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Targeted Interventions for Proficiency in Science |
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Physical Science
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| Students know elements in the periodic table are arranged into groups and periods by repeating patterns and relationships. E/S Some Historical Background about the Periodic
Table It wasn’t until the early 1900s that Mendeleev’s inconsistencies were explained. Henry Mosely used X-rays to measure the number of protons in atoms of the known elements. When he arranged the elements in order of increasing atomic number (number of protons), the patterns in the properties of the elements were consistent from row to row in the table of elements. According to Mosely’s Periodic Law, there was a periodic recurrence of the properties of the elements when the elements were arranged in order of increasing atomic number. Note: Many scientists attempted to organize the elements previous to Mendeleev’s attempt. For more information about the development of the periodic table, see http://www.wou.edu/las/physci/ch412/perhist.htm
The Organization of the Modern Periodic
Table
Elements in the same column exhibit similar chemical behaviors and reactivities. The columns are called families because of this. Just as the members of human families tend to have some similar behaviors, elements in the same family behave similarly. For example, all group 1A metals react vigorously with water. To see a demonstration of the vigorous reactions that group 1A metals have with water, go to http://chemed.chem.purdue.edu/demos/main_pages/9.1.html. Columns are labeled in one of two ways: (1) a number/letter combination (as is shown in Figure 1 above) or (2) numbers only, 1 through 18, from left to right. Some groups/families have specific names. Using the first column naming convention, the elements in group 1A (with the exception of hydrogen, H) are called alkali metals. Group 2A elements are called alkaline earth metals. Group 7A elements are called halogens, and Group 8A elements are called noble gases. Rows are called periods because physical and chemical properties repeat in each period. This is similar to the weeks on a monthly calendar. The days of the week change in the same manner on each row of the calendar just as the properties of elements change in similar ways across each row of the periodic table. The periods are labeled with numbers. Period 1 is the first row of the periodic table, period 2 is the second row of the periodic table, and so on. It is often useful to understand other features of the organization
of the periodic table. Elements on the periodic table can be generally
organized as metals, nonmetals, and semimetals
(metalloids). Metals are typically shiny, ductile, malleable, and
good conductors of heat and electricity. Nonmetals are elements that
do not have the properties of metals, and
metalloids have some of the properties of metals and some of the properties
of nonmetals. In Figure 1, the boxes containing symbols of metalloids
are shaded purple. Metals are found to the left of the metalloids,
and nonmetals are found to the right of the metalloids (although hydrogen
is found to the left, it is a nonmetal). For more information about the blocks of the periodic table and how they relate to the organization of electrons in the electron cloud, see http://www.chemsoc.org/viselements/Pages/data/intro_patterns.html.
Why Do Elements in The Same Group Have Similar
Chemical Behaviors? The number of valence electrons can be determined from the element’s electron configuration. For the representative elements (those in the s and p blocks), the number of valence electrons is equal to the group number. For example, all elements in group 7A have 7 valence electrons and have similar chemical behaviors and properties. All elements in group 1A have 1 valence electron and have similar chemical behaviors and properties. For a very detailed discussion of this topic, see http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch6/quantum.html
Periodic Properties Atomic Radius (Size). Atomic radius is the distance from the center of the nucleus to the valence, or outer, electrons. Figure 3 shows atomic radius plotted against atomic number. The positions of the group 1A elements are marked. Although there are some exceptions, atomic radius generally decreases across a period and increases down a group (which can be seen by focusing on just the marked group 1A elements). That the size of atoms increases down a column makes sense to most students. The atoms get larger as the atoms have more protons, electrons, and neutrons.
The trend across the periodic table makes less sense to students. Atomic radius decreases across a period because the valence electrons in a given period are all located in the same shell; however, the nuclear charge (number of protons in the nucleus) increases across the period. Moving from left to right across a given period, the valence electrons feel a stronger pull from the increased nuclear charge that pulls the valence electrons closer to the nucleus (atomic size decreases across the period). Ionization Energy. Ionization energy is the amount of energy required to remove the most loosely held (outermost) electron from an atom in the gas state. The closer an electron is to the nucleus, the more difficult it is to remove because of the attraction between the negatively charged electron and the positively charged nucleus. For this reason, smaller atoms have higher ionization energies. Accordingly, ionization energy increases towards the top of a column and, in general, increases from left to right on the periodic table, as can be seen in Figure 4 (ionization energy v. atomic number).
Electronegativity. Electronegativity is defined as the attraction an atom has for the shared electrons in a bond. In general, electronegativity increases toward the top of each group and from left to right across the periodic table (electronegativities are not reported for noble gas elements because they do not tend to form bonds with other elements). Figure 5 shows electronegativity plotted against atomic number (the positions of the group 1A metals are marked).
Performance Benchmark P.12.A.2 Students know elements in the periodic table are arranged into groups and periods by repeating patterns and relationships. E/S Common misconceptions associated with this benchmark: 1. Students confuse atomic number with the number of valence electrons in an atom. In a neutral atom, the total number of electrons is equal to the atomic number (number of protons). Valence electrons are those electrons found in the outermost shell of the electron cloud. The number of valence electrons can be found for any element on the periodic table by writing out the electron configuration for that element. For representative elements (those in the s and p blocks, groups 1A – 8A), the number of valence electrons an atom has is equal to the element’s group number. For example, oxygen, which is found in group 6A and has an atomic number of 8, has 6 valence electrons. To learn more about this, go to http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch8/index.php#valence
2. Students confuse periods (rows) and groups (columns). Some students are not aware that columns are vertical and periods are horizontal. This is most likely due to a lack of knowledge about the nomenclature associated with the periodic table or students’ confusing the meaning of the words horizontal and vertical. Teachers should be aware of this potential confusion and check students’ understanding of the terms period and group by asking students questions such as “in which group is the element sulfur (S)?” or “in which period is the element calcium (Ca)?” For a short, but clear discussion on periods and groups, go to http://www.chem4kids.com/files/elem_pertable.html
3. Students incorrectly think that the larger the mass or atomic number of an atom, the larger the radius of the atom. Atomic radius generally increases down a column in the periodic table, but it DECREASES across a period as valence electrons feel greater effective nuclear charge. To learn more about atomic radii and other periodic properties, go to http://itl.chem.ufl.edu/2045_s00/lectures/lec_12.html
4. Students incorrectly think that the periodic table is arranged by increasing atomic weight. The modern periodic table is arranged in order of increasing atomic
number. In most cases, this order also corresponds to increasing atomic
weight; however, there are cases in which an element with a lower
atomic number has a higher atomic weight than an element with a higher
atomic number (see, for example, argon, Ar, and potassium, K). Molar
mass (atomic weight) can be visually graphed against atomic number
on the American Chemical Society’s interactive periodic table:
http://www.chemistry.org/portal/a/c/s/1/acsdisplay.html?
5. Students believe that an element will have similar chemical behaviors to other elements that are found in the same region of the periodic table. It is more correct to say that elements in the same group/family will have similar chemical behaviors. Elements that are found in the same groups/families (columns) have similar chemical behaviors because they have the same number and arrangement of valence electrons (the electrons that are involved in chemical reactions). Elements that are found in the same region of the periodic table may share physical properties. For example, oxygen (O) and nitrogen (N) are found in the same region of the periodic table. They are both considered to be nonmetals and, as such, have similar physical properties (they are both gases, not malleable, poor electrical conductors, etc.). Chemically, however, oxygen’s reactions are much more similar to those of sulfur (S), which is in the same group as oxygen, than they are to those of nitrogen. To learn more about chemistry misconceptions, go to http://educ.queensu.ca/~science/main/concept/chem/c07/C07CDTL1.htm. Performance Benchmark P.12.A.2 Students know elements in the periodic table are arranged into groups and periods by repeating patterns and relationships. E/S Sample Test Questions
1. What is the symbol for the element in period
3 and group 5A? 2. What property is the element sulfur (S) most
likely to have? 4. In which of the following pairs are both elements
in the same period? 5. The periodic table is arranged in order of
increasing _________. 6. Which of the following elements will react
similarly to tellurium (Te)? 7. Which of the following elements would be expected
to have the largest atomic radius? 8. Which of the following elements would be expected
to have the largest atomic radius? 9. Which of the following elements would be expected
to have the largest ionization 11. Which of the following subatomic particles
determines how an atom will behave in a 12. How many valence electrons does oxygen (O)
have?
Students know elements in the periodic table are arranged into groups and periods by repeating patterns and relationships. E/S Answers to Sample Test Questions Students know elements in the periodic table are arranged into groups and periods by repeating patterns and relationships. 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. Interactive Periodic Tables
2. Periodic Table Videos To access these videos, go to http://www.gpb.org/public/education/classroom/chemistry/index.jsp?pcode=unit4
3. Periodic Table Concept Map Worksheet To download this worksheet, go to http://www.chemsoc.org/pdf/LearnNet/miscon2/The_periodic_table.pdf
4. Hands-on Minds-on Periodic Table This interactive activity is found at http://www.csupomona.edu/~ceemast/science/
5. Looking for Patterns Activity To access these activities go to , |
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