1. Suppose there were no limit to the power of our microscopes, so that we could examine a drop
Of water under stronger and stronger lenses indefinitely. What sort of microscopic world would we discover when the drop was enlarged, say a million times? Would we still see structureless, transparent, liquid water? Or would we perhaps see distinct particles, the building blocks, as it were, of the water that to our unaided senses is a completely uniform substance? Long ago people began to suspect that matter, despite it appearance of being continuous, actually possesses a definite structure on a microscopic level. This suspicion did not take on a more concrete form until early in the last century. Since then the existence of atoms and molecules, the ultimate particles of matter in its common forms, has been amply demonstrated, and their own ultimate particles had been identified and studied as well.
Elements and Compounds

2. Elements are substances that cannot be decomposed

or transformed into one another by ordinary chemical or physical means. The earth contains only a limited number of elements, and all other materials consist of two or more of them combined in various ways. Of the 105 known elements (not all found on the earth), 11 are gases, 2 are liquids, and the rest are solids at room temperature and atmospheric pressure. Hydrogen, oxygen, chlorine, and neon are familiar gaseous elements; bromine and mercury are the two liquids; iron, zinc, tin, aluminum, copper, lead, silver, gold, carbon, and sulfur arc among the solid elements.

3. Some materials consist of two or more elements united in a compound; water is a compound of the elements hydrogen and oxygen. The elements in compound are combined in definite, invariable proportions to form a new substance with characteristic properties of its own. In water, every gram
Of oxygen is combined with precisely 0.126 g of hydrogen, and it is a liquid at room temperature whereas hydrogen and oxygen are gases. Other materials consist of mixtures of elements or of compounds in which the separate substances do not lose their identities as they do in the case of a compound, and can be present in variable proportions.

4. The ultimate particles of an element are called atoms. A molecule is a group of atoms that stick together tightly enough to act as a unit. Many elemental gases consist of molecules instead of individual atoms. Thus gaseous oxygen contains molecules each of which is a pair of oxygen atoms bound together by forces whose nature we shall explore shortly. Other elemental gases, for instance helium and neon, consist of individual atoms.

5. Many compounds consist of molecules. The molecules of a compound have specific compositions and structures. Each water molecule contains two hydrogen atoms and one oxygen atom with the hydrogen atoms 105° apart, for example, while each ammonia molecule contains three hydrogen atoms 107.5° apart.

Chemical Symbols
6. By convention an atom of an element is represented by an abbreviation of the element’s name. For many elements the first letter is used; an atom of oxygen is 0, an atom of hydrogen is H, an atom of carbon is C. When the names of two elements begin with the same letter, two letters are used in the abbreviation for one or both: Cl stands for an atom of chlorine, He for helium, Zn for zinc. For some elements abbreviations of Latin names are used: a copper atom is Cu (cuprum), an iron atom Fe (ferrum), a mercury atom Hg (hydrargyrum). These abbreviations are called symbols of the elements.