The nature of M olecules Concept Outline 2.1 Atoms are nature's building material Atoms. All substances are composed of tiny particles called toms, each a positively charged nucleus around which orbit negatve Electrons determine the Chemical behavior of atoms Electrons orbit the nucleus of an atom: the closer electron' s orbit to the nucleus. the lowe 2.2 The atoms of living things are among the smallest Kinds of Atoms. Of the 92 naturally occurring element only 11 occur in organisms in significant amounts 2. 3 Chemical bonds hold molecules together. Ionic Bonds Form Crystals. Atoms are linked together into molecules, joined by chemical bonds that result from forces like the attraction of opposite charges or the sharing of electrons Covalent Bonds build Stable molecules. Chemical bonds formed by the sharing of electrons can be very strong, FIGURE 2.1 and require much energy to break. Cells are made of molecules. Specific, often simple, combina tions of atoms yield an astonishing diversity of molecules within 2.4 Water is the cradle of life the cell, each with unique functional characteristics. Chemistry of Water. Water forms weak chemical associations that are responsible for much of the organization of living chemistry. Water Atoms Act Like Tiny Magnets. Because electrons bout 10 to 20 billion years ago, an enormous explo- are shared unequally by the hydrogen and oxygen atoms of sion likely marked the beginning of the universe water, a partial charge separation occurs. Each water atom With this explosion began the process of evolution, which acquires a positive and negative pole and is said to be"polar. eventually led to the origin and div versification of life on Water Clings to Polar Molecules. Because the opposite earth. When viewed from the perspective of 20 billion of polar molecules attract one another, water tends to cling to itself and other polar molecules and to years, life within our solar system is a recent development, exclude nonpolar molecules but to understand the origin of life, we need to consider Water Ionizes. Because its covalent bonds occasionally events that took place much earlier. The same processes break, water contains a low concentration of hydrogen(H+) hat led to the evolution of life were responsible for the Ind hydroxide(OH-)ions, the fragments of broken water evolution of molecules(figure 2.1). Thus, our study of life on earth begins with physics and chemistry. As chemical machines ourselves, we must understand chemistry to19 2 The Nature of Molecules Concept Outline 2.1 Atoms are nature’s building material. Atoms. All substances are composed of tiny particles called atoms, each a positively charged nucleus around which orbit negative electrons. Electrons Determine the Chemical Behavior of Atoms. Electrons orbit the nucleus of an atom; the closer an electron’s orbit to the nucleus, the lower its energy level. 2.2 The atoms of living things are among the smallest. Kinds of Atoms. Of the 92 naturally occurring elements, only 11 occur in organisms in significant amounts. 2.3 Chemical bonds hold molecules together. Ionic Bonds Form Crystals. Atoms are linked together into molecules, joined by chemical bonds that result from forces like the attraction of opposite charges or the sharing of electrons. Covalent Bonds Build Stable Molecules. Chemical bonds formed by the sharing of electrons can be very strong, and require much energy to break. 2.4 Water is the cradle of life. Chemistry of Water. Water forms weak chemical associations that are responsible for much of the organization of living chemistry. Water Atoms Act Like Tiny Magnets. Because electrons are shared unequally by the hydrogen and oxygen atoms of water, a partial charge separation occurs. Each water atom acquires a positive and negative pole and is said to be “polar.” Water Clings to Polar Molecules. Because the opposite partial charges of polar molecules attract one another, water tends to cling to itself and other polar molecules and to exclude nonpolar molecules. Water Ionizes. Because its covalent bonds occasionally break, water contains a low concentration of hydrogen (H+) and hydroxide (OH–) ions, the fragments of broken water molecules. About 10 to 20 billion years ago, an enormous explo￾sion likely marked the beginning of the universe. With this explosion began the process of evolution, which eventually led to the origin and diversification of life on earth. When viewed from the perspective of 20 billion years, life within our solar system is a recent development, but to understand the origin of life, we need to consider events that took place much earlier. The same processes that led to the evolution of life were responsible for the evolution of molecules (figure 2.1). Thus, our study of life on earth begins with physics and chemistry. As chemical machines ourselves, we must understand chemistry to begin to understand our origins. FIGURE 2.1 Cells are made of molecules. Specific, often simple, combina￾tions of atoms yield an astonishing diversity of molecules within the cell, each with unique functional characteristics