Basic Structure of the Atom

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Transcript Basic Structure of the Atom

Basic Structure of
the Atom
Evidence Supporting the Atomic Theory
Democritus
 Ancient Greek
 Matter made up of atomos
 Atoms can not be:
 Created
 Destroyed
 Further Divided
Antoine Lavoisier (Late
1700’s)
 Founder of Modern Chemistry
 Explained O2 role in combustion
 Concept of element as basic substance
 Conservation of matter theory
 Measured the mass of substances before and
after a chemical reaction
 Performed carefully controlled experiments in
closed systems
 Beheaded during French Revolution
William Proust (1799)
 Devised the law of constant composition
 A given compound always contains the
same elements in the same proportions
Dalton
 Proposed Atomic Theory in 1803
 Half a Century until well accepted
 100 years until proven
 Each element :




Composed of indivisible and indestructible atoms
Atoms of different elements are different;
atoms of the same element are the same
Atoms of different elements combine to form
compound atoms (molecules)
Michael Faraday (1839)
 Developed fundamental theories of
electricity, magnetism, and light
 Theorized the structure of atoms are
somehow related to electricity
 1832 - Electrochemistry
J. J. Thomson
 Discovered electrons in 1897
 Plum Pudding Model – Positive and
negative charges scattered randomly
throughout atom
 Instantly accepted
Experiment
 Passed a cathode ray from a cathode to an
anode with a hole in it that allows a small
amount of the cathode ray to pass through
 Cathode ray passes through a tube surrounded
by both poles of a magnet and electrically
charged plates
 Magnetic field turned on, the ray is deflected
upwards
 Electric field is turned on, the ray is deflected
downward
 Magnetic and electric fields are off, the ray
travels straight
Becquerl (1896)
 Discovered spontaneous emission of radiation
from an element - radioactivity
 Accidently placed uranium salts on top of an
unexposed photographic plate which was
wrapped in paper and in a dark desk drawer
 Developed plate – saw silhouette of salts
 Led Pierre and Marie Curie to discover radium
and polonium by isolating them from
pitchblende
Millikan (1909)
 Measured charge and mass of the
electron
 Experiment
 measured the effect of an electric field on
the rate at which charged oil drops fell under
the effect of gravity
 used x-rays near the droplet to charge them
 the oil droplets fall could be accelerated,
retarded, or even reversed, depending on
the charge on the droplet and the polarity
voltage of the plate
Rutherford
 Gold foil experiment 1909 –
Mass concentrated in very small core at
the atom’s center (nucleus)
 Nucleus positive, negative electrons moving
around it
 Contributions:
 Field of Nuclear Physics (1898, alpha and beta
particles)
 Radioactive decay
 Rectify Periodic Table
Experiment
 Fired alpha particles at gold foil
 Most passed through the foil, a few were
deflected
 When alpha particles (+2) closely
approaches the gold nucleus (+79), it
undergoes a strongly repulsive
interaction
Bohr
 Doctoral Thesis 1911– Theory of Electrons
 In 1913, discovered electrons revolve around the
nucleus in energy levels (Einstein and Planck)
 Energy levels closest to nucleus have low energy
 Energy levels increase in energy with distance from the
nucleus
 Electrons gain and lose energy by moving between
energy levels (quantum)
 “This is an enormous achievement” Einstein
Moseley (1914)
 Determined the atomic numbers of each
element
 bombarded different elements with energetic
electrons and studied the x-rays they
emitted
 observed that the frequency of the x-rays
were different for each element
 arranged the frequencies in order by
assigning each element a unique, integral
number, which he called the atomic number
Modern Atomic Model
 The atom consists of three main particles:
 Protons (positive)
 Neutrons (neutral)
 Electrons (negative)
 Two main parts:
 Nucleus
 Electron cloud
Atomic Model (cont.)
 Nucleus contains:
 Protons (+)
 Neutrons (0)
 Nucleus surrounded by:
 electron cloud
 Negative charge due to electrons
Atomic Structure
Atomic Structure
Atomic Mass Units
 Mass measured in atomic mass units
 For protons and neutrons
 1 amu is defined as 1/12 the mass of a
carbon atom containing 6 protons and 6
neutrons
 1 amu is also the mass of 1 proton or 1
neutron
 An electron has a mass of 1/2000 amu
Key Terms
 Atomic number: the number of protons in
the nucleus of an atom.
 Mass number: the sum of the number of
protons AND the number of neutrons in
the nucleus.
 Mass # = # protons + # neutrons
Atomic Symbol and Mass
Number
Isotopes
 Isotopes are atoms of the same element
with differing numbers of neutrons.
 Isotopes have different masses
Isotopes
Isotopes of Carbon
How do you record the mass
of a group of isotopes?
 Because most elements have more than
one isotope, each element is given an
average atomic mass
 The average atomic mass is the average
mass of the mixtures of its isotopes
How do you calculate the
average atomic mass of an
atom?
 The number of naturally occurring
isotopes, their masses, and their percent
abundances must be known.
 Example: Lithium has 2 isotopes: Li-6
(mass 6.015 amu and 7.5% abundance),
and Li-7 (mass 7.017 amu and 92.5%
abundance). What is its average atomic
mass?
How do you calculate the
average atomic mass of an
atom?
 Calculate the average atomic mass of
silicon. The three silicon atoms have
masses of 27.98 amu, 28.98 amu, and
29.97 amu with relative abundances of
92.23%, 4.67%, and 3.10%, respectively.
What is radioactivity?
 Emission of high energy radiation or particles
from the nucleus of a radioactive atom
 The atoms of radioactive elements are held
together less securely than nonradioactive
elements
 Particles of energy can escape from all nuclei
with atomic numbers 84 or higher (radioactive
decay)
 The nuclei of these elements are unstable
 In elements < 20 Atomic Number, n:p 1:1
 In elements > 20 Atomic Number, n:p 1.5:1
 The atom’s nucleus is held together by the
strong force
How do you write the
symbol for a nuclide?
 The symbol gives atomic #, mass #, and
chemical symbol
mass #
39
atomic #
19
K
chemical symbol
What is nuclear radiation?
 Radiation given off by radioactive
nuclides
 There are three types:
 alpha particles ( particles)
 beta particles ( particles)
 gamma rays ( rays)
 Only gamma rays are a type of
electromagnetic radiation!!
What are alpha particles?
 Given off when a nucleus releases 2 neutrons
and 2 protons
 Same thing as a helium nucleus
 Has a charge of +2 and an atomic mass of 4
 Largest and slowest form of radiation
 Least penetrating – can be stopped by a sheet
of paper
 Used by smoke alarms (americium)
What are beta particles?
 Neutrons can spontaneously decay into a
proton and an electron
 The electron is the beta particle
 The proton can decay into a neutron and
a positron
What is a positron?
 A positron is similar to an electron, only
with a positive charge
 Positrons are considered beta particles
too
 Beta particles are much faster and more
penetrating than an  particle
What are gamma waves?
 Most penetrating and potentially dangerous
form of radiation
 Not made of particles
 Are electromagnetic waves with high frequency
and energy
 Have no mass, no charge, and travel at the
speed of light
 Usually released along with  and  particles
 thick blocks of lead and concrete are
commonly used for barriers
What is transmutation?
 Process of changing one element to
another through nuclear decay
 Atomic mass # of the decayed nuclide
equals the sum of the mass # of the
newly formed nuclide and the emitted
particle
How do you determine the
mass of the new nuclide?
 If the particle is an alpha particle, subtract the
mass of the ejected particle from the mass of
the old nuclide.
Alpha particle emission:
218
84
Po
214
82
4
2
Pb + He
How do I calculate the
mass of the nuclide
when it loses a beta
particle?
 Because a beta particle is the product of
the decay of a neutron, a proton will be
left behind when the –e is ejected.
214
82
Pb
214
83
Bi +
0
-1
e
Charged Atoms
 In a neutral atom, the number of protons
equals the number of electrons. The
positive and negative charges balance
out, leaving the atom with 0 net charge
 In a charged atom or ion, there is an
uneven number of protons and electrons,
so the atom will have either a positive or
negative net charge
Ions
What is half life?
 Some nuclides of radioactive isotopes may
require a long time to decay
 Half life is the amount of time it takes for half
the nuclides in a sample of a given radioactive
isotope to decay
 It can vary widely among the radioactive
isotopes
 Can determine amount of a radioactive sample
that will remain after a given amount of time
with the half life
Example
 Carbon 14:
At the beginning, there is 100%. It’s half life is
5730 years. So, after 5730 years, there will be
only half, or 50%, left. After another 5730
years (11,460 total), there will be half of 50%
left, or 25%. After another 5730 years (17,190
total), there will be half of 25% left, or only
12.5% of the original amount remaining.
Is there a formula for halflife calculations?
 Amount remaining = (initial amount)(1/2)n
 n = number of half-lives that have passed
 n also can equal t/T, where t = the elapsed time, T
= length of half-life
 Both t and T have to be in the same units
What is carbon 14 dating?
 Radioactive materials - In your body
 Carbon 14 emits beta particles and decays into
nitrogen
 Measuring % carbon 14 to carbon 12 allows
determination of approximate age of material
How can you measure
radioactivity?
 Cloud Chamber – contains a gas cooled
to a temperature below its condensation
point; droplets of the gas condense
around the radioactive particles, which
leave a trail that shows up along the
chamber lining.
 Geiger counter – produces an electric
current in the presence of a radioactive
substance.
What is fission?
 Nuclear fission is the splitting of an
atomic nucleus into two smaller nuclei
 Word “fission” means to divide
 Large nuclei with atomic numbers
above 90 can undergo nuclear fission
 U 235, when bombarded by a neutron,
splits to produce Ba 141, Kr 92, three
neutrons and ENERGY!
What is a nuclear chain
reaction?
 Neutrons released from one fission reaction
collide with another atom to cause another
fission reaction.
 A continuous series of fission reactions is
called a chain reaction.
 Huge quantities of energy are released with
many simultaneous nuclear reactions.
 An uncontrolled chain reaction causes a
nuclear explosion.
Nuclear Chain Reaction
Nuclear Reactor
 Nuclear reaction controlled with cadmium and
boron control rods that absorb neutrons
 Generate heat (energy) from U-235 and
heats the coolant water
 The “hot” coolant water then heats water that
is used to drive steam-driven turbines, which
produce electricity
 Problem – Spent fuel rods are VERY
hazardous waste and buried underground
 ½ life of U-235 is 713,000,000 years
Schematic of a Nuclear
Power Plant
What happened at Chernobyl?
What is Fusion?
 Fusion – Bind together
 Joining of 2 less stable nuclei (<60) into
one stable nuclei
 Example: Sun
 4 1H + 2 e --> 4He + 2 neutrinos + 6 photons
 A temperature of 5,000,000 K required to
overcome electrostatic repulsions
between the nuclei
Fusion Reaction
 In the picture to the
right, two types of
hydrogen atoms,
deuterium and tritium,
combine to make a helium
atom and an extra particle called a neutron
 This process releases four times as much
energy as the fission of a uranium nucleus
Medical Uses for Radiation
 Treating Cancer – Kills cancer and
healthy cells as well
 Radiotracers – emits non-ionizing
radiation and is used to signal the
presence of an element
 Used in studying blood flow patterns, uptake of
thyroid gland, emptying rate of gallbladder
 Used in research experiments to trace amounts of
chemicals in the system ( tertiary oil recovery)