atoms and molecules
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atomsTRANSCRIPT
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Atoms Element and Compounds
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A knowledge of chemistry is essential for understanding organisms
Important to biology are inorganic compounds, including water, simple acids and bases, and simple salts
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Elements Substances that cannot be
broken down into simpler substances by ordinary chemical reactions
Each has a chemical symbol
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Four elements comprise the mass of most organisms Oxygen, carbon, hydrogen, and
nitrogen In addition, other elements,
such as calcium, and trace elements are present
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Functions of Elements
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Atom The smallest portion of an
element that retains its chemical properties
Subatomic particles include Electron—carries a negative
charge Proton—carries a positive
charge Neutron—uncharged particle
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Every element has A fixed number of protons in
the atomic nucleus, known as the atomic number
The periodic table is a chart of the elements arranged by atomic number
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Periodic chart (including Bohr models)
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The atomic mass of an atom Is a number that indicates how
much matter it contains Is expressed by the atomic mass
unit (amu), also known as the dalton
Is indicated by a superscript to the left of the chemical symbol
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Characteristics of protons, neutrons, and electrons
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Isotopes Are two or more forms of atoms
of the same element Contain the same number of
protons and electrons, but the number of neutrons varies
Radioisotopes break down and emit radiation
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Carbon Isotopes
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Electrons move through orbitals
Electrons at the same principal energy level make up an electron shell
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Electrons in a shell distant from the nucleus have greater energy
Valence electrons occupy the valence shell
Changes in electron energy levels are important in energy conversions in organisms
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Atomic orbitals
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The chemical behavior of an atom is determined by the number and arrangement of its valence electrons
When the valence shell is not full, the atom tends to lose, gain, or share electrons
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A chemical compound consists of atoms of two or more elements
Atoms combine in a fixed ratio
Atoms may join to form a molecule
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A chemical formula describes the chemical composition of a substance Simplest formula Molecular formula Structural formula
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Molecular mass Sum of the atomic masses of
the component atoms of a single molecule
One mole is the amount of an element or compound whose mass in grams is equivalent to its atomic or molecular mass
The mole allows for comparison of atoms and molecules of very different mass
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Chemical reactions in an organism Described by chemical equations Reactants are written on the left Products are written on the right Reactions can proceed
simultaneously in both directions At dynamic equilibrium, forward
and reverse rates of reaction are equal
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Chemical bonds Forces of attraction that hold
atoms of a compound together The two principal types are
Covalent bonds Ionic bonds
Bond energy Energy necessary to break a
chemical bond
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Covalent bonds Share electrons between atoms Each atom has a filled valence shell
Covalent compound Compound consisting mainly of
covalent bonds Example is hydrogen gas molecule Bond can be single, double, or triple
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Covalent bonds
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Number of Covalent bonds
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Covalent bonds can be nonpolar or polar
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Ion Particle with one or more units
of electrical charge Results when an atom gains or
loses electrons Cations—positively charged ions Anions—negatively charged ions
Cations and anions are involved in biological processes, such as muscle contraction
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Sodium, potassium, and chloride ions are essential for this nerve cell to stimulate these muscle fibers
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Ion bonds Formed due to attraction
between a cation and an anion An ionic compound is a
substance consisting of cations and anions bonded together
An example of ionic bond is the attraction between sodium ions and chloride ions
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Ionic bonding
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Hydrogen bonds Tend to form between an atom
with partial negative charge and a hydrogen atom covalently bonded to oxygen or nitrogen
Readily formed and broken While individually weak,
hydrogen bonds are strong when present in large numbers
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Hydrogen bonding
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Many energy conversions in a cell involve an electron transfer from one substance to another
Known as oxidation-reduction, or redox reaction
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Large part of the mass of most organisms is water
Water is important as internal constituent and environmental factor
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Water facilitates chemical reactions Hydrophilic substances—
interact readily with water, such as table salt
Hydrophobic substances—not disrupted or dissolved by water, such as fats
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Water exists as gas, liquid, or solid
Hydrogen bonds are formed or broken as water changes state
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Acid Substance that dissociates in
solution to yield hydrogen ions and an anion
Base Substance that dissociates to
yield a hydroxide ion and a cation when dissolved in water
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The degree of a solution’s acidity is expressed in pH
Definition of pH The negative logarithm of the
hydrogen ion concentration Expressed in moles per liter
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Neutral solution pH of 7
Acidic solution pH value of less than 7
Base solution pH greater than 7
An acid and a base react to form a salt plus water
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pH valuesof commonsolutions