Atoms are the smallest units of matter that can undergo chemical change. They are much too small to be seen individually, even with the most powerful electron microscope. Through the efforts of generations of scientists, however, atomic structure is now well understood. At the center of an atom is its nucleus. The nucleus contains two types of particles—protons, which bear a positive charge, and neutrons, which carry no charge (are neutral). The mass of a proton is equal to the mass of a neutron, and the sum of the protons and neutrons in an atom is equal to the atomic mass of the atom. For example, an atom of carbon, which contains six protons and six neutrons, has an atomic mass of 12 (table 2.1). Note that the mass of electrons is not considered when calculating the atomic mass, because it is insignificantly small compared to the mass of protons and neutrons.

The number of protons in an atom is given as its atomic number. Carbon has six protons and thus has an atomic number of 6. Outside the positively charged nucleus are negatively charged subatomic particles called electrons. Since the number of electrons in an atom is equal to the number of protons, atoms have a net charge of zero.

Although it is often convenient to think of electrons as orbiting the nucleus like planets orbiting the sun, this simplified model of atomic structure is no longer believed to be correct. A given electron can occupy any position in a certain volume of space called the orbital of the electron. The orbitals form a "shell," or energy level, beyond which the electron usually does not pass.

There are potentially several such shells surrounding a nucleus, with each successive shell being farther from the nucleus. The first shell, closest to the nucleus, can contain only two electrons. If an atom has more than two electrons (as do all atoms except hydrogen and helium), the additional electrons must occupy shells that are more distant from the nucleus. The second shell can contain a maximum of eight electrons and higher shells can contain still more electrons that possess more energy the farther they are from the nucleus. Most elements of biological significance (other than hydrogen), however, require eight electrons to complete the outermost shell. The shells are filled from the innermost outward. Carbon, with six electrons, has two electrons in its first shell and four electrons in its second shell (fig. 2.1).

It is always the electrons in the outermost shell, if this shell is incomplete, that participate in chemical reactions and form chemical bonds. These outermost electrons are known as the valence electrons of the atom.

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