Chemical And Physical Bases Of Life

The Elements: Units of Matter

The basic "stuff of the universe," called matter, occurs in three states—solid, liquid, and gas. In simple terms, matter's characteristics are as follows:

1. It occupies space.

2. It has mass, which we commonly associate with weight.

3. It is composed of elements. There are 93 elements that occur naturally on our planet. At least 19 more elements have been produced artificially. Only a few of the natural elements (e.g., nitrogen, oxygen, gold, silver, copper) occur in pure form; the others are found combined together chemically in various ways. Each element has a designated symbol, often derived from its Latin name. The symbol for copper, for example, is Cu (from the Latin cuprum); and for sodium, Na (from the Latin natrium). The symbols for carbon, hydrogen, and oxygen are C, H, and O, respectively.

The smallest stable subdivision of an element that can exist is called an atom. Atoms are so minute that until the mid-1980s, individual atoms were not directly visible to us with even the most powerful electron microscopes. We have known for over 100 years, however, that atoms consist of several kinds of subatomic particles. Each atom has a tiny nucleus consisting of protons, which are particles with positive electrical charges, and other particles called neutrons, which have no electrical charges. Both protons and neutrons have a small amount of mass. If the nucleus, which contains nearly all of the atom's mass, were enlarged so that it was as big as a beach ball, the atom, which is mostly space, would be larger than a professional football stadium (Fig. 2.1).

^ neutron + proton ^ electron

^ neutron + proton ^ electron

Figure 2.1 Model of an oxygen atom. The nucleus in the center consists of eight electrically neutral neutrons and eight positively charged protons. Eight negatively charged electrons whirl around the nucleus. In a real atom, the electrons would not be spaced or confined as shown in this simple diagram.

Figure 2.1 Model of an oxygen atom. The nucleus in the center consists of eight electrically neutral neutrons and eight positively charged protons. Eight negatively charged electrons whirl around the nucleus. In a real atom, the electrons would not be spaced or confined as shown in this simple diagram.

Each atom of an element has a specific number of protons in its nucleus, ranging from 1 in hydrogen, the lightest element, to 92 in uranium, the heaviest natural element. This number is referred to as the atomic number. The atomic number is often shown as a subscript to the left of the chemical symbol. For example, nitrogen, which has seven protons in its nucleus, has its atomic number of 7 shown as 7N. The combined number of protons and neutrons in a single atom is referred to as its atomic mass (Table 2.1). The atomic mass number is commonly shown as a superscript to the left of the chemical symbol. For example, the atomic mass of nitrogen, which has seven protons and seven neutrons in its nucleus, is shown as 14N, and when both the atomic number and the atomic mass are shown, the chemical symbol appears as 174n.

Electrons, which are little more than negative electric charges, whirl around an atom's nucleus. Electron masses are about 1,840 times lighter than those of both protons and neutrons and are so minute that their masses are generally disregarded. Since opposite electric charges attract each other, the positive electric charges of protons attract the negative electric charges of electrons and determine the paths of the electrons whirling around the nucleus.

The region in which electrons whirl around the nucleus is called an orbital. Each orbital has an imaginary axis and is somewhat cloudlike, but it doesn't have a precise boundary, and so we can't be certain of an electron's position within an orbital at any time. This has led to an orbital being defined as a volume of space in which a given electron occurs 90% of the time.

Electrons may be located in one or more energy levels of an atom, and their distance from the nucleus depends on their energy level. Each energy level is usually referred to as an electron shell. The outermost electron shell determines how or if an atom reacts with another atom. Only two electrons can occupy the first and lowest energy level associated with the innermost orbital; this orbital is more or less spherical and is so close to the nucleus that it is often not shown on diagrams of atoms. One to several additional orbitals, which are mostly spindle shaped (like the tips of cotton swabs), generally occupy much more space. Up to eight electrons can be held by the second energy level, and although the third and fourth energy levels can hold more than eight electrons each, they can become unstable if more than eight electrons are present. If an electron in one orbital is provided with more energy, it can jump to an orbital farther away from the nucleus. Conversely, if an electron releases energy, it drops to an energy level closer to the nucleus.The electrons of each orbital tend to repel those of other orbitals, so that the axes of all the orbitals of an atom are oriented as far apart from each other as possible; the outer parts of the orbitals, however, actually overlap more than shown in diagrams of them. Orbitals usually have diameters thousands of times more extensive than that of an atomic nucleus (Fig. 2.2).

Because each atom usually has as many electrons as it does protons, the negative electric charges of the electrons balance the positive charges of the protons, making the

Chapter 2

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