209+Chapter+2

209+Chapter+2 - Chapter 2 I. HOW MATTER IS ORGANIZED A....

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Chapter 2 I. HOW MATTER IS ORGANIZED A. Chemical Elements 1. All forms of matter are composed of chemical elements which are substances that cannot be split into simpler substances by ordinary chemical means. 2. Elements are given letter abbreviations called chemical symbols . 3. Oxygen (O), carbon (C), hydrogen (H), and nitrogen (N) make up 96% of body weight. CHON B. Structure of Atoms 1. Units of matter of all chemical elements are called atoms . An element is a quantity of matter composed of atoms of the same type. 2. Atoms consist of a nucleus , which contains positively charged protons and neutral (uncharged) neutrons , and negatively charged electrons that move about the nucleus in energy levels (Figure 2.1a ). 3. Electrons revolve around the nucleus of an atom tending to spend most of the time in specific atomic regions, called shells (Figure 2.1b ). a. Each shell can hold a certain maximum number of electrons. b. The first shell, the one nearest the nucleus, can hold a maximum of 2 electrons; the second shell, 8; the third shell;18,the fourth shell, 18; and so on (Figure 2.1b ). 4. The number of electrons in an atom of a neutral element always equals the number of protons.
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C. Atomic Number and Mass Number 1. The number of protons in the nucleus of an atom a. The number of protons in the nucleus makes the atoms of one element different from those of another as illustrated in Figure 2.2 . b. Since all atoms are electrically neutral, the atomic number also equals the number of electrons in each atom. 2. The mass number of an atom is the total number of protons and neutrons. 3. Different atoms of an element that have the same number of protons but different numbers of neutrons are called isotopes . a. Stable isotopes do not change their nuclear structure over time. b. Certain isotopes called radioactive isotopes are unstable because their nuclei decay to form a simpler and thus more stable configuration. c. Radioactive isotopes can be used to study both the structure and function of particular tissues as described in the Clinical Connection on “Harmful and Beneficial Effects of Radiation.” D. Atomic Mass 1. The atomic mass , also called the atomic weight, of an element is the average mass of all its naturally occurring isotopes and reflects the relative abundance of isotopes with different mass numbers. E. Ions, Molecules, Free Radicals, and Compounds
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1. If an atom either gives up or gains electrons, it becomes an ion - an atom that has a positive or negative charge due to having unequal numbers of protons and electrons. + = cation - = anion 2. When two or more atoms share electrons, the resulting combination is called a molecule (Figure 2.3). 3. A free radical is an electrically charged atom or group of atoms with an unpaired electron in its outermost shell (Figure 2.3 ). a. Free radicals become stable by either giving up their unpaired
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209+Chapter+2 - Chapter 2 I. HOW MATTER IS ORGANIZED A....

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