EXTRACT A.

Date: 2015-10-07; view: 383.

In pairs ask and answer text-based questions.

Speaking.

18. Summarize the text ‘Covalent Bonds' in 150 words.

For example:

- How many elements are known to science today?

- Do you know that some elements do not exist at all in nature?

- How can we define valence electrons?

- And what if we consider atoms in a solid piece of pure silicon?

20. Divide into 2 groups. Group 1 translates Extract A and group 2 – extract B of the text “Semiconductors” with a dictionary in writing.

The two semiconductor materials used in the manufacture of semiconductor devices, such as diodes and transistors, are germanium and silicon. Both these materials fall into group IV of the Periodic Table of Elements. An atom of either substance may be represented by a central core having a positive charge and surrounded by orbiting electrons, each having a negative charge.

In its solid state silicon forms crystals of the diamond type, i.e. it forms a cubic lattice in which all the atoms except those at the surface are equidistant from their immediately neighbouring atoms. A study of crystal structure shows that the greatest number of atoms that can be neighbours to a particular atom at an equal distance away from that atom and yet be equidistant from one another is four. Hence each atom in a silicon crystal has four neighbouring atoms. In the crystal lattice each atom employs its four valence electrons to form covalent bonds with its four neighbouring atoms; each bond consists of two electrons, one from each atom. Each pair of electrons orbits around both its parent atom and a neighbouring atom.