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Learning objectives
- Familiarize yourself with the organization of the periodic table of elements.
Rutherford's nuclear model of the atom helped explain why atoms of different elements exhibit different chemical behaviors. The identity of an element is determined by itsatomic number (Z), the number of protons in the nucleus of an atom. Therefore, the atomic number is different for each element. Known items are arranged in ascending Z orderperiodic table(Drawing \(\PageIndex{1}\)). The rationale for the particular format of the periodic table will be explained later.miEach element is assigned a unique one-, two- or three-letter symbol. The names of the elements are listed on the periodic table along with their symbols, atomic numbers, and atomic weights. The chemical composition of each element depends on the number of protons and electrons. In a neutral atom, the number of electrons is equal to the number of protons.
The elements are arranged in Aperiodic table, which is probably the most important help in learning chemistry. It summarizes vast amounts of information about the elements in a way that makes it easy to predict many of their properties and chemical reactions. The elements are arranged in seven horizontal rows, in order of increasing atomic number from left to right and top to bottom. The rows are called periods and are numbered 1 through 7. The elements are arranged so that elements with similar chemical properties form vertical columns, called groups, numbered 1 through 18 (older periodic tables use a Roman numeral system). Groups 1, 2 and 13 to 18 are the main elements of the group, numbered A in the tables above. Groups 3 through 12 are in the middle of the periodic table and are transitional elements, labeled B in older tables. The two rows of 14 elements at the bottom of the periodic table are the lanthanides and actinides, whose positions in the periodic table are indicated in group 3.
A thick orange zigzag line running diagonally from top left to bottom right across groups of 13-16 inchesDrawing \(\PageIndex{1}\)Divide the elements into metals (in blue, below and to the left of the line) and non-metals (in bronze, above and to the right of the line). The gold-colored elements found along the diagonal line exhibit properties intermediate between metals and non-metals; They are called semi-metals.
The distinction between metals and non-metals is one of the most fundamental in chemistry. Metals such as copper and gold are good conductors of electricity and heat; they can be pulled into wires because they are malleable; they can be hammered or pressed into sheets or thin sheets because they are malleable; and most have a glossy appearance, so they are shiny. The vast majority of known elements are metals. Of the metals, only mercury is liquid at room temperature and pressure; all others are solid.
By contrast, non-metals are generally poor conductors of heat and electricity and are not shiny. Non-metals can be gases (such as chlorine), liquids (such as bromine), or solids (such as iodine) at room temperature and room pressure. Most solid non-metals are brittle, so they break into small pieces when struck with a hammer or pulled by a wire. As expected, semimetals exhibit properties intermediate between metals and nonmetals.
Example \(\PageIndex{1}\): Classification of elements
Given its position in the periodic table, would you expect selenium to be a metal, non-metal or semi-metal?
Dany: element
asked about: Classification
Strategy:
Find selenium in the periodic table shown in the pictureDrawing \(\PageIndex{1}\)and then sort the item based on its location.
Solution:
Selenium's atomic number is 34, which places it in period 4 and group 16. InDrawing \(\PageIndex{1}\), selenium is above and to the right of the diagonal line that marks the boundary between metals and nonmetals, so it should be a nonmetal. However, keep in mind that since selenium is close to the dividing line between metals and non-metals, it would not be surprising if selenium is similar to a semi-metal in some of its properties.
Exercise \(\PageIndex{1}\)
Based on its position in the periodic table, do you expect indium to be a non-metal, metal or semi-metal?
- Answer
metal
As mentioned above, the periodic table of elements is arranged so that elements with similar chemical properties are in the same group. Chemists often make general statements about the properties of the elements in a group using descriptive names with historical origins. For example, the elements in group 1 are known as the alkali metals, group 2 are the alkaline earth metals, group 17 are the halogens, and group 18 are the noble gases.
Group 1: Alkali metals
The alkali metals are lithium, sodium, potassium, rubidium, cesium and francium. Hydrogen is unique in that it is generally placed in group 1, but it is not a metal. Alkali metal compounds are common in nature and in everyday life. An example is table salt (sodium chloride); Lithium compounds are used in fats, batteries, and as drugs to treat patients who exhibit manic-depressive or bipolar behavior. Although lithium, rubidium, and cesium are relatively rare in nature, and francium is so unstable and highly radioactive that it only occurs in trace amounts, sodium and potassium are the seventh and eighth most abundant elements in the earth's crust, respectively.
Group 2: Alkaline earth metals
alkaline earth metalsThese are beryllium, magnesium, calcium, strontium, barium and radium. Beryllium, strontium and barium are rare, and radium is unstable and highly radioactive. In contrast, calcium and magnesium are the fifth and sixth most common elements on Earth, respectively; they are found in huge deposits of limestone and other minerals.
Group 17: Halogens
halogensThese are fluorine, chlorine, bromine, iodine and astatine. The name halogen comes from the Greek words for "salt formation", meaning that all halogens react quickly with metals to form compounds such as sodium chloride and calcium chloride (used in some areas as road salt).
Compounds containing a fluoride ion are added to toothpaste and water to prevent cavities. Fluoride is also found in Teflon coatings on cookware. Although chlorofluorocarbon propellants and refrigerants are believed to lead to the depletion of the Earth's ozone layer and contain fluorine and chlorine, the latter is responsible for the adverse effects on the ozone layer. Bromine and iodine are present in smaller amounts than chlorine, and astatine is so radioactive that it occurs in nature in minute amounts.
Group 18: Noble gases
The noble gases are helium, neon, argon, krypton, xenon and radon. Since the noble gases consist of only single atoms, they are called monatomic. They are unreactive gases at room temperature and pressure. Due to their lack of reactivity, they were called inert gases or noble gases for many years. However, the first chemical compounds containing noble gases were prepared in 1962. Although noble gases are relatively minor components of the atmosphere, natural gas contains significant amounts of helium. Due to its low reactivity, argon is often used as a non-reactive (inert) soldering atmosphere and in light bulbs. The red light emitted by neon in a discharge lamp is used in neon signs.
Noble gases are not reactive at room temperature and pressure.
summary
The periodic table is used as a predictive tool. Put the elements in order of increasing atomic number. Elements with similar chemical compositions appear in vertical columns called groups (numbered 1 to 18 from left to right); the seven horizontal rows are called periods. Some groups have commonly used common names, including the alkali metals (group 1) and alkaline earth metals (group 2) on the left, and the halogens (group 17) and noble gases (group 18) farthest away. Normal. Elements can be broadly divided into metals, non-metals and semi-metals. Semi-metals exhibit properties intermediate between those of metals and non-metals. Metals are on the left side of the periodic table and non-metals are on the top right. They are separated by a diagonal strip of semi-metals. Metals are shiny, good conductors of electricity, and easy to shape (they are ductile and ductile), while solid non-metals are generally brittle and poor conductors of electricity. Other important groups of elements in the periodic table are the main group elements, the transition metals, the lanthanides, and the actinides.