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)