Chapter 3, Section 2 Organizing the Elements Thursday, November 12, 2009 Pages 81 -- 88

Objectives State key events in the historical development of the periodic table. List the information in the periodic table and describe how it’s organized. Compare the valence electrons in a period and in a family, and explain how they influence the properties of elements.

Vocabulary Words Atomic mass Periodic table Group Family Period

Historical Development By 1830, 55 elements had been discovered. A few were gases. 2 were liquids. Most were metals. Some were volatile, some were not.

Organization With the discovery of atoms, scientists were looking for a way to organize the elements in a useful way. It wasn’t until 1860 that a system was discovered that could be applied to all the elements.

Dimitri Mendeleev He observed that some elements have chemical and physical properties. He noted melting and boiling points. He noted density and color. He looked at atomic mass (average mass of one atom). He looked at the number of possible chemical bonds.

The Periodic Table Mendeleev noticed that patterns appeared when the elements were arranged in order of increasing atomic mass. Elements were then rearranged again based on similarities (characteristics).

Modern Periodic Table Properties of the elements repeat in each period (row). It wasn’t until the early 1900’s that the atomic number became an important part of the periodic table.

The Modern Periodic Table

Reading the Periodic Table The periodic table contains over 100 squares. Each square represents one element. Each square contains information about the element’s atomic number, chemical symbol, name, and atomic mass.

Organization of the Periodic Table An element’s properties can be predicted from its location in the periodic table. Groups The main body of the periodic table is arranged in 18 vertical columns. Each column is called a group. Groups are also called families. Groups are numbered from left to right. Elements in each group have similar charactistics.

Organization of the Periodic Table -- cont’d Each horizontal row across the table is called a period. There are 7 periods in the periodic table. A period contains a series of different types of elements from different families. Elements in a period have different properties. When you move from Left to right, the properties change in a pattern.

Why the Period Table Works Mendeleev was successful in placing these elements in a way that help us understand them better. Mendeleev, however, wasn’t able to explain why it worked. It words because it is based on the structure of atoms -- in particular, the valence electrons.

Why the Period Table Works -- cont’d When you move from left to right in the periodic table, you will notice that atomic number increases by 1. This means that an element has one more valence electron than the one to its left. By contrast, all the elements in a family have the same number of valence electrons.

Homework Workbook 3.2 due Monday, 11/16. Vocabulary quiz 3.2 on Tuesday, 11/17.