chapter 2 the chemistry of life€¦ · the chemistry of life 2.2 – properties of water . water...
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Chapter 2
The Chemistry of Life
2.1 – The nature of
matter
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Atoms make up all matter Composed of three parts:
Protons, Neutrons, and Electrons
Protons and Neutrons are found in the atom’s neucleus
Electrons are found in the space surrounding (orbiting)
the neucleus
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Each type of atom makes up
a separate element Element – a pure substance made up of only one type
of atom.
Represented by a 1 or 2 letter symbol. (EX: Oxygen=O,
Sodium=Na)
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Break it down… The atomic number tells you the number of protons
The number of protons is the same as the number of
electrons ( THEY DO NOT ADD TOGETHER – THEY
ARE JUST THE SAME NUMBER).
The atomic mass tells you the number of protons and
neutrons – which are added together.
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Try These…How Many (Protons,
Neutrons, Electrons)?
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Answers..
Sodium = 11 Protons (11 Electrons), 11 Neutrons (Atomic # 22 = Protons + Neutrons).
Lithium = 3 Protons (3 electrons), 4 Neutrons (Atomic # 7 = Protons + Neutrons).
Oxygen = 8 Protons (8 electrons), 8 Neutrons (Atomic # 16 = Protons + Neutrons).
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Time for some math!
Boron has 5 protons, and an atomic mass of
11. How many neutrons? How many electrons?
Silicon has 14 protons and 14 neutrons. What
is the atomic mass? How many electrons?
Aluminum has 13 electrons and 14 neutrons.
What is the atomic mass? How many protons?
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Answers..
6 neutrons, 5 electrons
Atomic mass 28, 14 electrons
13 protons, Atomic mass 27
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Isotopes Atoms of an element may have different numbers of
neutrons. For example, although all atoms of carbon have
six protons, some have six neutrons, some seven, and a
few have eight.
Atoms of the same element that differ in the number of
neutrons they contain are known as isotopes.
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The total number of protons and neutrons in the
nucleus of an atom is called its mass number. Isotopes
are identified by their mass numbers; for example,
carbon-12, carbon-13, and carbon-14.
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The weighted average of the masses of an element’s
isotopes, in which the abundance of each isotope in
nature is considered, is called its atomic mass.
Because they have the same number of electrons, all
isotopes of an element have the same chemical
properties.
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Radioactive Isotopes Some isotopes are radioactive, meaning that their nuclei are
unstable and break down at a constant rate over time.
Although radiation can be dangerous, radioactive isotopes have a number of important scientific and practical uses.
Geologists can determine the ages of rocks and fossils by analyzing the isotopes found in them.
Radiation from certain isotopes can be used to detect and treat cancer and to kill bacteria that cause food to spoil.
Radioactive isotopes can also be used as labels or “tracers” to follow the movements of substances within organisms.
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Atoms form bonds with each other to
form molecules and compounds..
A compound is a substance formed by the chemical
combination of two or more elements in definite
proportions
Represented by a chemical formula: (made of two
hydrogen and one oxygen atom) is written as H2O
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Two types of
chemical bonds..
Ionic bonds: formed between
oppositely charged ions
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Covalent bonds: formed when two or
more atoms share electrons
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Van der Waals Forces Because of their structures, atoms of different
elements do not all have the same ability to attract
electrons.
Some atoms have a stronger attraction for electrons
than do other atoms.
When the atoms in a covalent bond share
electrons, the sharing is not always equal.
Even when the sharing is equal, the rapid
movement of electrons can create regions on a
molecule that have a tiny positive or negative
charge.
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Van der Waals Forces When molecules are close together, a slight
attraction can develop between the oppositely
charged regions of nearby molecules. (think
magnets)
These intermolecular forces of attraction are called
van der Waals forces.
These forces are not as strong as ionic bonds or
covalent bonds, but they can hold molecules
together, especially when the molecules are large.
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Chapter 2
The Chemistry of Life
2.2 – Properties of Water
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Water is vital to all organism because it has
‘special’ properties
Water is Polar
Polarity – characteristic of a molecule
that involves and uneven distribution
(sharing) of charges (has one or more
positive end and negative end)
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Water is polar because oxygen pulls more electrons
towards itself so it has a slightly negative charge,
leaving the hydrogen(s) with a slight positive charge
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Polarity allows 3 properties: Adhesion – H2O molecules stick to other polar
molecules, but repel nonpolar molecules
Cohesion – H2O molecules stick to each other
High Heat Capacity – H2O can absorb a large amount of energy before its temperature increases, and holds energy longer than other substances
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Water is used in many
solutions. Solution – a mixture in which all parts are evenly
distributed, such as saltwater
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Solute – the part of a solution that is dissolved
the salt in salt water
Solvent – the substance in a solution that dissolves the
solute
the water in salt water
Water is the most common solvent because it can
dissolve many things
Often called the ‘universal solvent’
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Acids, Bases, and pH
Water molecules sometimes split apart to form
hydrogen ions and hydroxide ions.
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Acids, Bases, and pH
In pure water, about 1 water molecule in 550
million splits to form ions in this way.
Because the number of positive hydrogen ions
produced is equal to the number of negative
hydroxide ions produced, pure water is neutral.
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The pH Scale
Chemists devised a measurement system
called the pH scale to indicate the
concentration of H+ ions in solution.
The pH scale ranges from 0 to 14.
At a pH of 7, the concentration of H+ ions
and OH– ions is equal. Pure water has a pH
of 7.
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The pH Scale Solutions with a pH below 7 are called acidic because
they have more H+ ions than OH– ions.
The lower the pH, the greater the acidity.
Solutions with a pH above 7 are called basic because
they have more OH– ions than H+ ions. The higher the
pH, the more basic the solution.
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The pH Scale Each step on the pH scale represents a factor of 10.
For example, a liter of a solution with a pH of 4 has 10
times as many H+ ions as a liter of a solution with a pH
of 5.
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Acids An acid is any compound that forms H+ ions in solution.
Acidic solutions contain higher concentrations of H+
ions than pure water and have pH values below 7.
Strong acids tend to have pH values that range from 1
to 3. Hydrochloric acid (HCl) is a strong acid produced
by the stomach to help digest food.
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Bases A base is a compound that produces hydroxide (OH–)
ions in solution.
Basic, or alkaline, solutions contain lower
concentrations of H+ ions than pure water and have pH
values above 7. Strong bases, such as the lye
(commonly NaOH) used in soap making, tend to have
pH values ranging from 11 to 14.
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Buffers The pH of the fluids within most cells in the human
body must generally be kept between 6.5 and 7.5 in
order to maintain homeostasis. If the pH is lower or
higher, it will affect the chemical reactions that take
place within the cells.
One of the ways that organisms control pH is through
dissolved compounds called buffers, which are weak
acids or bases that can react with strong acids or bases
to prevent sharp, sudden changes in pH.
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Buffers Adding acid to an un-buffered solution causes the pH of
the un-buffered solution to drop. If the solution contains
a buffer, however, adding the acid will cause only a
slight change in pH.