Transcript Chapter 2 Study Guide Dr. Joseph Silver

Chapter 2
Study Guide
Dr. Joseph Silver
this chapter deals with
- structure of atoms
- how atoms form molecules
- the periodic table
- the uniqueness of water
- acids and bases
anything that has mass (made up of atoms)
and
occupies space (no matter how small)
is called
matter
the word atom
was used 2000 years ago in Greece and
we still are learning more
about atoms today
we have not yet been able to actually
clearly see an atom
but
scientists have done many experiments
which
show that one or more electrons (-)
circle about a nucleus
containing protons(+) and neutrons(0 no charge)
atomic number = number of protons in an atom
atomic mass = mass of protons+neutrons+electrons
electron mass is so small that it contributes
very little to the mass of an atom
remember mass represents the amount of matter in any
atom, molecule or anything else
weight represents the amount of force gravity
exerts on a substance
if you go to Mars your weight changes
but your mass does not change
the nucleus of an atom has protons and neutrons
packed into the center of the atom
and
the electrons are located in circular orbits (orbitals)
or layers about the nucleus
-electrons travel so quickly that we cannot see one
-they do not fly off due to their attraction to the nucleus
-atoms with large atomic numbers have many orbital layers
when protons are equal to electrons - neutral atom
when protons do not equal electrons – atom is an ion
if there are more protons than electrons – positive cation
if there are more electrons than protons – negative anion
C12 = 6P + 6E + 6 neutrons
C13 = 6P +6E + 7 neutrons
C14 = 6P + 6E + 8 neutrons
atoms of an element with different number of neutrons
are called isotopes
some isotopes release radioactive energy as the extra
neutrons are released
isotopes have many uses in industry, medicine,
chemistry, and biology
one use is to date the age of fossils
isotopes have a half life
we know how long it takes for one-half of the energy in
an isotope to disappear and we use this information to
compare the amount of an isotope in new material and
in a fossil and arrive at the age of the fossil
electrons determine the chemical behavior of atoms
how
when the outer electron shell of an atom is not full
it will give away electrons,
or get electrons from other atoms,
or share electrons with other atoms
in order to have the outer shell filled
the farther an electron is from the nucleus
the greater is the energy of the electron
likewise
the closer to the nucleus
the less energy in an electron
look at the drawings on pages 20 to 25
the first electron layer can have 2 electrons
the second layer can have 8 electrons
these 8 are made up 4 orbitals each with 2 electrons
(see 2.5C)
for the purpose of this class any layer or orbital past the
second orbital is filled by 8 electrons
the electrons in the outer shell of an atom
are referred to as valence electrons
and these
are the electrons available for chemical reactions
-electrons are first added to the first shell (level)
-when it is full electrons are added to the next shell
-when the 2nd shell is full electrons are added to the 3rd
and so on for the
90 naturally occurring elements
as well as the laboratory manufactured elements
an atom with 1 electron in its first outer shell
would love to give it away or gain one
so that
the outer shell a would be complete
an atom with 7 electrons in the outer shell
would love to find an atom to give it 1 electron
and
an atom with 6 electrons in the outer shell
would love to find an atom to give it 2 electrons
or 2 atoms to each give it 1 electron
in atoms with multiple shells
gaining electrons can fill the last orbital level
or giving away electrons can empty the last shell
and
the lower level shell which is already filled
will become the outer layer
see fig 2.9 page 23
on page 22
there is a picture of a periodic table
it shows the 90 naturally occurring elements
and the 28 man made elements
in the periodic table
the elements are arranged
according to their atomic number
(the number of protons or electrons)
they are arranged in columns
according to the number of electrons
in the outermost orbital shell
the electrons in the outer shell
are called
valance electrons
these are the electrons which
are involved in chemical reactions
look at the periodic table in the textbook
-put a 1 over the H
-put a 2 over Be
-put a 3 over B
-put a 4 over C
-put a 5 over N
-put a 6 over O
-put a 7 over F
-put an 8 over He
the number represents the number of electrons present in the
outer shell of the elements in the column
(H,Li,Na,K,Rb,Cs,Fr all have 1 electron in the outer shell)
(Be,Mg,Ca,Sr,Ba, Ra all have 2 electrons in the outer shell)
an element with 1 electron in the outer shell can readily
interact with an element having 7 valence electrons
an element with 1 valence electron can also readily
interact with elements having 6 valence electrons
by exchanging or sharing electrons they fill
the outer shells and form compounds with stable atoms
ionic bonds form when atoms exchange electrons
and become positive and negative ions
which are attracted to each other and form
a new molecule or compound
look at fig 2.9
sodium gives up its 1 outer electron
chlorine adds 1 electron to its 7 and fills the shell
sodium becomes positive by giving away an electron
chlorine adds an electron and becomes negative
the + and – ions attract to become sodium chloride
carbon has 4 electrons in its outer shell
two carbons share their outer 4 electrons
making an outer shell in each atom with 8 electrons
thus each has a full outer shell with 8 electrons
forming a covalent bond
the electrons travel so fast that it is as if
the 8 electrons are in both atoms at the same time
the ability of atoms to form chemical bonds
is influenced by
- temperature (2x for every 10C)
- concentration
-catalysts (enzymes)
- pressure (keeps atoms closer together)
WATER
liquid at room temperature
¾ of earth’s surface covered by water
polar covalent bond (see page 26 fig 2.11)
water has 2 single covalent bonds
water molecules bond to each other by hydrogen bonds
water becomes or has
cohesive – adhesive – high specific heat
high heat of vaporization – low density of ice
- universal solvent
each water molecule has 2 weak positive
and 2 weak negative charges
these charges attract water molecules and form weak
hydrogen bonds – trillions of them – actually more
- causing all water molecules to be attached to all of the
other water molecules allowing water to have a high
surface tension – a pin gently placed on water will float
– a lizard can run across a pond – and insects can walk
on water
the weak polar charges of water are attracted
to other polar charges
water makes polar surfaces wet meaning they interact
with each other – they dissolve unlike non polar oils
which form a ball and do not interact with water
these charges allow water to climb up a capillary tube
see fig 2.14
a gram of water can hold more heat (specific heat)
than any other biologically active compound
if the heat generated by the chemical processes which
keep you alive were not removed your body would
literally go up in flames
water in your blood and sweat picks up this heat
and brings it to the surface where it leaves through
your skin and lungs
it takes more heat to evaporate 1 gram of water
(high heat of vaporization)
than any other biologically active molecule
so
water is able to remove large amounts of heat
as sweat is evaporated from your body and
blood vessels bring heat to your skin
a gallon of water weighs more than a gallon of ice
ice is less dense than water
that is why ice floats
this is very important
think about this
you live in upper Michigan
it is 20 below zero
the ice on the lake is really thick
now
what will happen if the ice gets thicker and thicker
and sinks to the bottom of the lake
YES
all plants and animals in the lake just got crushed to death
as you walk on the ice covering the lake
the air temperature is 20 below zero
what is the temperature of the water under the ice?
Since water freezes at 0C or 32F
then the water below the ice
is much warmer than the temperature of the air
in the middle of winter the water can be 50, 60, 70
degrees warmer than the air
the insulation of the ice preserves life in the lake
water is a universal solvent
more things dissolve or get wet in water
than any other molecule
think of all the chemicals and molecules that
are in your body which dissolve in water
here is a new word
mole
a mole is the weight of any substance in grams
that corresponds to the atomic masses
of all of the atoms in that molecule
of that substance
1 mole of any substance contains 6.02 x 1023 molecules
scientists use this information to know how much
reactant and product are present
if we have a glass of pure water
what do we have in the glass?
if you said pure water
that is a logical answer
but
it is not the correct answer
Look on page 29 and read
the section
“water can form ions”
water can spontaneously
split to form ions
H2O splits to form H+ and OHH = a positive hydrogen ion (a proton)
OH = a negative hydroxyl ion
for every 10 million molecules of water
1 molecule splits into H and OH
1 out of 10 million
is
1/10,000,000
or scientifically
10-7
thus
in pure water
the concentration of H is 10-7
and
the concentration of OH is 10-7
they are equal
what is an acid?
a substance which adds H+ ions
to a solution
if there are more H ions in a solution than OH ions
then it is an acid
what is a base?
a substance which add OH ions
to a solution
if there are more OH ions
than H ions then it is a base
in pure water
the H ions and the OH ions are equal
so
water is not a base nor is it an acid
so
pure water is neutral
because most people do not want to be speaking
in scientific terminology like negative 6 or negative 12
scientists removed the negative attributes and called
pure water 7.000000
where did the 7 come from
from 1 part in 10 million
look at fig 2.16
the pH scale goes from 0 to 14
the amount of H and OH always adds up to 14
so
at a pH of 4
the concentration of H is 10-4
and
the concentration of OH is 10-10
remember minus 4 is more than minus 10
at a pH of 10-12
the concentration of H is 10-12
and
the concentration of OH is 10-2
minus 2 is much greater than minus 12
so
an acid has more H than OH
and
a base has more OH and H
and
the farther away from 7 the
more concentrated an acid
or base becomes
pH is calculated on a log scale
as you move 1 number (from 7 to 6 or 7 to 8)
the concentration increased 10 times
and
as you move 2 numbers
the concentration increases 10 x 10 or 100 times
think back to math class and working with exponents
in your body you have many chemical processes which
produce acids and bases
your body in order to work efficiently must keep
the pH of your body around 7.35
this is done by a series of chemicals
known as buffers
buffers are compounds which when
reacted with a base release an acid
and when
reacted with an acid release a base
the
goal is to keep the pH of the body optimum
HCl + NaOH  HOH + NaCl
acid + base  water + salt
a buffer releases H ions when a base is added
or
releases OH ions when an acid is added
the action of a buffer
is to resist a change in pH
in our bodies there are 2 important buffers
carbonic acid and bicarbonate