This simplified representation of an atom of lithium includes three electrons in motion around a nucleus that contains three protons and four neutrons. Circles depict electron shells.

number of protons plus the number of neutrons in each of an element's atoms essentially equals the atomic weight of that atom. Thus, the atomic weight of a hydrogen atom, which has only one proton and no neutrons, is approximately 1. The atomic weight of a carbon atom, with six protons and six neutrons, is approximately 12 (table 2.3).

of electrons outside the nucleus equals the number of protons, so a complete atom is said to have no net charge and is electrically neutral.

The atoms of different elements contain different numbers of protons. The number of protons in the atoms of a particular element is called its atomic number. Hydrogen, for example, whose atoms contain one proton, has atomic number 1; carbon, whose atoms have six protons, has atomic number 6.

The weight of an atom of an element is primarily due to the protons and neutrons in its nucleus, because the electrons have so little weight. For this reason, the


All the atoms of a particular element have the same atomic number because they have the same number of protons and electrons. However, the atoms of an element vary in the number of neutrons in their nuclei; thus, they vary in atomic weight. For example, all oxygen atoms have eight protons in their nuclei. Some, however, have eight neutrons (atomic weight 16), others have nine neutrons (atomic weight 17), and still others have ten neutrons (atomic weight 18). Atoms that have the same atomic numbers but different atomic weights are called isotopes (i'so-topz) of an element. Because a sample of an element is likely to include more than one isotope, the atomic weight of the element is often presented as the

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Essentials of Human Physiology

Essentials of Human Physiology

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