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2.1 Atoms are nature's building material.
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• The smallest stable particles of matter are protons, neutrons, and electrons, which associate to form atoms.
• The core, or nucleus, of an atom consists of protons and neutrons; the electrons orbit around the nucleus in a cloud. The farther an electron is from the nucleus, the faster it moves and the more energy it possesses.
• The chemical behavior of an atom is largely determined by the distribution of its electrons and in particular by the number of electrons in its outermost (highest) energy level. There is a strong tendency for atoms to have a completely filled outer level; electrons are lost, gained, or shared until this condition is reached.
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1. An atom of nitrogen has 7 protons and 7 neutrons. What is its atomic number? What is its atomic mass? How many electrons does it have?
2. How do the isotopes of a single element differ from each other?
3. The half-life of radium-226 is 1620 years. If a sample of material contains 16 milligrams of radium-226, how much will it contain in 1620 years? How much will it contain in 3240 years? How long will it take for the sample to contain 1 milligr
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2.2 The atoms of living things are among the smallest.
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• More
than 95% of the weight of an organism consists of oxygen, hydrogen, carbon,
and nitrogen, all of which form strong covalent bonds with one another.
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4. What is the octet rule, and how does it affect the chemical behavior of atoms?
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2.3 Chemical bonds hold molecules together.
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• Ionic bonds form when electrons transfer from one atom to another, and the resulting oppositely charged ions attract one another.
• Covalent bonds form when two atoms share electrons.
They are responsible for the formation of most biologically important
molecules.
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5. What is the difference between an ionic bond and a covalent bond? Give an example of each.
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2.4 Water is the cradle of life.
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• The chemistry of life is the chemistry of water (H2O). The central oxygen atom in water attracts the electrons it shares with the two hydrogen atoms. This charge separation makes water a polar molecule.
• A hydrogen bond is formed between the partial positive charge of a hydrogen atom in one molecule and the partial negative charge of another atom, either in another molecule or in a different portion of the same molecule.
• Water is cohesive and adhesive, has a great capacity for storing heat, is a good solvent for other polar molecules, and tends to exclude nonpolar molecules.
• The H+ concentration in a solution is expressed by
the pH scale, in which pH equals the negative logarithm of the H+ concentration.
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6. What types of atoms participate in the formation of hydrogen bonds? How do hydrogen bonds contribute to water's high specific heat?
7. What types of molecules are hydrophobic? What types are hydrophilic? Why do these two types of molecules behave differently in water?
8. What is the pH of a solution that has a hydrogen ion concentration of 10-3 mole/liter? Would such a solution be acidic or basic?
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