Download Chapter 14 ~ Atoms

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.
135 Chapter 14 Atoms and the Periodic Table
Section 14.1
Atoms
Terms:
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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.
136 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
137 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.
138 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 onetwelfth 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
139 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.
140 Section 14.2
The Periodic Table
Terms:
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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..
141 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.
142 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
143 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.
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