Chemical elements and compounds

An element is a pure substance containing only one kind of atom. When two distinct elements are chemically combined the result is termed a compound.

The term element was first used by the Greek philosopher Plato in about 360 B.C. Plato believed that the elements introduced a century earlier by Empedocles were composed of small polyhedral forms: tetrahedron (fire), octahedron (air), icosahedrons (water), and cube (earth).

There are 118 elements known today. Some elements occur pure in nature and have been known for thousands of years. Elements like iron, silver, gold, mercury, lead, tin and sulphur were known to the ancients. They were given Latin names by the early chemists. For example, iron was called ferrum, silver was called argentum, and gold was called aurum.

The first modern list of chemical elements was given in Antoine Lavoisier’s 1789 Elements of chemistry, which contained thirty-three elements, including light and caloric. During the 19^th century there was an increase in the number of elements discovered as chemists began to adopt quantitative methods. Dmitri Mendeleev had sixty-six elements in his periodic table of 1869. By the end of the 19^th century 81 elements had been known. Almost twice as many elements* were discovered in the 20^th century as were discovered up to that time. Credit for proof of elemental character goes to scientists in England, France, Sweden, Germany, Portugal, and the United States. Three of those scientists were: Marie Skladovska Curie (radium, polonium), Maquerite Perey (francium), and Ida Tacke (rhenium).

Each element has a name and has been given a shortened symbol of one or two letters. The element carbon is symbolized by the letter C, the element neon by letters Ne. Ten of the elements have symbols derived from the capitalized first letter of the Latin name of the element and, if necessary, by a second letter. Other elements have symbols that can be derived from their English names. But now consider a substance like water. Water can be identified by its properties: colour, taste, melting and boiling temperatures, and ability to dissolve sugar and salt. A demonstration of electrolysis can easily show that water can be changed into other substances, hydrogen gas and oxygen gas. Since water can be decomposed into two other substances, water must contain at least two kinds of atoms. Water is a compound.

Sugar is another substance that you know. One property of sugar is its sweetness. Another property is that it dissolves in water. Still another is the way it behaves when heated. Under definite temperature sugar not only begins to melt to a liquid but it also begins to decompose. Sugar can be decomposed to form water and charcoal in definite amounts. Sugar is a compound.

Actually there is a set of characteristics, called properties that characterize each substance. Chemists sometimes speak about chemical changes and physical changes. Physical changes do not mean a change in the composition of a substance. For example breaking, freezing, melting, boiling, and deforming a substance such as lead are considered physical changes. A chemical change or chemical reaction, however, changes the composition and characteristic or special properties of a substance.

Note: almost twice as many elements - почти в два раза больше элементов.

Exercise 1. Translate the following word combinations.

To dissolve sugar and salt, to adopt quantitative methods, a set of characteristics, under definite temperature, to dissolve in water, to produce in definite amounts, to identify the substance by its properties, to derive the name from, melting and boiling temperatures, to consider a substance, to produce in definite amounts, elements occurring in nature.

Exercise 2. Insert instead of gaps the proper words from the text.

1. The term “element” was first introduced by ….. 2. By the end of the 19^th century ….. elements had been known. 3. There are … elements known today. 4. Elements like ……, …., …., …., ….,….., ….. were known to the ancient people. 5. Iron was called …, silver was called…, and gold was called… in ancient times. 6. The first modern list of chemical elements was given by …. 7. Radium and polonium were discovered by Polish scientist …….. in 1898. 8. Francium was discovered by …… in 1939. 9. Ida Tacke, a German scientist, discovered ….. in 1925.

Exercise 3. Define the following notions.

1. An element. 2. A compound. 3. A periodic table. 4. A physical change 5. A chemical reaction. 6. Sugar. 7. Water.

Exercise 4. Answer the following questions.

1. Who was the first to use the term “element”? 2. What is an element according to the current definition? 3. What is a compound? 4. What names did ancients give to such elements as iron, silver and gold? 5. What languages are chemical names of elements based on? 6. Why was there an increase in the number of elements discovered in the 19^th century? 7. The scientists of what countries contributed to the discovery of elements? Give their names. 8. What is the most convenient and the most traditional presentation of elements? 9. How can water be identified? 10. In what way one can prove that water is a compound? 11. What are the main properties of sugar? 12. What is the product of decomposition of sugar under definite temperature? 13. Are heating, freezing, melting, boiling and deforming chemical or physical changes? 14. What is a chemical change?

Exercise 5. Make up the outline to the text and speak about chemical elements and compounds according to it.

Exercise 6. Read the text, entitle it and tell what contribution the mentioned scientists made in the field of chemistry.

As chemists discovered new elements, they also discovered that certain elements behave in a similar manner. Beginning in the mid-1800’s, several chemists suggested ways to organize the known elements into a periodic table so that (так чтобы) those with similar properties would be grouped together. Chief among these researchers were Dmitri Ivanovich Mendeleev, the Russian chemist, and the German chemist Julius Lotcher Meyer. D.I. Mendeleev developed a form of a periodic law, a basic principal in chemistry. His law states that the properties of chemical elements reoccur in regular pattern when the elements are arranged according to their atomic weight. Mendeleev’s work together with that of Meyer’s led to the periodic table, a systematic arrangement of the elements.

In 1869, Mendeleev proposed his arrangements of the elements in order of increasing atomic weight and according to similarity in properties. Mendeleev’s table has blank spaces for unknown elements. Later using the periodic law, he predicted (предсказал) the properties of these unknown elements. His predictions were confirmed by the discovery between 1875 and 1886 of three elements with these properties. Mendeleev also discovered the phenomenon of critical temperature, the temperature at which a gas or vapour may be liquefied by pressure.

Exercise 7. Read the texts and translate them with the help of a dictionary.

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