Chapter 3 - Beginnings of Chemistry

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This chapter limits the discussion to some of the elementary aspects of chemical reactions. Of the four types of forces that direct all reactions in the universe, only electromagnetic forces are involved in chemical reactions. All atoms contain a centrally located nucleus with protons and neutrons. Surrounding the nucleus in a series of approximately concentric shells are the electrons. Electrons in the innermost shell are the hardest to remove from the atom, while electrons in the outermost shell are the easiest to remove. The outermost shell is called the valence shell because in chemical reactions among atoms, electrons are usually added to or removed from this shell. Atoms of a single type, each with the same number of protons, constitute an element. Elements are ordered from left to right by atomic number. Except the inert gases, atoms are prone to interaction with other atoms to form molecules. Molecules are described by writing the symbols of the constituent elements and indicating the numbers of atoms of each element in the molecule as subscripts. Atoms of some of the elements most prevalent in living organisms are most stable chemically when they have eight electrons in their valence shells. The result of gaining or losing electrons, while the proton number remains unchanged, is an atom with a net positive or negative charge, respectively. Such an atom is called an ion. Atoms that have three, four, or five electrons in their valence shells are more likely to share electrons in a covalent bond.