[ New messages · Members · Forum rules · Search · RSS ]
Page 1 of 11
Forum moderator: ProfVietanh 
Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson twenty one (The Periodic Table of the Elements)

Lesson twenty one
ProfVietanhNgày: Thứ bảy, 2010-05-08, 12:36 PM | Tin nhắn # 1
Thượng úy
Chức vụ: Tổ trưởng chuyên môn
Tổng số bài viết: 160
Phần thưởng: 0
Danh tiếng: 5
Trạng thái: Offline
The Periodic Table of the Elements

More chemical reactions exist than anyone can imagine. Nevertheless, certain patterns of chemical reactivity have been recognized for more than 100 years. These patterns remain valid even though new reactions are always being discovered. Each chemical element has characteristic chemical properties. Moreover, certain groups of elements display similar chemical properties. In 1869, the Russian chemist Dmitri Mendeleev and the German chemist Julius Lothar Meyer independently discovered how to arrange the chemical elements in a table so that the elements in each column have similar chemical properties. This arrangement, the periodic table, contains all the known chemical elements.

Arrangement

The periodic table lists all the known elements in numerical order, starting with the lightest (hydrogen) and proceeding to the heaviest (uranium, among naturally occurring elements). The list is broken into seven rows. Each row is placed below the previous row in a way that places elements with similar chemical properties in the same column of the table. Moving across a row of the periodic table, the elements generally increase in mass and change dramatically in their chemical properties. Moving down a column, mass also increases, but the elements have similar chemical properties.

As we have known that the periodic table is based on the structure of atoms rather than on their masses. Elemental masses correlate closely with atomic structure, however, so ordering by mass is almost the same as ordering by structure. There are only three exceptions among more than 100 elements.

Figure 1shows the periodic table. Notice that rows 6 and 7 are quite long, which makes the table rather cumbersome. For convenience, 14 elements in the sixth row and 14 in the seventh row usually are separated from the rest of the table and placed beneath the main portion, as shown in Figure 2 and on the inside front cover of the book. This is the most common format for the periodic table.


Figure 1: Long form of Periodic table

Figure 2 Periodic table of the elements as used in common in practice

Metals, Nonmetals, and Metalloids

The elements can be divided into categories: metals, nonmetals, and metalloids. Examples of each appear in Figure 2. Except for hydrogen, all the elements in the left and central regions of the periodic table are metals. Metals display several characteristic properties. For example, they are good conductors of heat and electricity and usually appear shiny. Metals are malleable, meaning that they can be hammered into thin sheets, and ductile, meaning that they can be drawn into wires. Except for mercury, which is a liquid, all metals are solids at room temperature.

As Figure 3 shows, the nonmetals are found in the upper right corner of the periodic table. The properties of nonmetals are highly variable, but most nonmetals are poor conductors of electricity and heat. Running diagonally across the table between the metals and the nonmetals are six elements that are categorized as metalloids (B, Si, Ge, As, Sb, and Te). These dull-appearing, brittle solids are sometimes called semiconductors because they conduct electricity better than nonmetals but not as well as metals. Silicon and germanium are used in the manufacture of semiconductor chips in the electronics industry.


Figure 3: The six metalloids occupy a diagonal region of the periodic table between the metals and the nonmetals.


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson twenty one (The Periodic Table of the Elements)
Page 1 of 11
Search:

Powered by uCoz | Website Builder Templates | Designed by game cheats