2. Some facts about atoms

An atom may be spoken of as the smallest particle of any substance. If atoms cannot be seen it does not necessarily mean that they do not exist. It indicates that any particle, if pre­sent, must be extremely small. There are methods by means of which the sizes of atoms and their arrangement in molecules can be determined. One of these methods uses X-ray diffraction.

In the course of many investigations, chemists came to a conclusion that the atoms of different elements are all made essentially of three simple types of units, which were referred to as protons, neutrons, and electrons. We now believe that an atom is composed of a cloud of electrons that revolve about a central core of protons or of protons plus neutrons. Repeated experiments show that every atom has the same number of electrons as well as protons. The posi­tively charged protons form the nucleus of the atom, and balance the positive charges of the protons in the core of the atom. The neutrons are also found in the nucleus of the atom.

It was also found that many elements and compounds are com­posed of small numbers of atoms which are held together in a regular arrangement. These groups of atoms are referred to as molecules.

It is known that ninety-two elements occur in nature, and a number of others have been made by man in the laboratory. Every element is a special combination of protons, neutrons, and electrons. Each element is identified by the number of protons in its nucleus and is designated by a name and a symbol.

Element Number 1 is a combination of one proton and one electron. Long before its atomic structure was known this ele­ment was referred to as hydrogen, or "water-former", because water forms when hydrogen burns in air. Its symbol is H.

3. The measurements in chemistry

In order to understand the quantitative relationships which exist between

various kinds of matter, the chemist who is in­terested in matter and changes which

it undergoes, has to mea­sure the quantities of matter with which he works, that is since mass is the measure of the quantity of matter, he is to measure mass. The measuring device the chemist is to employ in this determination should be the balance.

Since for every chemical change there is always accompanying energy change which the chemist has to take into account, the calorimeter and the thermometer have to be used.

The chemist usually employs graduated cylinders, burettes, pipettes and volumetric flasks for the measurements of volumes of liquids, and the gas burette for the measurement of volumes of gases.

The chemist employs the barometer if he has to measure the pressure.

The analytical chemist and the physical chemist employ such devices as calorimeters, refractometers, and a number of electrical devices.

If the chemist is to examine very small samples of matter, he should use a microscope. The microscope is an instrument which by the combination of lenses permits man to see objects which are too small to be seen with a naked eye; it is an instru­ment which is useful in many sciences and which, although more frequently used in a qualitative way, can also be used quantitavely.