Atomic Theory DEMOCRITUS 460 - 370 BC The Greek philosopher Democritus proposed that all matter was...
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Transcript of Atomic Theory DEMOCRITUS 460 - 370 BC The Greek philosopher Democritus proposed that all matter was...
DEMOCRITUS 460 - 370 BC
• The Greek philosopher Democritus proposed that all matter was made of small, unbreakable particles he called atoms which means unbreakable.
• He believed that atoms were too small to be seen.
• Philosophers are not scientists. They do not test their ideas. Instead they use reasoning to back up their beliefs.
• To them, human reasoning was superior to experimentation.
ARISTOTOLE
• The famous philosopher Aristotle, who also lived at that time, argued that all matter was made of only four elements.
• For the next two thousand years, Aristotle overshadowed Democritus.
• Finally, in the early 1800s, the atomist’s theory was revived by John Dalton.
John Dalton 1766-1844
• In 1809, Dalton by proposing the following:a) All matter was made of atoms.b) Atoms were solid spheres.c) Atoms of different elements differed in
mass.d) Atoms were indivisible and
indestructible.e) Atoms combine to form compounds.
J.J. THOMSON 1856-1940• Before you can understand Thomson’s
experiment, 3 properties about electrical charges:a) There are two types of electrical charge:
positive and negative.b) Opposite charges attract.
c) Like charges repel.• Thomson took Cathod ray tube and added
two plates inside the tube and connected them with a wire.
• When the plates were not charged, the ray shot straight.
Passing an electric current makes a beam appear to move from the negative to the positive end
Thomson’s Experiment
Voltage source
+-
Voltage source
By adding an electric field he found that the moving pieces were negative
+
-
Thomson’s Experiment
Cathod Ray Tube Conclusion
• Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.
• Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons
• Electrons have so little mass that atoms must contain other particles that account for most of the mass
• Thomson’s model was called the Plum Pudding Model was named after a popular dessert in England at that time.
• It was the first model to propose that smaller charged particles make up the atom.
• Thomson’s model lasted less than two decades but it was first to propose the existence of subatomic particles.• In 1911 another scientist who worked in Thomson’s lab improved on his atomic model.
ERNEST RUTHERFORD 1871-1937• One type of radioactivity
is when an atom throws out a positively charged particle from the nucleus.
• This particle was called an alpha particle (α).
• Rutherford used this alpha particle to investigate the structure
Rutherford and Geiger in the Cavendish Lab
Rutherfold’s Gold Foil Experiement• Uranium is a radioactive element that gives off positive particles
(alpha particles).• Rutherford used these positive particles to invest• Rutherford encased uranium in lead (which absorbs alpha
particles).• This produced a beam of alpha particles traveling in a straight line.
• He fired these positive particles at a thin piece of gold (dense metal).• A screen around the gold to detect where the alpha particles were traveling.
• Rutherford shot alpha particles at a thin sheet of gold to observe what happened when the positive α particles passes through the gold atoms.
• If Thompson’s model was correct the alpha particles should pass through the diffused positive cloud with ease.
Rutherfold’s Gold Foil Experiement
Rutherfold’s Conclusion
• From his observations Rutherford concluded that the atom had a dense, positive central nucleus composed of + charged protons.
• He stated that the electrons orbited the nucleus - like planets orbiting the Sun.
• In 1909 Rutherford proposed his Planetary Model of the Atom.
• His model created positively charged protons located in the nucleus and placed electrons in orbit around the nucleus – like planets around the sun.
Checking for understandingExplain Thompson’s conclusions in 3 points:1.2.3.
Explain Rutherford’s conclusions in 3 points:1.2.3.
Subatomic Particles
• Over the past century scientist have discovered that the atom is composed of 3 subatomic particles:
ProtonsNeutronsElectrons
Checking for understandingDraw this diagram. Label all subatomic particles and include their charges.
The Proton1. Symbol = p+2. Relative Mass =
1 Atomic Mass Unit (AMU).
3. Actual mass = 1.674 x 10 -24 g
4. Location: Inside the nucleus
5. Electrical charge: Positive.6. Importance: The atomic
number which is the identity of the element.
7. Discovered by: Ernest Rutherford in 1909
The Electron1. Symbol = e-2. Relative Mass = 1 /1836
Atomic Mass Unit.3. Actual mass =
9.11 x 10 -28 g4. Location: Energy level
outside the nucleus
5. Electrical charge: Negative.6. Importance: The number of
electrons located in the last energy level determine the chemical activity of the element.
7. Discovered by: J.J.Thomson in 1897
The Neutron1. Symbol = n2. Relative Mass = 1 Atomic
Mass Unit (AMU).3. Actual mass =
1.675 x 10 -24 g4. Location: Inside the
nucleus
5. Electrical charge: Neutral.6. Importance: Is responsible for isotopes (atoms of the same element with different numbersof neutrons.7. Discovered by: James Chadwick in 1932
Atomic NumberAtomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.
Element # of protons Atomic # (Z)
Carbon 6 6
Phosphorus 15 15
Gold 79 79
Mass NumberMass number is the number of protons and neutrons in the nucleus of an isotope.
Mass # = p+ + n0
Nuclide p+ n0 e- Mass #
Oxygen - 10 - 33 42 - 31 15
8 8 1818Arsenic 75 33 75
Phosphorus 15 3116
IsotopesIsotopes are atoms of the same element having different masses due to varying numbers of neutrons.
Isotope Protons Electrons Neutrons NucleusHydrogen–1
(protium)1 1 0
Hydrogen-2(deuterium)
1 1 1
Hydrogen-3(tritium)
1 1 2
Atomic Masses
Isotope Symbol Composition of the nucleus
% in nature
Carbon-12
12C 6 protons6 neutrons
98.89%
Carbon-13
13C 6 protons7 neutrons
1.11%
Carbon-14
14C 6 protons8 neutrons
<0.01%
Atomic mass is the average of all the naturally occurring isotopes of that element.
Carbon = 12.011
Weight Average Atomic Mass
• The atomic masses given on the periodic table are WEIGHT-AVERAGED masses.
• This is calculated using both the masses of each isotope and their percent abundances in nature.
• For the purposes of simplicity, we will round weight-average mass to the THOUSANDTHS place.
• The weight-average mass is based on the abundance of the naturally occurring isotopes of that element
• To find the weight-average mass of an element given the mass of each isotope and each isotopes percent abundance:
WAM =
(massisotope 1 X % ) + (massisotope 2 X % ) + (massisotope 3 X % ) + etc…
Weight Average Atomic Mass
Atomic Mass Unit (AMU)• amu = atomic mass unit– the ratio of the average mass per atom of
the element to 1/12 of the mass of 12C in its nuclear and electronic ground state.
• An atomic mass unit is actually an average mass, found by taking the mass of a C-12 nucleus and dividing it by 12–Hydrogen = 1amu, 1/12 of C
39
Carbon has two stable isotopesCarbon-12 has natural abundance of 98.89% and 12.000 amuCarbon-13 has natural abundance of 1.11% and 13.003 amu
Calculate the atomic mass
1. GivensCarbon-12 m=12.000 amu Abundance= 98.89%=0.9889Carbon-13 m = 13.003 amu Abundance = 1.11%=0.0111
2. Formula atomic mass of carbon-avg = (mass C-12 x nat.abund) + (mass C-13 x nat.abund.)
3. Plug in the #s(12.000amu x 0.9889) + (13.003 amu x 0.0111)= 12.011 amu= 12.0 amu
1. Shell Configuration
• Shows how many electrons are found in each shell (principal energy level).
• This is the configuration Niels Bohr would have come up with as the discoverer of the energy level!
Shell Number (Principle Electron Level)
Number of Electrons to hold
1 2
2 8
3 8
4 18
5 18
6 32
7 32
Shell Configuration (Bohr Diagrams)
C
1) Draw a nucleus with the element symbol inside.
2) Carbon is in the 2nd period, so it has two energy levels, or shells.
3) Draw the shells around the nucleus.
1) Add the electrons.
2) Carbon has 6 electrons.
3) The first shell can only hold 2 electrons.
C
Shell Configuration (Bohr Diagrams)
Shell Configuration (Bohr Diagrams)
1) Since you have 2 electrons already drawn, you need to add 4 more.
2) These go in the 2nd shell.
3) Add one at a time -starting on the right side and going counter clock-wise.
C
Shell Configuration (Bohr Diagrams)
1) Check your work.2) You should have 6 total
electrons for Carbon.3) Only two electrons can
fit in the 1st shell.4) The 2nd shell can hold
up to 8 electrons.5) The 3rd shell can hold
18, but the elements in the first few periods only use 8 electrons.
C
2. Sublevel Electron Configuration• Principal energy levels are made up of
sublevels, much as a town is made up of streets.
• The expanded configuration tells you how many electrons are found in each sublevel of each PEL.
• Most of the time (and for all of the configurations you will be responsible for), one sublevel must fill up completely before the next one can get any electrons.
Electrons in atoms are arranged as
SHELLS (n)
SUBSHELLS (l)
ORBITALS (ml)
row #shell #
possibilities are 1-77 rows
Arrangement of Electrons in an Atom
subshellpossibilities are
s, p, d, or f4 subshells
group ## e-
s subshell : 1 orbital , total 2 e-p subshell : 3 orbital, total of 6 e-d subshell :5 orbital, total of 10 e-
f subshell: 7 orbital, total of 14 e-
Each orbital can be
assigned no more than 2 electrons!
1
2
3
4
5
6
7
6
7
1A
2A
3B 4B 5B 6B 7B 8B 8B 8B 1B 2B
3A 4A 5A 6A 7A
8Agroup # = # valence (outside) e-
d p
f
sRow
=# shells
1
2
3
4
5
6
7
6
7
perio
d #
= #
e- s
hells
1A
2A
3B 4B 5B 6B 7B 8B 8B 8B 1B 2B
3A 4A 5A 6A 7A
8Agroup # = # valence e-
d
f
3d4d5d6d
4f5f
Subshells d and f are “special”
Electron Configuration – Spdf notation
Is2row #shell #
possibilities are 1-77 rows
subshellpossibilities are
s, p, d, or f4 subshells
group ## valence e-
possibilities are:s: 1 or 2p: 1-6
d: 1-10f: 1-14
Total e- should equalAtomic #
HELIUM – 2 electrons
Electron Configuration – Spdf notation
Is2row #shell #
possibilities are 1-77 rows
subshellpossibilities are
s, p, d, or f4 subshells
group ## valence e-
possibilities are:s: 1 or 2p: 1-6
d: 1-10f: 1-14
Total e- should equalAtomic #
HELIUM – 2 electrons
3. Orbital Box Diagram • Shows how many electrons are in each ORBITAL of
each sublevel, and what each electron’s SPIN is. • Orbitals are all the same size, they can all fit up to
two electrons in them. • The spin of electrons is indicated by arrows up and
down.• If the orbital has two electrons in it, the first will
have an up spin, and the second will have a down spin.
• The number of arrows will equal the number of electrons in the sublevel.
Drawing Orbital DiagramDraw the orbital diagram for nitrogen.Step 1 Draw boxes to represent the occupied
orbitals. Nitrogen has an atomic number of seven, which means it has seven electrons. Draw boxes to represent the 1s, 2s, and 2p orbitals.
1s 2s 2p
Drawing Orbital Diagram
Step 2 Place a pair of electrons in the last occupied sublevel in separate orbitals. We place the remaining three electrons in the 2s orbitals.
1s 2s 2p
Drawing Orbital Diagram
Step 3 Place remaining electrons with opposite spins in each filled orbital. First we place a pair of electrons with opposite spins in the 2p orbitals, with arrows in the same direction.
1s 2s 2p
HONORS CHEMISTRY ONLY3a. Quantum Numbers
• Electron energies are addressed in a similar way to a ZIP code. Many addresses in Ulster and northern Orange
• county have 125 as the prefix, with the last two digits signifying the actual postal box.\
• For example, New Paltz is 12561, Wallkill is 12589, Newburgh is 12550, Pine Bush is 12566.
3a. Quantum Numbers
• There are four identifying characteristics of the energy of a specific electron in an atomic, each more specific than the last.
• They are:– n (principal quantum number) = Principal Energy
Level (1, 2, 3, 4, etc.)– l (levarotary) = Sublevel (s, p, d, f)– m (magnetic) = Orbital– s (spin) = Spin (+ 1/2, - 1/2)
3a. Quantum Numbers
• n , principal quantum number–based on Bohr’s observations of line
spectra for different elements–‘n’ relates to the main energy of an
electron–allowable values: n = 1, 2, 3, 4, …–electrons with higher ‘n’ values have
more energy
3a. Quantum Numbers
• l , The Secondary Quantum Number – based on the observation that lines on line spectra
are actually groups of multiple, thin lines– ‘l ’ relates to the shape of the electrons’ orbits– allowable values: l = 0 to l = n - 1• i.e. for n = 4: l = 0, 1, 2, or 3
– the ‘l ’ values 0, 1, 2, and 3 correspond to the shapes we will call s, p, d and f, respectively
3a. Quantum Numbers
• ml , the Magnetic Quantum Number– based on the observation that single lines on line
spectra split into new lines near a strong magnet– ‘ml ’ relates to the direction/orientation of the
electrons’ orbits– allowable values: ml = - l to + l • i.e. for l = 2: ml = -2, -1, 0, 1, or 2
– electrons with the same l value but different ml values have the same energy but different orientations
3a. Quantum Numbers
• ms , The Spin Quantum Number– based on the observation that magnets could
further split lines in line spectra, and that some elements exhibit paramagnetism
– ‘ms ’ relates to the ‘spin’ of an electron– allowable values: ms = - ½ or + ½ • i.e. for any possible set of n, l, and ml
values, there are two possible ms values– when two electrons of opposite spin are paired,
there is no magnetism observed; an unparied electron is weakly magnetic
4. Electron (Lewis) Dot Diagram• VALENCE ELECTRONS– the electrons in the outermost shell (furthest
energy level from the nucleus), which is also called the valence shell.
– The number of valence electrons that an atom has can be determined by the last number in the basic electron configuration.
The number of valence electrons that an atom has determines its physical and chemical
properties
Lewis Dot Diagram
• using dots in groups of 2 around the symbol of the atom to represent the valence electrons.
• For every atom, the valence electrons will occupy only s and p orbitals.
• The s electrons fill up first, then the p electrons fill, up electrons first, followed by the downs, just like in the box diagram.
The Electron Dot diagram for Nitrogen
Nitrogen has 5 valence electrons.
First we write the symbol.
NThen add 1 electron at a time to each side.
Until they are forced to pair up.
Nuclear Chemistry• Nucleus of an atom contains
protons and neutrons• Strong forces (nuclear force) hold
nucleus together– Protons in nucleus have electrostatic
repulsion– however, strong force overcomes this
repulsion– Strong force: the interaction that
binds nucleus together– Nuclear force (strong force) is MUCH
stronger than electrostatic force– Strong force increases over short
distances
Radioisotopes
• Radioisotopes- unstable isotopes that gain stability by releasing particles.
Unstable isotope stable
alpha
betagamma
Characteristics of Some RadiationProperty Alpha
radiationBeta radiation
Gamma radiation
Composition Alpha particle (He nucleus)
Beta particle (electron)
High energy EM radiation
Symbol , 42 He , 0
-1 e
Penetrating power
low moderate Very high
Shielding Paper, clothing
Metal foil Lead, concrete
Atomic number (Z) = # protons in nucleus
Mass number (A) = # protons + # neutrons
= atomic # (Z) + # neutrons
Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei
XAZ
U23592 U238
92
Mass Number Atomic Number
Element Symbol
Review
Alpha emission (decay) 4 2 He
238 92 U
Identify the product formed when uranium-238 alpha decays
4 2 He + 234
90 Th
Determines which atom from the periodic table
Beta emission (decay) 0-1 e
14 6 C
Identify the product formed when carbon-14 emits beta particle
0 -1 e + 14
7 N
Gamma emission (decay) 00 γ
14 6 C
Identify the product formed when carbon-14 releases gamma rays
0 0 γ + 14
6 C
Nuclear Fission• Reaction splits a large
nucleus apart to form two smaller ones.
• Reaction is unknown in the natural world, is a form of artificial transmutation
• Reaction can take place at any temperature or pressure
• Reaction is currently being used to produce electricity for our use
• Requires mining to extract uranium ore• Produces THOUSANDS of times more energy than conventional chemical explosives• Produces radioactive wastes
Nuclear Fusion• Reaction combines two small
nuclei together to form one larger one.
• All stars are powered by nuclear fusion
• Reaction requires temperatures of millions of degrees and vast pressures
• Reaction requires temperatures of millions of degrees and vast pressures
• Hydrogen is the most abundant element in the universe• Produces MILLIONS of times more energy than conventional chemical explosives• Produces essentially no radioactive waste
Common to Both Fission and Fusion
• Both generate their energy the same way by converting small amounts of mass (MASS DEFECT) into extraordinary amounts of energy.
Half- Life• The half-life of a radioactive isotope is defined as the
period of time that must go by for half of the nuclei in the sample to undergo decay.
- Half of the radioactive nuclei/isotope in the sample decay into new, more stable nuclei/isotope
• After one half-life, half (50%) of the original amount of the sample will have undergone radioactive decay.
• After a second half-life, one quarter (25%) of the original sample will remain undecayed.
• After a third half-life, one eighth (12.5%) of the original sample will remain undecayed.
The blue grid below represents a quantity of C14. Each time you click,one half-life goes by and turns red. C14 – blue N14 - red
As we begin notice that no time has gone by and that 100% of the material is C14
Halflives
% C14 %N14 Ratio of C14 to N14
0 100% 0% no ratio
97
The grid below represents a quantity of C14. Each time you click,one half-life goes by and you see red. C14 – blue N14 - red
Halflives
% C14 %N14 Ratio of C14 to N14
0 100% 0% no ratio
1 50% 50% 1:1
After 1 half-life (5730 years), 50% ofthe C14 has decayed into N14. The ratioof C14 to N14 is 1:1. There are equalamounts of the 2 elements.
98
The blue grid below represents a quantity of C14. Each time you click,one half-life goes by and you see red .C14 – blue N14 - red
Halflives
% C14 %N14 Ratio of C14 to N14
0 100% 0% no ratio
1 50% 50% 1:1
2 25% 75% 1:3
Now 2 half-lives have gone by for a totalof 11,460 years. Half of the C14 that waspresent at the end of half-life #1 has nowdecayed to N14. Notice the C:N ratio. Itwill be useful later.
99
The blue grid below represents a quantity of C14. Each time you click,one half-life goes by and you see red. C14 – blue N14 - red
Halflives
% C14 %N14 Ratio of C14 to N14
0 100% 0% no ratio
1 50% 50% 1:1
2 25% 75% 1:3
3 12.5% 87.5% 1:7
After 3 half-lives (17,190 years) only12.5% of the original C14 remains. Foreach half-life period half of the materialpresent decays. And again, notice the ratio, 1:7 100
Radioactive Dating
• Radioactive Decay is a RANDOM process. It is not possible to predict when a particular nucleus will decay, but we can make fairly accurate predictions regarding how many nuclei in a large sample will decay in a given period of time.
Radioactive Dating
• used to determine the age of a substance that contains a radioactive isotope of known half-life.
• Step 1: Determine how many times you can cut your original amount in half in order to get to your final amount. This is the number of half-lives that have gone by.
• Step 2: Multiply the number of half-lives by the duration of a half-life
Age of Sample = # Half-Lives X Half-Life DurationSee Reference
chart
• The oldest rocks on Earth have been found to contain 50% U-238 and 50%Pb-206 (what U-238 ultimate decays into). What is the age of these rocks?
First, find out how many half-lives have had to go by so that you have gone from 100% U-238 to 50% U-238:
100 50 ONE half-life has gone by!
Age of Sample = # Half-Lives X Half-Life Duration = 1 half-life X (4.51 X 109 years) = 4.51 X 109 years old!