N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
Chemistry
Unit 2: Atomic Structure
Clarifying Statements
May, 2006
The focus of this unit, atomic structure, is very important to the study of
chemistry; as it lays the foundation for many other concepts. Students must “know
that atomic structure is determined by nuclear composition, allowable electron
cloud, and subatomic particles” (TEKS 6). The student is also expected to
“describe the existence and properties of subatomic particles” (TEKS 6.A).
The current definition of an atom is the “smallest particle of an element that
retains the chemical properties of the element” (Modern Chemistry HRW, p. 10).
Students must understand the development of this definition by learning about the
history of various atomic theories. In about 400 BC, Democritus called the basic
part of nature the “atom”, meaning “indivisible.” However, it was not until the late
1700s that chemists began to understand the atom’s properties in more detail.
Improved technology and measuring scales allowed chemists to develop the law of
conservation of mass, law of definite proportions, and the law of multiple
proportions. John Dalton unified these laws into a theory which we now know as
Dalton’s atomic theory.
Dalton’s atomic theory was eventually modified into the Modern Atomic Theory.
This theory states that matter is composed of atoms; and atoms of one element
have different properties than atoms of another element.
The atom is divided into two main regions: the nucleus and the electron cloud. The
nucleus, which contains protons and neutrons, is located in the center of the atom
and is surrounded by the electron cloud.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for instructional
design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
Chemistry
Unit 2: Atomic Structure
Clarifying Statements
May, 2006
Students must be able to describe the characteristics of the three main types of
subatomic particles; protons, neutrons, and electrons. This description should
include the correct mass number and charge, such as in the figure below. Students
can further investigate the properties of the electron by studying J. J. Thomson’s
experiments with cathode ray tubes. They can do the same for the nucleus by
studying E. Rutherford’s gold foil experiment.
The atomic number of an element can be found on the periodic table and is equal to
the number of protons in one atom. The mass number of an element can also be
found on the periodic table and is equal to the number of protons and neutrons in
one atom. The average atomic mass is calculated from the abundance of isotopes
found in nature. Students must be able to determine the atomic number, mass
number, and average atomic mass of various elements using the periodic table and
simple calculations. Students must also know that an isotope is an atom of the
same element, with the same number of protons, but with a different number of
neutrons.
Teachers may wish to include an introduction to the concept of a mole in this unit.
(The mole is the SI unit for an amount of any substance). Chemists use the mole,
Avogadro’s number, and molar mass to relate masses (measured in grams) to the
actual number of atoms in a sample. These concepts are significant in the study of
quantitative analysis and will be studied with more detail in future units.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for instructional
design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
Chemistry
Unit 2: Atomic Structure
Clarifying Statements
May, 2006
Next, the students will investigate the location of the electrons in an atom and
recognize their significance in chemical bonding. Students will study the Quantum
Model of the Atom in order to understand that electrons exist in specific energy
levels. They must also know that valence electrons are located in the outermost
(highest) energy level. Students should begin to understand the importance of
valence electrons in determining the chemical behavior of an element.
Students should know the following background information leading to the
development of the periodic table. Scientists initially used the information learned
from the discovery of the subatomic particles in order to create a table of
elements. Mendeleev was the first to notice a repeating, periodic pattern of
properties when he arranged the elements according to increasing atomic mass.
About 40 years later, Moseley rearranged the table by increasing atomic number,
an arrangement which lead to the modern periodic table shown below; as well as the
periodic law.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for instructional
design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
Chemistry
Unit 2: Atomic Structure
Clarifying Statements
May, 2006
Students must understand the periodic table’s arrangement and should be able to
identify the families such as noble gases, halogens, alkaline earth metals, etc.
Students should also be familiar with the characteristics of the families, including
number of valence electrons. They must know that all members of a family have
the same number of valence electrons and therefore have similar properties.
Students will also be able to compare and contrast the properties of metals,
nonmetals, and metalloids.
Using the characteristics of the families, atomic numbers, and their knowledge of
valence electrons, students must be able to make predictions of periodic
properties. Students will investigate the periodic trends of atomic radii,
ionization energy, electron affinity, and electronegativity as they are related to
the chemical behavior of the element. These concepts are important as they are
often referred to in the remaining units.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for instructional
design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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