Content Benchmark P.8.A.4
Students know atoms often combine to form molecules, and that compounds form when two or
more different kinds of atoms chemically bond. E/S
Molecules
An understanding of the nature of molecules provides an essential conceptual framework for
future understanding of the physical and chemical properties of matter, and the physical and
chemical changes that occur in matter. Atoms of most elements do not exist independently. They
either form molecules or ions.
A molecule is the smallest particle of a substance that exists independently and retains the
properties of that substance. A molecule is formed when two or more atoms chemically combine.
Elements and compounds can form molecules. A compound is a molecule that contains at least
two different elements or kinds of atoms. All compounds are molecules, but not all molecules are
compounds. For example, molecular hydrogen (H2) is not a compound, and other molecular
elements include oxygen (O2), nitrogen (N2), fluorine (F2), chlorine (Cl2), bromine (Br2), and
iodine (I2). Water (H2O), carbon dioxide (CO2) and glucose (C6H12O6) are compounds because
each is made from more than one element. Remember, the smallest unit of each of these
substances is a molecule. Figure 1 is a diagram of a water molecule that contains one oxygen
atom and two hydrogen atoms chemically bonded by sharing electrons.
Figure 1. Model of a water molecule
(From http://www.historyoftheuniverse.com/shelmod6.html)
To learn more about the difference between molecules and compounds, go to
http://education.jlab.org/qa/compound.html
Ions
Ions are electrically charged atoms or molecules. If an atom or molecule has gained electrons, it
has a negative charge. If an atom or molecule has lost electrons, it has a positive charge. The
following diagram shows sodium (Na+) having a positive charge because it has given an electron
to the chlorine, and thereby, gives the chorine (Cl-) a negative charge. The resulting opposite
charges of the ions causes the formation of a bond between the two ions.
Na
+
Cl
-
Figure 2. A model of the ionic bond between sodium and chlorine in table salt.
For an animation of the formation of an ionic bond between sodium and chlorine visit
http://www.promotega.org/UGA06004/ionic_bonds.html
Ionic Compounds
Metals often react with nonmetals to form ionic compounds. Ionic compounds have positive and
negative ions formed by gaining or losing electrons, which in turn, forms neutral atoms and
molecules. For example 2Na + Cl2
2NaCl. This equation can be restated as two
atoms of sodium react with one diatomic molecule of chlorine to form two ions of sodium
chloride. Individual molecules do not exist in ionic compounds. Instead the positive and negative
ions are arranged in a specific pattern to form a unit called a crystal.
Figure 3. Sodium Chloride, an ionic compound
(From http://www.americanchemistry.com/s_chlorine)
Molecular Compounds
Nonmetals combine with each other to form covalent compounds that exist as neutral molecules.
For example 2H2 + O2
2H2O. This equation can be restated as two diatomic
molecules of hydrogen react with one diatomic molecule of oxygen to form two molecules of
water. In covalent bonding, electrons are shared. The electron clouds of the bonded atoms
overlap. Figure 4 is the diagram of another covalent compound, methane, showing the sharing of
electrons between one carbon atom and four hydrogen atoms.
Figure 4. A model of covalent bonding in methane molecule.
Each hydrogen atom can share one electron and the carbon atom can share four electrons.
(From http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_106.gif)
To learn more about chemical compounds and covalent bonds, go to
http://www.elmhurst.edu/~chm/vchembook/143Aioniccpds.html
Chemical Formulas
Chemical formulas can be subdivided into empirical formulas, molecular formulas, and
structural formulas. Empirical formulas are based on the mass of the elements represented in the
formula. This type of formula shows the relative number of atoms of each element in a
compound. For example, in the ionic compound of sodium chloride (NaCl) the ratio of sodium
(Na+) ions to Chlorine (Cl-) ions is one to one. Now consider the molecular compound glucose.
The empirical formula for glucose is CH6O. This formula is written to indicate there is one
carbon atom and one oxygen atom for every six atoms of hydrogen.
The molecular formula for glucose however, is C6H12O6. Molecular formulas show the actual
number of atoms of each element in a molecule of a compound. Middle school students should
become familiar with molecular formulas. Being able to identify the kinds and numbers of atoms
in a molecular formula will allow them to take greater conceptual strides in understanding
chemical formulas when they enter high school.
For a guide on determining molecular formula, go to http://pages.towson.edu/ladon/empiric.html
The third type of chemical formula the structural formula shows the number of atoms, their
placement in a molecule, and the bonds between. The structural formula for glucose is shown in
figure 5.
Figure 5. Structural formula of a glucose molecule
(From http://www.purchon.com/biology/sugars.htm#glucose)
For more information about chemical formulas, go to
http://www.science.uwaterloo.ca/~cchieh/cact/cl20/formula.html
Coefficients and Subscripts in Molecular Formulas
Three molecules of carbon dioxide are represented by the molecular formula 3CO2. In this
formula the coefficient is represented by the number 3 preceding the C. The subscript is the
small number two to the lower right of the O. With this molecule, there are three molecules, as
indicated by the coefficient: with a total of three carbon atoms and six oxygen atoms (3 groups of
oxygen pairs). Students benefit greatly from both building and drawing molecules and matching
them to their correct molecular formulas. Once they have mastered this they are ready to build
models for chemical reactions.
More information about coefficients and subscripts, and their use in balancing chemical
equations, can be found at
http://dbhs.wvusd.k12.ca.us/webdocs/Equations/Balance-Equation.html
Properties of Matter
Atoms are recycled. They come together and bond to form molecules. When those molecules
decompose, the same atoms rearrange with other atoms to form new molecules. The chemical
make-up of a substance formed when bonding occurs determines what the physical and chemical
properties of that substance will be. The properties or characteristics of matter are generally
divided in two groups: physical and chemical. Whereas chemical composition cannot directly be
observed, we can use our senses to observe properties. From our observation of the properties of
a substance, we can infer the chemical nature of the substance.
Physical Properties of Matter
Physical properties do not change the chemical structure or composition of matter. Altering
physical properties will not change the basic nature of the atom or molecule. Physical properties
can be determined both quantitatively and qualitatively. Quantitative properties can be measured
and include quantities such as distance, mass, and time. Qualitative properties are those that can
be observed using our senses of smell, taste, sight, hearing, and touch. Examples of qualitative
properties include odor, color, and texture.
Groups of elements are often categorized by their common properties. For example, metals are
generally described by their tendency to share the physical properties of a shiny luster, ductility,
malleability and being good conductors of heat and electricity. The table below provides a
partial list of common physical properties of matter. This list has been limited to those properties
most likely to be observed and discussed within the middle school classroom.
Common physical properties of matter
Mass
Volume
Density/Specific gravity
Melting/Freezing point
Boiling/Condensing point
Texture
Color
Odor
Solubility/Dissolving
Polarity
Appearance
Specific heat
Conductivity
Viscosity
Surface tension
Malleability
Ductility
Electrical conductivity
Thermal conductivity
Hardness
Luster
Elasticity
Attraction or repulsion of magnets
Ability to transmit light (transparency/translucency/opacity)
Chemical Properties of Matter
Chemical properties do change the chemical structure of matter. More specifically, chemical
properties refer to the ability of matter to undergo chemical reactions to form new substances.
Below is a list of some common chemical properties of matter.
Chemical Properties of Matter
Reactivity with water
Reactivity with oxygen
Reactivity with metals
pH/reactivity with acid or base
Electromotive force
Flammability/Heat of combustion
Oxidation
Reduction
Physical Changes
Physical changes take place without any change in molecular composition. The same elements or
compounds are present before and after the change. The same molecules are present through out
the change. Physical changes are related to physical properties.
Changes in the state of matter are physical changes. To learn more about the states of matter and
how the change from one state to another, go to MS TIPS Benchmark P.8.A.1.
Another form of physical change is the separation or combination of substances without
affecting their chemical composition. You can stir, crush, cut, tear, dissolve, and smash a
substance without changing the fundamental composition of the substance, the molecule being
that substance in the case of bonded atoms.
Chemical Changes
Chemical changes alter the composition of the original matter. A different arrangement elements
or compounds are present at the end of the chemical change. In other words, in a chemical
change, a chemical reaction has occurred. The atoms in molecules/compounds are rearranged to
make new and different molecules/compounds.
Some chemical changes are reversible, but more often, they are difficult to reverse or cannot be
reversed. Chemical changes may also be accompanied by such characteristics as a color change,
different odor being given off, and light being emitted. Other characteristics can be popping or
fizzing sounds due to the release of gas and/or the release or absorption of heat. Endothermic
reactions are those that absorb or require heat, such as baking a cake or photosynthesis.
Exothermic reactions release heat to the surroundings, such as the reaction between calcium
chloride and water or when a substance is burned.
The rate at which a chemical reaction occurs can be affected by a number of variables including
temperature, concentration, amount, surface area, stirring, and the presence or absence of a
catalyst. Catalysts are substances that can either increase or decrease the rate at which a chemical
reaction occurs without being changed by the chemical reaction. Enzymes are examples of
catalysts that facilitate chemical reactions in the human body and other living organisms.
To learn more about the difference between chemical and physical changes, go to
http://www.chem4kids.com/files/matter_chemphys.html
Chemical Reactions and Changes in pH (acids, bases, and neutralization).
Acids and bases are most commonly identified by their properties. The following table
summarizes these properties.
Acids
Taste sour
Turn blue litmus red
Have a pH below 7
Are corrosive
Conduct electricity
Generate H+ ions in solution
React with metals
Bases
Taste bitter
Turn red litmus blue
Have a pH above 7
Are corrosive
Conduct electricity
Generate OH- ions in solution
Feel slippery
As an acid dissolves in water, its molecules separate and hydrogen ions (H+) are produced. The
H+ ions then combine with water (H2O) to form hydronium ions (H3O+). As a base dissolves in
water, its molecules separate and hydroxide ions (OH-) are produced. Pure water (HOH) is
neutral. The following equation shows the reaction between hydronium and hydroxide ions to
produce pure water: H3O+ + OH2H2O.
To learn more about how acids and bases react in water, go to
http://www.iun.edu/~cpanhd/C101webnotes/chemical%20reactions/acidbase.html
Some common acids include vinegar and citric acid. Common bases include ammonia and
baking soda. Care should be taken to ensure that students understand that not all substances that
contain H are acids, and not all substances that contain OH are bases. For example, sugar
(C6H12O6) contains H and OH, but because sugar molecules do not ionize in water, they are not
acidic or basic. The following diagram shows the dissolving of an acid and a base and
subsequent formation of ions in water.
Hydrochloric acid (HCL) dissolved in water
H+
Cl+
Cl H
H+
Cl-
Sodium hydroxide (NaOH) dissolved in water
Na+ OHOHNa+
Na+ OH-
A substance that changes color in the presence of an acid or base is an indicator. Indicators vary
in their ability to react with concentrations of acids or bases. Litmus is a common indicator used
to identify acids and bases. Litmus turns blue in the presence of a base and red in the presence of
an acid. Some common household substances can be used as acid-base indicators, including: (1)
red cabbage juice, (2) beet juice, (3) grape juice, (4) berry juices, and (5) red onion juice. These
substances work as indicators because they contain anthocyanin pigments that dissolve in water
and their color is dependent upon the level of acidity. Anthocyanin pigments in plants are
generally not seen due to the presence of chlorophyll which is green except in the fall when
chlorophyll breaks down and their red, blue and purple colors become visible.
More about natural acid and base indicators are found at
http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/natural-indicators.shtml
Indicators can identify substances as acids or bases but to determine how acidic or basic a
solution is the pH scale is used. pH stands for power of hydrogen and is a measure of the
concentration of hydronium ions (H3O+) in a solution. The pH scale ranges from 0-14. A pH of 7
is neutral (neither acid nor base). A pH below 7 is acid and a pH above 7 is base. The scale is
logarithmic, so each number represents a difference 10 times greater or smaller than the previous
number. For example, a solution with a pH of 8 would be 10 times more basic than a neutral
solution with a pH of 7. Figure 6 summarizes the pH scale, some common examples of
substances at each pH level, and the affects of an acid or base on the environment.
Figure 6. The pH scale with some common substances.
(From http://www.elmhurst.edu/~chm/vchembook/184ph.html)
Chemical reactions can result in pH changes. A neutralization reaction takes place when acids
and bases react with each other to produce a salt and water. The H+ ions of the acid and the OHions of the base react to form water. Acids can be added to bases to lower the pH and bases can
be added to acids to raise the pH. Many commercial chemicals are designed to affect pH
changes. For example, antacids are used to raise the pH in the stomach. Acid is also used to
neutralize lime deposits on and in pipes.
Acid Rain
One way humans affect chemical changes in the environment is by the burning of fossil fuels.
The burning of high-sulfur coal releases sulfur dioxide (SO2) into the atmosphere. When sulfur
dioxide dissolves in water sulfuric acid (H2SO3) forms. Oxides of nitrogen (NOx) are released
into the atmosphere from vehicle engines and power plants. When nitrogen dioxide dissolves in
atmospheric water vapor, nitric acid forms. These acids return to the earth in precipitation (rain,
snow or fog) and cause a decrease in the pH of bodies of water and soil, which in turn adversely
affects living organisms. These acids also dissolve the calcium carbonate in the limestone of
statues, monuments and buildings, causing physical damage.
To learn more about acid rain, go to http://www.epa.gov/acidrain/
Content Benchmark P.8.A.4
Students know atoms often combine to form molecules, and that compounds form when two or
more different kinds of atoms chemically bond. E/S
Common misconceptions associated with this benchmark
1. Students commonly fail to differentiate between atoms and molecules and also,
between elements and compounds.
Students need to explore atoms, molecules, elements and compounds in-depth in order to
confidently differentiate among these fundamental substances. The smallest particle of an
element, and also that retains the properties of that element, is an atom. The smallest
particle of a compound, and also that retains the properties of that compound, is a
molecule. The atoms of many elements form diatomic molecules. For example, a diatomic
molecule of oxygen (O2) contains two oxygen atoms that are covalently bonded. Atoms
combine to form molecules and elements combine to form compounds. The concept map
below summarizes the relationship between the fundamental building blocks of matter.
Matter
Pure substances
Elements
Smallest
Particles
Atoms
Pure substances
.
Compounds
Chemically bond to form
Chemically bond to form
Chemically bond to form
Smallest
Particles
Molecules
For a listing of this misconception and other student misconceptions about matter, visit the
Operation Physics American Institute of Physics website at
http://www.amasci.com/miscon/opphys.html
2. Students frequently fail to relate the properties of pure substances to the behavior of
certain types of particles.
During typical science lab investigations into chemical changes, students use their senses
to observe the signs of chemical change. Often, the investigation is followed by making a
list of the chemical and physical properties observed. Students then memorize the list of
properties without making the connection between those properties and what is happening
at the particle level during the reaction. Lesson plans should consistently be designed to
lead students to make connections between what they observe about a substance and what
is happening at the particle level. Having students memorize physical and chemical
properties of matter without making this connection often results in their viewing the study
of atoms and molecules as independent of the properties of matter they observe in their
daily interactions with substances and chemical reactions.
More information about this misconception, along with a free on-line publication dealing
with chemistry misconceptions, can be found at
http://www.rsc.org/education/teachers/learnnet/miscon2.htm
3. Students often incorrectly think that the formation of new substances just happens
rather than resulting from particle rearrangement.
Students often mistakenly perceive the products of a chemical reaction as existing in
addition to the reactants, rather than being formed by the rearrangement of the atoms and
molecules of the reactants. For example, students might view the reactants of the
photosynthesis process (water and carbon dioxide) as existing independently of the
products of the reaction (glucose and oxygen). They believe that the glucose and oxygen
are formed from different atoms than those that compose the water and carbon dioxide.
Using models to build the reactants of a chemical reaction atom by atom and then
rearranging those atoms to build the products provides students with concrete evidence of
particle rearrangement.
For more details on misconceptions associated with chemical reactions, go to
http://educ.queensu.ca/~science/main/concept/chem/c07/C07CDTL1.htm
4. Students perceive particles as being created or destroyed during chemical changes.
The Law of Conservation of Mass is very difficult for students to understand. Many
students memorize the definition, but do not apply it to specific chemical changes. For
example, when things burn or an explosion occurs, students often incorrectly believe that
matter is destroyed. In addition to building the reactants out of atom models and
rearranging them to form the products, some chemical reactions can be conducted on a
balance to show that mass is conserved. For example, putting a balloon containing baking
soda over a flask containing vinegar and then tipping the balloon up so the baking soda
falls into the vinegar. Since the reactants and products are contained within a closed system
the balance will show no change in mass.
More information about misconceptions related to the conservation of mass and chemical
reactions can be found at
http://intro.chem.okstate.edu/ChemSource/ChemRx/chemrx11.htm#Com
For a more detailed discussion of the Law of Conservation of Mass refer to MS TIPS
Benchmark P.8.A.5.
5. Students often believe that all acids are strong and will burn through things
Students often think of acids only in terms of chemicals used in a laboratory. They also
think of acids in terms of their ability to “eat” through skin, clothing, metal, and wood, etc.
Although acids do react with metals and are defined as corrosive, students need to
understand that acids are defined by the presence of H+ ions and vary in concentration.
For teacher tips on addressing student misconceptions related to the behavior of acids and
bases visit http://www.okstate.edu/jgelder/acidPage10.html#teacherlab
6. Many students think that with acids and bases, strength and concentration mean the
same thing.
Acids and bases are defined as strong or weak based on the number of molecules that
separate and ionize when dissolved in water. More specifically, strength refers to the
number of H+ and OH- ions present in the solution. Hydrochloric acid is a strong acid and
citric acid is a weak acid. Sodium hydroxide is a strong base and ammonia is a weak base.
Concentration refers to the amount of acid or base that is dissolved in water.
An excellent site that discusses student difficulties with acids and bases, and more
generally, chemical reaction is found at
http://www.education.vic.gov.au/studentlearning/teachingresources/science/scicontinuum/l
5physchem.htm
Content Benchmark P.8.A.4
Students know atoms often combine to form molecules, and that compounds form when two or
more different kinds of atoms chemically bond. E/S
Sample Test Questions
Questions and Answers to follow on a separate document
Content Benchmark P.8.A.4
Students know atoms often combine to form molecules, and that compounds form when two or
more different kinds of atoms chemically bond. E/S
Answers to Sample Test Questions
Questions and Answers to follow on a separate document
Content Benchmark P.8.A.4
Students know atoms often combine to form molecules, and that compounds form when two or
more different kinds of atoms chemically bond. E/S
Intervention Strategies and Resources
The following is a list of intervention strategies and resources that will facilitate student
understanding of this benchmark.
1. Structure of Matter
TutorVista has a comprehensive website that covers many chemistry topics, including
chemical and physical changes and acid-base reactions. The site is intended as a tutorial for
high school students, but much of the information is appropriate for middle school. The site
could be used to supplement homework and prepare students for quizzes and tests.
To access the chemistry section of the site, including excellent animations, visit
http://www.tutorvista.com/content/chemistry/chemistry-i/chemical-reactions/chemicalreactionsindex.php
2. Matter Lesson Plans
The Mathematics and Science Activity Center has a database rich in information and lesson
plans about the structure and nature of matter. This site has the added dimension of accessing
information specifically by state.
For access to these lessons visit http://edinfo.securesites.net/math_science/
3. Visual Approach to Teaching Ionic Compounds
David Harazin from Jamieson Elementary School in Chicago, Illinois offers a lesson plan for
teaching an understanding of how ionic compounds form. This lesson is very visual and easy
to understand and implement.
For access to this lesson plan that uses a visual approach to understand atomic structure and
bonding visit http://www.iit.edu/~smile/ch9308.html
4. Graphic Representation of Covalent Bonding
The National Health Museum has an excellent graphics gallery that provides easily
understood examples of ionic and covalent bond formation.
To access this website for information about bonding, plus free PowerPoint templates and
downloadable demonstrations visit
http://www.accessexcellence.org/RC/VL/GG/ecb/covalent_ionic_bonds.php
5. Physical and Chemical Properties
Indiana University Northwest provides a basic review of the meaning of the physical and
chemical properties of matter. A highlight of this website is an image of the periodic table
that can be clicked on to access the elements and their individual properties.
The Indiana University website address is
http://www.iun.edu/~cpanhd/C101webnotes/matter-and-energy/properties.html
Elmhurst College offers a website known as The Virtual Chembook. This site provides a
concise review of physical and chemical properties and physical and chemical changes. Very
clear and informative pictures are also included.
The website can be accessed at
http://www.elmhurst.edu/~chm/vchembook/105Achemprop.html
6. Physical and Chemical Changes Activities and Resources
Saskatchewan Schools in California have a colorful and interactive lab with video clips and
extensions.
To access their chemistry section visit
http://www.saskschools.ca/curr_content/science9/chemistry/lesson8.html
Quia website has flashcards, a word search, and matching and concentration games to help
reinforce vocabulary related to physical and chemical properties and changes.
Visit the website at http://www.quia.com/jg/540845.html
The State of Utah, Office of Education offers several activities that include video clips,
animations, and pictures related to physical and chemical properties and changes. This site is
targeted specifically for middle school physical science students.
To access these activities visit
http://www.schools.utah.gov/curr/science/core/8thgrd/sciber8/matter/html/intro.htm
7. Acid & Base pH Lab
Hampshire County Schools in West Virginia offers an acid and base lab using household
substances. A variety of techniques including a lab activity, Venn diagram and a concept map
to teach the concepts of acid, base, and pH are presented. A data table is also included.
Access this activity at http://rms.hamp.k12.wv.us/alkire/pHlab.html