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Unit 7 The Nature of Matter

• Chapter 14 ~ Atoms o Section 1 ~ Modeling an Atom o Section 2 ~ Mass of an Atom o Section 3 ~ The Periodic Table

• Chapter 15 ~ Elements, Compounds, and Mixtures

o Section 1 ~ Types of Substances • Chapter 16 ~ States of Matter

o Section 1 ~ Kinetic Molecular Theory o Section 2 ~ Forms of Matter

• Chapter 17 ~ Physical or Chemical Properties o Section 1 ~ Physical and Chemical Properties

Unit 7 covers the following framework standards: PS 5, 6 and 7. Content was adapted the following:

McLaughlin, C. W., & Thompson, M. (1999). Physical science. Columbus, Ohio/USA: Glencoe/McGraw-Hill.

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Chapter 14 Atoms and the Periodic Table Section 14.1 Atoms Terms:

• Chemical Symbol • Nucleus • Electron • Proton • Neutron • Atomic Number • Electron Cloud • Mass Number • Isotope • Average Atomic Mass

Chemical Symbols Do the letters C. Al, Ne, and Ag mean anything to you? Each letter of pair of letters is a chemical symbol, which is an abbreviated way to write the name of an element. The black material on a burned match is carbon—C. You may wrap food in foil made of aluminum—Al. Have you noticed the bright glow of electrical signs? Many are filled with neon—Ne. You often use coins that contain copper—Cu. Chemical symbols consist of one capital letter or a capital letter plus one or two small letters. For some elements, the symbol is the first letter of the element’s name. For other elements, the symbol is the first letter of the name plus another letter from its name. Some symbols, such as Ag, are derived from Latin. Matter and Atoms Over 2,400 years ago, Greeks defined atoms as the smallest part of matter. Atoms consist of three subatomic particles—protons (which are positively charged particles), neutrons (have no charge; are neutral), and electrons (negatively charged). The center of the atom, or nucleus, is positively charged. This is because the nucleus contains protons and neutrons, which also make up most of the mass of an atom. Electrons have a mass 1/2000 of protons, thus the electrons mass is negligible when finding the mass of an atom.

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Counting in Atoms The atomic number of an atom is the number of protons in its nucleus. Every atom of the same element has the same number of protons. For example, every carbon atom has six protons. Therefore, it has the atomic number 6. Atoms of different elements have different numbers of protons. For example, every carbon atom has six protons, but every oxygen atom has eight protons. In a neutral carbon atom, the number of electrons would also be six, cancelling the charge. Models of the Atom As scientists continued to study matter and atoms, they tried to form a mental picture or model of what an atom might look like. A model helps us understand something we cannot see directly, usually because it is too large or too small. As more information was collected, scientists changed their models. Therefore, the model of the atom we use today is the result of the work of many scientists.

In 1962, scientists developed a better model of the atom. In this model, the electrons moved about in a region called an electron cloud. This cloud surrounds the nucleus of the atom. It describes the region where an electron is likely to be at any time. The diameter of the nucleus is about 1/100 000

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the diameter of the electron cloud. To better understand this scale, suppose you built a model of an atom with an electron cloud as wide as a football field. The atom’s nucleus would be about the thickness of the wire in a paper clip! Because an electron’s mass is so small, it is impossible for you—or anyone—to describe exactly where it is as it moves in the atom. All anyone can give is its probable location. You may have heard the expression, “you can’t be everywhere at once.” The multiple-exposure photo to the right shows what it might be like if you could. The electron cloud model of the atom is based on the same idea. Scientists make calculations of the electron’s most probable locations around the nucleus. If each location were marked with a dot, the closer spacing of the dots would indicate the most probable area for an electron. This is called the electron cloud because the dots give a cloudlike appearance when taken altogether. Energy Levels and Electrons The figure below illustrates another way to look at the placement of electrons. The electrons in the atom make up the electron cloud. Within the electron cloud, electrons are at various distances from the nucleus. Electrons closest to the nucleus have low energy. Electrons farther away from the nucleus have higher energy. You can represent the differences of the electrons by picturing the atom as having energy levels.

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The Figure above illustrates the maximum that each energy level can hold in an atom. The lowest energy level can hold just two electrons. The second energy level can hold eight electrons, and the third energy level, a maximum of 18 electrons.

Masses of Atoms Atomic Mass When thinking about the small masses of atoms, scientists found that even grams were not small enough. The unit of measurement of those particles is the atomic mass unit (u). In fact, the mass of a proton or a neutron is almost equal to 1 u. This is not a coincidence—the unit was defined that way. The atomic mass unit is defined as one-twelfth the ass of a carbon atom containing six protons and six neutrons. Remember that the mass of a carbon atom is in its nucleus because the atom’s six electrons have a negligible mass. Therefore, each of the 12 particles in the nucleus must have a mass nearly equal to one-twelfth the mass of the carbon atom. Thus, a proton or a neutron has a mass of about 1 u. Mass Number The mass number of an atom is the sum of the number of protons and the number of neutrons in the nucleus of an atom. As you can see in the table below, the mass number of an atom is almost equal to the mass of its most common form, expressed in atomic mass unit. If you know the mass number and the atomic number of an atom, you can then calculate the number of the neutrons. The number of neutrons is equal to the atomic number subtracted from the mass number. Number of neutrons = mass number – atomic number. Isotopes Not all the atoms of an element have the same number of neutrons. Atoms of the same element that have different numbers of neutrons are called isotopes. Scientists use the average atomic mass of an element, which is the average mass of the mixture of

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its isotopes.

Summary

• There are more than 100 elements that combine in a multitude of ways to produce compounds that make up all of the living and nonliving things that we encounter.

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Section 14.2 The Periodic Table Terms:

• Periodic Table • Group • Period • Nonmetal • Metal • Metalloid

Structure of the Periodic Table Dimitri Mendeleev, a Russian chemist, searched for a way to organize all the known elements. In the late 1800s, he arranged the elements by increasing atomic masses. He discovered that there was a pattern—chemical properties found in lighter elements could be shown to repeat in heavier elements. Because the pattern repeated, it could be considered periodic. Today we call this arrangement a periodic table of elements.

An Improved Table Although Mendeleev’s arrangement of elements was successful it needed some changes. On Mendeleev’s table, the atomic mass gradually increased from left to right in each row. If you look at the modern periodic table, you will see several examples, such as cobalt and nickel, where the mass decreases from left to right. However, you may notice that the atomic number always increases from left to right. The work of Henry G.J. Moseley, a young English scientists, in 1913 led to the arrangement of elements based on their properties and atomic numbers instead of an arrangement based on atomic masses..

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Each box in the periodic table contains information about the elements that you studied earlier in this chapter. Look at the figure to the right. This box represents the element boron. The atomic number, chemical symbol, name and average atomic mass are included in this box. The boxes for all the elements are arranged in order of their atomic numbers. Groups of Elements The vertical columns in the periodic table are called groups, or families. The groups are number 1 through 18. Elements in each group have similar properties. For example, the elements copper, silver, and gold are all found in Group 11 on the periodic table. Each is a shiny metal and a good conductor of electricity and heat. Atoms of different elements have different number of electrons. However, atoms of different elements may have the same number of electrons in their outer energy levels. It is the number of electrons in the outer energy level that determines the chemical properties of the element. Different elements with the same number of electrons in their outer energy level have similar chemical properties. These outer electrons are so important that a special way to represent them has been developed. A dot diagram uses the symbol of the element and dots to represent the electrons in the out energy level. Family Traits The dot diagrams of the atoms of elements in Group 17, called halogens, are shown in the figure below. They all have seven electrons in their outer energy levels. One similar property of the halogens is the ability to form compounds with elements in Group 1. The elements in Group 18 are known as noble gases. Noble gases do not usually form compounds. We say they are stable, or unreactive. The atoms of all the noble gases except helium have outer energy levels that contain eight electrons. New Elements You have now learned that each element can be specifically identified by its atomic number—the number of protons in the nucleus of an atom. The number of neutrons may vary, as with isotopes. The number of electrons also may vary if some outer ones are removed or added. However, if the number of protons of an atom changes, then the atom has a new identity. Periods The horizontal rows of elements in the periodic table are called periods.

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Notice the staircase line on the right side of the periodic table. All the elements to the left of this line, except hydrogen, are metals. Iron, zinc, and copper are examples of metals. Most metals have the common properties of existing as solids at room temperature and being shiny and good conductors of heat and electricity. Metals are also good conductors of heat and electricity. The atoms of metals generally have from one to three electrons in their outer energy levels. Metals tend to give up electrons easily. If a metal combined with a nonmetal, then the atoms of the metals will lose electrons to the atoms of nonmetals because they have a weaker gravity, or attraction to their electrons. This would result in an ionic bond. If two nonmetals combined to form a molecule, they would have a covalent bond.

Those elements to the right of the staircase line on the periodic table are classified as nonmetals. Oxygen, nitrogen, and carbon are examples of nonmetals. At room temperature, most nonmetals are gases and some are brittle solids. Most nonmetals do not conduct heat and electricity well.

The elements next to the staircase line are metalloids because they have properties of both metals and nonmetals. Boron and silicon are examples of metalloids.

Elements in Groups 3 through 12 are called the transition elements. They are metals but have properties not found in elements of other groups. Copper and iron are examples of common transition elements.

Metals in the Crust When we examine pictures of metals and their compounds w are really seeing only half the story. Where are metals found? How do we obtain them? Earth’s crust contains many compounds and a few examples of uncombined metals such as gold and copper. Metals must be dug, or mined from Earth’s hardened outer layer. Due to varying conditions in different areas, some metals are deposited more in one place than in another. For example, most of the world’s platinum is found in South Africa. Large amounts of cobalt can be found in Morocco and Canada.

Ores: minerals and Mixtures Metals in Earth’s curst that are in a combined forms are found in ores. Typically, an ore consists of a metal compound, or mineral, within a mixture of clay or rock. Lead metal is usually found combined with sulfur in the form of galena, or lead sulfide. After an ore is mined and brought from Earth’s surface, minerals must be separated from the rock. Then the mineral is often converted to another chemical form. Galena is converted to lead oxide. This step involves heat and is called roasting. finally, the metal is refined into a pure form. It may

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later be alloyed with other metals. Removes the waste rock an be very expensive. If the cost o removing the waste rock gets higher than the value of the desired material, the mineral will no longer be classified as an ore.

Summary

• The periodic table is organized by atomic number. The rows and periods group elements by properties.