Matter, Elements and Atoms

Matter consists of one or more elements. Elements are substances that cannot be divided into other substances. An atom is the smallest particle of an element, which has all the properties of the element and can take part in chemical reactions. In each atom there is a nucleus containing a number of protons (each proton has a positive electrical charge) and a number of neutrons, having no electrical charge. The nucleus is surrounded by a number of electrons. Each electron has a negative electrical charge. The electrons orbit around the nucleus. An atom has as many protons as it has electrons. This results in a zero electrical charge of the atom. Electrons orbit around the nucleus in one of the several possible orbits. These orbits are called shells. In a silicon atom there are fourteen protons in the nucleus. Fourteen electrons orbit around the nucleus in three orbits. The first or inner shell contains two electrons and cannot hold any more electrons. In the second shell there are eight electrons. It cannot hold additional electrons either (также). In the third shell there are only four electrons. This shell can hold more electrons. The outer shell of any atom is called its valence shell. The number of electrons in the valence shell is known as the valency of the atom.

15. Read the text again and complete the sentences.

1. In each atom there is ….

2. … is known as the valency of the atom.

3. An atom is the smallest particle of an element, which ….

4. The nucleus is surrounded by ….

5. The outer shell of any atom is called ….

6. An atom has as many protons as ….

7. In a silicon atom there are ….

8. … are substances that cannot be divided into other substances.

9. … orbit around the nucleus in one of the several possible orbits.

Match the terms in Table A with their definitions in Table B.