Atoms and Elements

Atoms, the basic units of all matter, are made up of three major components: the negatively charged electrons; positively charged protons; and uncharged neutrons (figure 2.1). The protons and neutrons, the heaviest components, are found in the heaviest part of the atom, the nucleus. The very light electrons orbit the nucleus. The number of protons normally equals the number of electrons, and so the atom as a whole is uncharged. The relative sizes and motion of the parts of an atom can be illustrated by the following analogy. If a single atom were enlarged to the size of a football stadium, the nucleus would be the size of a marble and it would be positioned somewhere above the 50-yard line. The electrons would resemble fruit flies zipping around the stands. Their orbits would be mostly inside the stadium, but on occasion they would travel outside it.

An element is a substance that consists of a single type of atom. Although 92 naturally occurring elements exist, four elements make up over 99% of all living material by weight. These elements are carbon (abbreviated C), hydrogen (H), oxygen (O), and nitrogen (N). Two other elements, phosphorus (P), and sulfur (S), together make up an additional 0.5% of the elements in living systems (table 2.1). All of the remaining ele-

Electrons travel primarily in this volume of space around the nucleus.

Figure 2.1 Atom The proton has a positive charge, the neutron has a neutral charge, and the electron has a negative charge.The electrons that orbit the nucleus are arranged in orbitals of different energy levels.

ments together account for less than 0.5% of living material. In general, the basic chemical composition of all living cells is remarkably similar.

Each element is identified by two numbers: its atomic number and its atomic weight or mass. The atomic number is the number of protons, which equals the number of electrons. For example, hydrogen has 1 proton, and thus its atomic number is 1; oxygen has 8 protons, and its atomic number is 8. The atomic weight is the sum of the number of protons and neutrons, since electrons are too light to contribute to the weight. The atomic weight of hydrogen is 1, which is abbreviated :H, reflecting 1 proton and no neutrons. It is the lightest element known. The atomic weight of oxygen is approximately 16 and is abbreviated 16O, reflecting 8 protons and 8 neutrons.

It is convenient to consider electrons as being arranged in orbitals of differing energy levels. The electrons farthest from

Table 2.1 Atomic Structure of Elements Commonly Found in the Living World

Atomic Number Atomic Weight Number of Possible Approximate

Table 2.1 Atomic Structure of Elements Commonly Found in the Living World

Atomic Number Atomic Weight Number of Possible Approximate



(Total Number of Protons)

(Protons and Neutrons)

Covalent Bonds*

% of Atoms in Cells


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