calculate the atomic mass of

an element?

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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.

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▪ Some Elements and Their Isotopes

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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.

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▪ Weighted Average Mass of a Chlorine Atom

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for Conceptual Problem 4.3

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.

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▪ 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%.

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The Periodic Table—A Preview Why is a periodic table useful?

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A periodic table is an arrangement of elements in which the elements are separated into groups based on a set of repeating properties. A periodic table allows you to easily

compare the properties of one element (or a group of elements) to another element (or group of elements).

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▪ The Periodic Table

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Each horizontal row of the periodic table is called a period.

Within a given period, the properties of the elements vary as you move across it from element to element.

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▪ A Period

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Each vertical column of the periodic table is called a group, or family.

Elements within a group have similar chemical and physical properties.

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▪ A Group or Family

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Marie Curie was a Polish scientist whose research led to many discoveries about radiation and radioactive elements. In 1934 she died from leukemia caused by her long-term exposure to radiation. You will learn about the various types of radiation and their effects.

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Radioactivity How does an unstable nucleus release

energy?

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Marie Curie (1867-1934) and Pierre Curie (1859-1906) were able to show that rays emitted by uranium atoms caused fogging in photographic plates. Marie Curie named the process by which

materials give off such rays radioactivity. The penetrating rays and particles emitted

by a radioactive source are called radiation.

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Nuclear reactions differ from chemical reactions in a number of important ways. In chemical reactions, atoms tend to

attain stable electron configurations by losing or sharing electrons.

In nuclear reactions, the nuclei of unstable isotopes, called radioisotopes, gain stability by undergoing changes.

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An unstable nucleus releases energy by emitting radiation during the process of radioactive decay.

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Types of Radiation What are the three main types of nuclear radiation?

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▪ The three main types of nuclear radiation are alpha radiation, beta radiation, and gamma radiation.

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Alpha Radiation▪Alpha radiation consists of helium nuclei that have been emitted from a radioactive source. These emitted particles, called alpha particles, contain two protons and two neutrons and have a double positive charge.

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Beta Radiation▪ An electron resulting from the breaking apart

of a neutron in an atom is called a beta particle.

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Carbon-14 emits a beta particle as it undergoes radioactive decay to form nitrogen-14.

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Gamma Radiation▪ A high-energy photon emitted by a

radioisotope is called a gamma ray. The high-energy photons are electromagnetic radiation.

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▪ Alpha particles are the least penetrating. Gamma rays are the most penetrating.

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Radon-222 is a radioactive isotope that is present naturally in the soil in some areas. It has a constant rate of decay. You will learn about decay rates of radioactive substances.

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Nuclear Stability and Decay What determines the type of decay a

radioisotope undergoes?

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The nuclear force is an attractive force that acts between all nuclear particles that are extremely close together, such as protons and neutrons in a nucleus

At these short distances, the nuclear force dominates over electromagnetic repulsions and hold the nucleus together.

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More than 1,500 different nuclei are known. Of those, only 264 are stable and do not decay or change with time. These nuclei are in a region called the band of stability.

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The neutron-to-proton ratio determines the type of decay that occurs.

A positron is a particle with the mass of an electron but a positive charge. During positron emission, a proton changes to a neutron.

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Half-Life How much of a sample of a radioisotope

remains after each half-life?

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A half-life (t1/2) is the time required for one-half of the nuclei of a radioisotope sample to decay to products. After each half-life, half of the existing

radioactive atoms have decayed into atoms of a new element.

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Stable Isotope

▪ The ratio of Carbon-14 to stable carbon in the remains of an organism changes in a predictable way that enables the archaeologist to obtain an estimate of its age.

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Transmutation Reactions What are two ways that transmutation

can occur?

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The conversion of an atom of one element to an atom of another element is called transmutation. Transmutation can occur by radioactive

decay. Transmutation can also occur when particles bombard the nucleus of an atom.

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▪ The first artificial transmutation reaction involved bombarding nitrogen gas with alpha particles.

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▪ The elements in the periodic table with atomic numbers above 92, the atomic number of uranium, are called the transuranium elements.

All transuranium elements undergo transmutation.

None of the transuranium elements occur in nature, and all of them are radioactive.

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▪ Transuranium elements are synthesized in nuclear reactors and nuclear accelerators.

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The sun is not actually burning. If the energy given off by the sun were the product of a combustion reaction, the sun would have burned out approximately 2000 years after it was formed, long before today. You will learn how energy is produced in the sun.

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Nuclear Fission What happens in a nuclear chain

reaction?

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When the nuclei of certain isotopes are bombarded with neutrons, they undergo fission, the splitting of a nucleus into smaller fragments.

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In a chain reaction, some of the neutrons produced react with other fissionable atoms, producing more neutrons which react with still more fissionable atoms.

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▪ Nuclear Fission

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A Nuclear Power Plant

Neutron Moderation▪ Neutron moderation is a process that slows

down neutrons so the reactor fuel (uranium-235 or plutonium-239) captures them to continue the chain reaction.

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Neutron Absorption▪ Neutron absorption is a process that

decreases the number of slow-moving neutrons. Control rods, made of a material such a cadmium, are used to absorb neutrons.

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Nuclear Waste Why are spent fuel rods from a nuclear

reaction stored in water?

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Water cools the spent rods, and also acts as a radiation shield to reduce the radiation levels.

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Nuclear Fusion How do fission reactions and fusion

reactions differ?

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▪ Fusion occurs when nuclei combine to produce a nucleus of greater mass. In solar fusion, hydrogen nuclei (protons) fuse to make helium nuclei and two positrons.

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Fusion reactions, in which small nuclei combine, release much more energy than fission reactions, in which large nuclei split.

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The use of controlled fusion as an energy source on Earth is appealing. The potential fuels are inexpensive and

readily available. The problems with fusion lie in achieving

the high temperatures necessary to start the reaction and in containing the reaction once it has started.

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In a smoke detector, radiation from the Americum nuclei ionizes the nitrogen and oxygen in smoke-free air, allowing a current to flow. When smoke particles get in the way, a drop in current is detected by an electronic circuit, causing it to sound an alarm. You will learn about some of the other practical uses of radiation.

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Detecting Radiation What are three devices used to detect

radiation?

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▪ Ionizing radiation is radiation with enough energy to knock electrons off some atoms of the bombarded substance to produce ions. ▪ Devices such as Geiger counters, scintillation

counters, and film badges are commonly used to detect radiation.

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▪ Radiation can produce ions, which can then be detected, or it can expose a photographic plate and produce images.

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Geiger Counter▪ A Geiger counter uses a gas-filled metal

tube to detect radiation.

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Scintillation Counter▪ A scintillation counter uses a phosphor-

coated surface to detect radiation.

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Film Badge▪ A film badge consists of several layers of

photographic film covered with black lightproof paper, all encased in a plastic or metal holder.

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Using Radiation How are radioisotopes used in medicine?

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Neutron activation analysis is a procedure used to detect trace amounts of elements in samples.

Neutron activation analysis is used by museums to detect art forgeries, and by crime laboratories to analyze gunpowder residues.

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Radioisotopes can be used to diagnose medical problems and, in some cases, to treat diseases.

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▪ This scanned image of a thyroid gland shows where radioactive iodine-131 has been absorbed.

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