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ATOMS AND MOLECULES: THE CHEMICAL BASIS OF LIFE
Elements and Atoms
Name the principal chemical elements in living things and provide an important function
of each
An element is a substance that cannot be decomposed into simpler substances
by normal chemical reactions. About 96% of an organism’s mass consists of
carbon the backbone of organic molecules; hydrogen and oxygen, the
components of water; and nitrogen, a component of proteins and nucleic acids.
Compare the physical properties and locations of electrons, protons, and neutrons.
Distinguish between the atomic number and the mass number of an atom
Each atom is composed of a nucleus containing positively charged protons and
uncharged neutrons. Negatively charged electrons encircle the nucleus.
An atom is identified as belonging to a particular element by its number of
protons (atomic number). The atomic mass of an atom is equal to the sum of its
protons and neutrons.
A single proton or a single neutron each has a mass equivalent to an atomic mass
unit (amu). The mass of a single electron is only about 1/1800 amu.
Define the terms orbital and electron shell. Relate electron shells to principal energy
levels.
In the space outside the nucleus, electrons move rapidly in electron orbitals. An
electron shell consists of electrons in orbitals at the same principal energy level.
Electrons in a shell distant from the nucleus have greater energy than those in a
shell closer to the nucleus.
Chemical Reactions
Explain how the number of valence electrons of an atom is related to its chemical
properties
The chemical properties of an atom are determined chiefly by the number and
the arrangement of its most energetic electrons, known as valence electrons.
The valence shell of most atoms is full when it contains 8 electrons; that of
hydrogen or helium is full when it contains 2. An atom tends to lose, gain, or
share electrons to fill its valence shell.
Distinguish among simplest, molecular, and structural chemical formulas
Different atoms are joined by chemical bonds to form chemical compounds. A
chemical formula gives the types and relative numbers of atoms in a substance.
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A simplest formula gives the smallest whole number ratio of the component
atoms. A molecular formula gives the actual numbers of each type of atom in a
molecule. A structural formula shows the arrangement of the atoms in a
molecule.
Explain why the mole concept is so useful to chemists
One mole (the atomic or molecular mass in grams) of any substance contains
6.02 x 10
23
atoms, molecules or ions, enabling scientists to “count” particles by
weighing a sample. This number is known as Avogadro’s number.
Chemical Bonds
Distinguish among covalent bonds, ionic bonds, hydrogen bonds, and van der Waals
interactions. Compare them in terms of the mechanisms by which they form and their
relative strengths.
Covalent bonds are strong, stable bonds formed when atoms share valence
electrons, forming molecules. When covalent bonds are formed, the orbitals of
the valence electrons may become rearranged in a process known as orbital
hybridization. Nonpolar covalent bonds are formed if the electrons are shared
equally between the two atoms. Polar covalent bonds are formed if one atom is
more electronegative (has a greater affinity for electrons) than the other.
An ionic bond is formed between a positively charged cation and a negatively
chafed anion. Ionic bonds are strong in the absence of water but relatively weak
in aqueous solution.
Hydrogen bonds are relatively weak bonds formed when a hydrogen atom with a
partial positive charge is attracted to an atom (usually oxygen or nitrogen) with a
partial negative charge already bonded to another molecule or in another part of
the same molecule.
Van der Waals interactions are weak forces based on fluctuating electric charges.
Redox reactions
Distinguish between the terms oxidation and reduction, and relate these processes to
the transfer of energy
Oxidation and reduction reactions (redox reactions) are chemical processes in
which electrons (and their energy) are transferred from a reducing agent to an
oxidizing agent. In oxidation, an atom, ion, or molecule loses electrons (and their
energy. In reduction, an atom, ion, or molecule gains electrons (and their
energy).
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Water
Explain how hydrogen bonds between adjacent water molecules govern many of the
properties of water
Water is a polar molecule because one end has a partial positive charge and the
other has a partial negative charge. Because its molecules are polar, water is an
excellent solvent for ionic or polar solutes.
Water molecules exhibit the property of cohesion because they form hydrogen
bonds with one another; they also exhibit adhesion through hydrogen bonding
to substances with ionic or polar regions.
Water has a high heat of vaporization. Hydrogen bonds must be broken for
molecules to enter the vapor phase. These molecules carry a great deal of heat,
which accounts for evaporative cooling.
Because hydrogen bonds must be broken to raise its temperature, water has a
high specific heat, which helps organisms maintain a relatively constant internal
temperature; this property also helps keep the ocean and other large bodies of
water at a constant temperature.
The hydrogen bonds between water molecules in ice cause it to be less dense
than liquid water. Because ice floats, the aquatic environment is less extreme
than it would be if ice sank to the bottom.
Acids, Bases, and Salts
Contrast acids and bases, and discuss their properties
Acids are proton (hydrogen ion, H
+
) donors, while bases are proton acceptors. An
acid dissociates in solution to yield H
+
and an anion. Many bases dissociate in
solution to yield hydroxide ions (OH
-
), which then accept protons to form water.
Convert the hydrogen ion concentration (moles per liter) of a solution to a pH value and
describe how buffers help minimize changes in pH.
pH is the negative log of the hydrogen ion concentration of a solution (expressed
in moles per liter). A neutral solution with equal concentrations of H
+
and OH
-
has a pH of 7, an acidic solution has a pH less than 7 and a basic solution has a
pH greater than 7.
A buffering system is based on a weak acid or weak base. A buffer resists
changes in the pH of a solution when acids or bases are added.
Describe the composition of a salt and explain the ways in which salts are important in
organisms
A salt is a compound in which the hydrogen atom of an acid is replaced by some
other cation. Salts provide the many mineral ions essential for life functions.

Unformatted Attachment Preview

ATOMS AND MOLECULES: THE CHEMICAL BASIS OF LIFE Elements and Atoms Name the principal chemical elements in living things and provide an important function of each • An element is a substance that cannot be decomposed into simpler substances by normal chemical reactions. About 96% of an organism’s mass consists of carbon the backbone of organic molecules; hydrogen and oxygen, the components of water; and nitrogen, a component of proteins and nucleic acids. Compare the physical properties and locations of electrons, protons, and neutrons. Distinguish between the atomic number and the mass number of an atom • Each atom is composed of a nucleus containing positively charged protons and uncharged neutrons. Negatively charged electrons encircle the nucleus. • An atom is identified as belonging to a particular element by its number of protons (atomic number). The atomic mass of an atom is equal to the sum of its protons and neutrons. • A single proton or a single neutron each has a mass equivalent to an atomic mass unit (amu). The mass of a single electron is only about 1/1800 amu. Define the terms orbital and electron shell. Relate electron shells to principal energy levels. • In the space outside the nucleus, electrons move rapidly in electron orbitals. An electron shell consists of electrons in orbitals at the same principal energy level. Electrons in a shell distant from the nucleus have greater energy than those in a shell closer to the nucleus. Chemical Reactions Explain how the number of valence electrons of an atom is related to its chemical properties • The chemical properties of an atom are determined chiefly by the number and the arrangement of its most energetic electrons, known as valence electrons. The valence shell of most atoms is full when it contains 8 electrons; that of hydrogen or helium is full when it contains 2. An atom tends to lose, gain, or share electrons to fill its valence shell. Distinguish among simplest, molecular, and structural chemical formulas • Different atoms are joined by chemical bonds to form chemical compounds. A chemical formula gives the types and relative numbers of atoms in a substance. • A simplest formula gives the smallest whole number ratio of the component atoms. A molecular formula gives the actual numbers of each type of atom in a molecule. A structural formula shows the arrangement of the atoms in a molecule. Explain why the mole concept is so useful to chemists • One mole (the atomic or molecular mass in grams) of any substance contains 6.02 x 1023 atoms, molecules or ions, enabling scientists to “count” particles by weighing a sample. This number is known as Avogadro’s number. Chemical Bonds Distinguish among covalent bonds, ionic bonds, hydrogen bonds, and van der Waals interactions. Compare them in terms of the mechanisms by which they form and their relative strengths. • Covalent bonds are strong, stable bonds formed when atoms share valence electrons, forming molecules. When covalent bonds are formed, the orbitals of the valence electrons may become rearranged in a process known as orbital hybridization. Nonpolar covalent bonds are formed if the electrons are shared equally between the two atoms. Polar covalent bonds are formed if one atom is more electronegative (has a greater affinity for electrons) than the other. • An ionic bond is formed between a positively charged cation and a negatively chafed anion. Ionic bonds are strong in the absence of water but relatively weak in aqueous solution. • Hydrogen bonds are relatively weak bonds formed when a hydrogen atom with a partial positive charge is attracted to an atom (usually oxygen or nitrogen) with a partial negative charge already bonded to another molecule or in another part of the same molecule. • Van der Waals interactions are weak forces based on fluctuating electric charges. Redox reactions Distinguish between the terms oxidation and reduction, and relate these processes to the transfer of energy • Oxidation and reduction reactions (redox reactions) are chemical processes in which electrons (and their energy) are transferred from a reducing agent to an oxidizing agent. In oxidation, an atom, ion, or molecule loses electrons (and their energy. In reduction, an atom, ion, or molecule gains electrons (and their energy). Water Explain how hydrogen bonds between adjacent water molecules govern many of the properties of water • Water is a polar molecule because one end has a partial positive charge and the other has a partial negative charge. Because its molecules are polar, water is an excellent solvent for ionic or polar solutes. • Water molecules exhibit the property of cohesion because they form hydrogen bonds with one another; they also exhibit adhesion through hydrogen bonding to substances with ionic or polar regions. • Water has a high heat of vaporization. Hydrogen bonds must be broken for molecules to enter the vapor phase. These molecules carry a great deal of heat, which accounts for evaporative cooling. • Because hydrogen bonds must be broken to raise its temperature, water has a high specific heat, which helps organisms maintain a relatively constant internal temperature; this property also helps keep the ocean and other large bodies of water at a constant temperature. • The hydrogen bonds between water molecules in ice cause it to be less dense than liquid water. Because ice floats, the aquatic environment is less extreme than it would be if ice sank to the bottom. Acids, Bases, and Salts Contrast acids and bases, and discuss their properties • Acids are proton (hydrogen ion, H+) donors, while bases are proton acceptors. An acid dissociates in solution to yield H+ and an anion. Many bases dissociate in solution to yield hydroxide ions (OH-), which then accept protons to form water. Convert the hydrogen ion concentration (moles per liter) of a solution to a pH value and describe how buffers help minimize changes in pH. • pH is the negative log of the hydrogen ion concentration of a solution (expressed in moles per liter). A neutral solution with equal concentrations of H + and OHhas a pH of 7, an acidic solution has a pH less than 7 and a basic solution has a pH greater than 7. • A buffering system is based on a weak acid or weak base. A buffer resists changes in the pH of a solution when acids or bases are added. Describe the composition of a salt and explain the ways in which salts are important in organisms • A salt is a compound in which the hydrogen atom of an acid is replaced by some other cation. Salts provide the many mineral ions essential for life functions. Name: Description: ...
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