Under natural conditions, most elements do not exist alone; atoms of most elements can readily combine with the same or different atoms or elements to make compounds. Compounds are made up of atoms of two or more elements in fixed proportions. A chemical formula shows the kinds and proportions of atoms of each element that forms a particular compound. For example, water's chemical formula, H2O, shows that the atoms always combine in a proportion of two hydrogen (H) atoms to one oxygen (O) atom.

The physical and chemical properties differ between the compounds and elements that compose them. In nature, the elements oxygen and hydrogen are usually found as gases with the formulas O2 and H2. However, when oxygen gas and hydrogen gas combine at room temperature, they form liquid H2O. How elements combine and form compounds depends on the number and arrangement of electrons in their orbitals. An atom is chemically stable when the orbitals that correspond to its highest energy level are filled with the maximum number of electrons. Some elements, such as helium and neon, consist of atoms that have the maximum number of electrons in the orbitals of their highest energy levels. These elements, also called noble or inert elements, do not react with other elements under normal conditions.

Most atoms are not stable in their natural state, so they tend to react with other atoms in different ways to become more stable. Carbon, nitrogen, and oxygen atoms have unfilled orbitals that correspond to their highest energy levels. Similar to these elements, most elements tend to interact with other atoms to form chemical bonds. Chemical bonds are the attractive forces that hold atoms together.

Covalent Bonds

A covalent bond forms when two atoms share one or more pairs of electrons. For example, water is made up of one oxygen atom and two hydrogen atoms held together by covalent bonds. In Figure 2-3, step O, an atom of hydrogen needs a second electron to achieve stability. Having two electrons in the orbital that corresponds to hydrogen's highest energy level allows the atom to be more stable. The oxygen atom needs two more electrons to give it a stable arrangement of eight electrons, which fill oxygen's orbitals to its highest energy level. Thus, hydrogen atoms and oxygen atoms share pairs of electrons in a ratio of two atoms of hydrogen to one atom of oxygen. The resulting stable compound, H2O (water), is shown in step ©.

A molecule is the simplest part of a substance that retains all of the properties of that substance and can exist in a free state. For example, one molecule of the compound water is H2O, and one molecule of oxygen gas is O2. Some molecules that biologists study are large and complex.

Word Roots and Origins compound from the Latin componere, meaning "to put together"

figure 2-3

Two atoms of hydrogen and one atom of oxygen share electrons in covalent bonds and thus become stable. Covalent bonding results in the formation of molecules.

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