Elements and Compounds

More than 100 chemical elements—substances that cannot be decomposed or broken into more elementary substances by ordinary chemical means—are known to exist in the universe. Russian chemist Dmitry Ivanovich Mendeleyev and German physicist Julius Lothar Meyer independently developed the periodic law of the chemical elements at about the same time in the late 19th century. Both discovered that arranging the elements in order of increasing atomic mass produced a table of chemical properties and reactivity patterns that were regularly repeated. This phenomenon—known as the periodic law—is most often represented in the periodic table of the elements.

Elements

Hydrogen, oxygen, chlorine, sodium, and iron are examples of elements. Elements cannot be resolved into simpler substances by ordinary heat, light, electricity, or attack by other substances. It requires either special equipment, such as a particle accelerator, or temperatures like those in the interior of the sun. Ninety elements are known to occur in nature, and 22 more have been made artificially. Out of this limited number of elements, all the millions of known substances are made. Abbreviating the names of the elements is often convenient. For each element, a symbol has been chosen that consists of one or two letters. The symbols are derived from the names of the elements; for example, H stands for hydrogen. The abbreviations are not always derived from the English names, however. The symbol Fe for iron comes from the Latin ferrum, and W for tungsten comes from the German wolfram.

Compounds

Salt, water, iron rust, and rubber are examples of compounds. A compound is made up of elements, but it looks and behaves quite differently, as a rule, from any of its component elements. Iron rust, for example, does not look and feel like its components: oxygen gas and iron metal. This individuality of properties, as well as other qualities, distinguishes a compound from a simple mixture of the elements it contains. Another important characteristic of a compound is that a compound has a definite, invariable composition, always containing the same elements in the same proportions by weight; this is the law of definite proportions.

Vocabulary

Substance

universe

physicist

atomic mass

heat

Equipment

occur

iron rust

rubber

contain

composition

weight

1. Ask three questions to each of the passages. Let your group mates answer them.

2. Retell the text.

3. Give brief characteristics of the elements that you have met in the text.

Your description should include: atomic number, atomic mass, group, period, row, chemical and physical properties.

Modal Verbs

Основные значения модальных глаголов - необходимость (долженствование), возможность и предположение. Некоторые глаголы имеют формы прошедшего времени: can - could, may - might (иногда называют и will - would, shall - should), тем не менее, все эти формы могут иметь самостоятельные, присущие только им оттенки значения.

Вместо недостающих неличных и аналитических форм модальных глаголов используют их эквиваленты: to have, to be (=must), to be able (=can), to be allowed, to be permitted (=may).