1. Section 2: A Guided Tour of the Periodic Table
Preview
Key Ideas
Bellringer
Recognizing a Pattern
Changing the Arrangement
The Periodic Table of the Elements

2. Key Ideas
How did Mendeleev arrange the elements in his periodic table?
How are elements arranged in the modern periodic table?

3. Bellringer
One way to organize a large group of objects is to arrange them into groups of similar objects. This is how scientists organize all of the many elements. Practice the skill of categorizing by arranging the magazines listed below into similar groups.
Calling All Girls Homeopathic Medicine Modern Housekeeping
Computer World Sports and Scores Easy Car Repairs
Beautiful Homes Calling All Boys The Health Newsletter
Auto Racing All About Computing Football Stories
The Healthy Man Home Decorating Read Aloud Stories
Sporting Times Classic Cars Building a Web Site
Child’s Play The Healthy Woman Home Makers Magazine
Family Computing Golf for Everyone The Sports Car Story
Beautiful Homes Nursery Rhymes Good Nutrition
Car Trends How to Use the Internet Tennis Tips

4. Bellringer, continued 1. Arrange the magazines into similar groups.
2. What criteria did you use for grouping the magazines?
3. Once you arrange the magazines into groups, could you sort the material further to make it even more organized?
Calling All Girls Homeopathic Medicine Modern Housekeeping
Computer World Sports and Scores Easy Car Repairs
Beautiful Homes Calling All Boys The Health Newsletter
Auto Racing All About Computing Football Stories
The Healthy Man Home Decorating Read Aloud Stories
Sporting Times Classic Cars Building a Web Site
Child’s Play The Healthy Woman Home Makers Magazine
Family Computing Golf for Everyone The Sports Car Story
Beautiful Homes Nursery Rhymes Good Nutrition
Car Trends How to Use the Internet Tennis Tips

5. Recognizing a Pattern
How did Mendeleev arrange the elements in his periodic table?
In his periodic table, Mendeleev arranged elements in rows by increasing atomic mass.

6. Recognizing a Pattern, continued
Mendeleev was able to predict new elements.
Mendeleev left spaces in his
table to make the pattern fit.
He used the spaces to
successfully predict the
existence and properties of
elements not yet discovered.
A few elements did not fit the pattern.

7. Changing the Arrangement
How are elements arranged in the modern periodic table?
The modern periodic table organizes elements by atomic number. When the elements are arranged in this way, elements that have similar properties appear at regular intervals.

8. Changing the Arrangement, continued
As scientists learned more about the structure of the atom, they improved Mendeleev’s table.
Arranging the table by atomic number (number of protons) rather than by atomic mass fixed the discrepancies in Mendeleev’s table.
periodic law: the law that states that the repeating chemical and physical properties of elements change periodically with the atomic numbers of the elements

9. The Periodic Table of the Elements

10. Changing the Arrangement, continued
Elements become less metallic across each period.
period: a horizontal row of elements in the periodic table
Elements in a group have similar properties.
group: a vertical column of elements in the periodic table; elements in a group share chemical properties

11. Visual Concept: Periodic Table Overview
Click the button below to watch the Visual Concept.

12. The Role of Electrons
Why do elements within a group of the periodic table have similar chemical properties?
The periodic trends in the periodic table are the result of electron arrangement.

13. The Role of Electrons, continued
Valence electrons account for similar properties.
An element’s location in the periodic table is related to electron arrangement.
Example: Lithium and sodium, in Group 1, each have one valence electron.

14. Ion Formation What happens to an atom that gains or loses electrons?
If an atom gains or loses electrons, it no longer has an equal number of electrons and protons. Because the charges do not cancel completely, the atom has a net electric charge.

15. Ion Formation, continued
Group 1 elements form positive ions.
The single valence electron is easily removed.
Group 17 elements form negative ions.
The addition of one valence electron fills the outer energy level.

16. Atomic Number and Mass Number
What do atoms of an element have in common with other atoms of the same element?
Atoms of each element have the same number of protons, but they can have different numbers of neutrons.

17. Atomic Number and Mass Number, continued
The atomic number equals the number of protons.
atomic number: the number of protons in the nucleus of an atom
The mass number equals the total number of subatomic particles in the nucleus.
mass number: the sum of the numbers of protons and neutrons in the nucleus of an atom

18. Atomic Number and Mass Number, continued

19. Isotopes
Why do isotopes of the same element have different atomic masses?
Isotopes of an element vary in mass because their numbers of neutrons differ.

20. Isotopes, continued

21. Isotopes, continued The number of neutrons can be calculated.
number of neutrons = mass number – atomic number

22. Atomic Masses What unit is used to express atomic mass?
Because working with such tiny masses is difficult, atomic masses are usually expressed in unified atomic mass units.
unified atomic mass unit: a unit of mass that describes the mass of an atom or molecule; it is exactly 1/12 the mass of a carbon atom with mass number 12 (symbol, u)

23. Atomic Masses, continued
Average atomic mass is a weighted average.
Isotope abundance determines the average atomic mass.
Example: Chlorine-35 is more abundant than chlorine-37, so chlorine’s average atomic mass ( u) is closer to 35 than to 37.