ADDITIONAL TEXT. I. Read and translate the text

I. Read and translate the text. Be ready to fulfil the tasks that follow.

PROPERTIES OF CHEMICAL REACTIONS

An atom is the smallest particle of an element that can retain the chemical properties of that element. It is composed of a nucleus, which contains positively charged protons and neutral neutrons, around which negatively charged electrons revolve in orbits. For example, a helium atom contains 2 protons, 2 neutrons, and 2 electrons.

An ion is a positively or negatively charged atom or group of atoms. An ion which is negatively charged is called an anion, and a positively charged ion is called a cation.

Isotopes are alternate forms of the same chemical element. A chemical element is defined in terms of its atomic number, which is the number of protons in its nucleus. Isotopes of an element have the same number of protons as that element, but a different number of neutrons. Since atomic mass is determined by the number of protons plus neutrons, isotopes of the same element have varying atomic masses.

All substances are composed of matter, they have mass and occupy space. Elements and compounds constitute two general classes of matter. Elements are substances that consist of identical atoms (i.e., atoms with identical atomic numbers). This definition of an element includes all isotopes of that element. Hence O¹⁸ and O¹⁶ are both considered to be elemental oxygen. A compound is a substance that is composed of two or more different kinds of atoms (two or more different elements) combined in a definite weight ratio. This fixed composition of various elements, according to law of definite proportions, differentiates a compound from a mixture. Elements are the substituents of compounds. For example, water is a compound composed of the two elements hydrogen and oxygen in the ratio 2:1, respectively. This compound may be written as H₂O, which is the molecular formula of water. The subscript “2” that appears after the hydrogen (H) indicates that in every molecule of water there are two hydrogen atoms. There is no subscript after the oxygen (O) in the molecular formula of water, which indicates that there is only one oxygen atom per molecule of water. Hence water is a compound whose molecules are each made up of two hydrogen atoms and one oxygen atom.

II. PO ST-READING TASKS.

1. Define the following terms: atom, isotope, ion, element, compound.

2. Describe the differences between an element and a compound.

3. Talk about different constituents of matter.

UNIT 2

Дата добавления: 2015-09-03