elements, atoms, & the discovery of atomic structure
TRANSCRIPT
Elements, atoms, & the
discovery of atomic structure
EARLY MODELS OF THE ATOMChapter 4
What is an atom?
The smallest particle of an element
that can keep the same properties
of the element. What is the smallest number of carbon atoms
that have all the properties of a carbon atom?
One
Democritus(Ancient Greece, 440 B.C.)
He stated that atoms are the smallest particles of all matter.
Democritus
Who was the first person to use the word atom?
Atomic Structure- outline
Dalton’s Atomic Theory Thomson’s model vs.
Rutherford’s model of the atomDiscovery of subatomic
particles:Protons, neutrons, and electrons
IsotopesQuantum mechanical model
Early model of the atom
Dalton’s Model was a dense solid sphere; indivisible and unchanged in chemical reactions.
Where did he get his ideas?
John dalton (1766-1844)
Berzelius’s ExperimentLaw of Definite Proportions
Proust’s experimentsLaw of Definite Proportions
Dalton’s Atomic Theory (pg 103)
All matter is made of indivisible atoms; they can be neither created nor destroyed during chemical reactions
All atoms of a given element are identical in their physical and chemical properties; they differ from atoms of every other element
Atoms of different elements combine in simple whole-number ratios to form compounds (can form more than one compound together)
Chemical reactions consist of the combination, separation, or rearrangement of atoms
Which of these are no longer valid?
Can we see an atom?
We can now view individual atoms! Scanning electron microscope (SEM)
J.J. Thomson’s Experiments 1897
Used a cathode ray tube: metal is placed at the positive end (anode) and the negative end (cathode).
J.J. Thomson’s Experiments
Rays produced from the cathode end.
The beam bends toward positive plates.
A small paddle wheel spins when hit by the cathode rays.
Conclusions-
Particles were bent by the charged plates
particles are charged. Particles set the wheel in motion
particles have mass. Particles were the same size no matter what metal was used
particles are the same, no matter the element.Atoms are neutral, but are made of negatively charged
particles
atoms are made of negative and positive charge.
What happened to the Dalton model?
The Plum Pudding Model
An early and now obsolete attempt to describe the interior structure of atoms
Electrons scattered throughout positively charged matter
Electrically neutral
electron
sphere of positive charge
Rutherford – Thomson’s student
Rutherford's gold foil experiment
Alpha particles (positively charged) bombarded foil of various metals.
A fluorescent screen was placed around to detect the particles as they passed through the metal.
Rutherford's expected vs. actual results Rutherford expected α-particles to pass undeflected through
atoms. But, he observed that a small fraction of the α-particles were
deflectedEvidence that the positively charged part of the atom
consisted of a tiny, dense object at the atom's center.He proposed the nuclear model of the atom.
AnimationAnimated Tutorial
Rutherford model- nuclear
What is the problem with this model?Charge and mass of atom did not work out!
Chadwick- discovers neutrons
Act as a kind of glue to hold the nucleus together. Positively charged protons are in a very confined
space but shouldn’t because they repel each other. Protons and neutrons are all attracted to each other
as a result of another force - the strong nuclear force.
The neutrons don't contribute any repulsive effects, so having more around can help to hold the nucleus together.
Subatomic particles: summary
Name Location Charge Mass Discovered
Proton
Neutron
Electron
Chemical reactions involve changes Nuclear reactions involve changes
Subatomic Particles
Mass of nucleus comes from the mass of protons and neutrons (= the nucleus).
The nuclear atom
How small is an atom?
An atom is so small, a single water droplet contains about 5 sextillion(1021) atoms
Electrons are on the outside of the atom with very little mass. Most of the mass of the atom is in a central nucleus. Therefore, an atom is mostly empty space
You can think of it as being like a marble in the middle of a football stadium. the marble is the nucleus-on the 50-yard line;
spectators are the electrons.
Size of an atom
Atomic Number and Mass Number
Atomic Number: number of protons in the nucleus of one atom - number of electrons in a neutral atom
Mass Number: total number of protons and neutrons in an atom’s nucleus.
Atomic Mass
Average Atomic Mass: average mass of all known atoms of an element. Unit: amu (atomic mass unit)
Atomic Mass
Isotopes
Atoms of the same element that contain different numbers of neutrons.
What is the chemical symbol? What is their atomic number? What is the mass number of the atom on the left?
Naturally occurring isotopes
Stable vs. Unstable isotopes
Radioactive Isotopes: unstable atoms due to a nucleus with too many or too few neutrons
No amount of neutrons can hold a nucleus together once it has more than 82 protons. Elements with an atomic number greater than 82 have unstable isotopes.
Unstable atoms emit energy in the form of radiation when they break down (decay)
Large nucleus (unstable) nucleus + energy
Reaction gives off LOTS of energy (= nuclear energy)
Discoveries lead to more about atomic theory
1890’s X-rays given off from anode when
cathode operating (light energy) Radioactivity- , , , rays
“Quantization of Energy” – 1900 Max Planck. E = hv
1905 Light as a wave and particle Einstein's Ideas about Light
Electromagnetic spectrum
Waves
If 1= 4s-1 = ?
Electromagnetic Spectrum
Speed of light
c = speed of light (3.0 x 108 m/s)
Types of light energy:
= wavelength = frequencyE = energy c =
Electromagnetic spectrum
Diffraction grating/prism
Note: A light bulb is an example of blackbody radiation (continuous spectrum). Most densely
packed solids will emit a continuous spectrum when heated to a certain temperature.
Absorption or Emission of light
The atom can absorb or emit light. Examples of absorption –
the color of shirt.Photosynthesis
Examples of emissionGas discharge tubesFlame testsNeon lightsLamps
Excited Electrons and Spectra
Line spectrum - can be used to identify an element – it is a characteristic property of that element. Examples of practical use: determine
the chemical make-up of the stars and plants’ atmospheres.
FIREWORKS! SIMILAR CONCEPT TO OUR
FLAME TEST
Different metal will burn different colors.
-What metallic elements do you think are in
these fireworks?
Continuous vs. Line Spectrum
Hydrogen’s line spectrum
Another great student…Niels Bohr (student of Rutherford)
1913 - Revised Rutherford’s model to include newer discoveries about how an atom could absorb or emit light!
Here’s his thoughts: Electrons are found in “distinct energy
levels”. This means electrons can’t be found in-between these levels.
Like Rutherford he proposed e- orbited the nucleus.
Bohr Model
Electrons absorb energy and move to outer energy levels. When they relax, they give off energy.
“Your theory is crazy, but it's not crazy enough to be true”. Niels Bohr
Quantum Theory
vs.
Classical Theory Quantum Theory
Bohr Model
1
23
456 Energy of photon depends on the difference in energy levels
Bohr’s calculated energies matched the IR, visible, and UV lines for the H atom
Bohr Model
Each element has a unique bright-line emission spectrum.
“Atomic Fingerprint”
HeliumBohr’s calculations only
worked for hydrogen! Did not agree with classical physics.
Electrons and energy
An electron’s P.E. & K.E both change when it relaxes (down orbital/s) or is in an excited state (up orbital/s)
EXCITED STATE: Absorbs a photon or quantum of energy
elevates to higher energy levelGROUND STATE:
Electrons in their lowest energy levels
Atomic structure- Bohr model
Energy level=n Lowest energy state is closest to
nucleus-attracted to the protonsWhen one energy level is filled,
electrons are found at higher levels. Each energy level can hold a maximum
number of electrons (2n2 electrons) First shell = two electrons Second shell = eight electrons Third shell = eighteen electrons
Quantum Mechanical Model
Electrons have properties of waves and light(De Broglie)
It is impossible to know both the position and momentum of an electron (Heisenberg)
The probability of finding in electron in a certain area around the nucleus. (Schrödinger) Sublevels- defined by energy level/distance from nucleus Orbitals- mathematical function corresponding to a region
within atom each with a maximum of 2 e- with opposite spin
Quantum Mechanical Model
Determines the allowed energies an electron can have and how likely it is to find the electron in various locations around the nucleus of the atom.
Where is an electron?
Heisenberg Uncertainty Principle
– It is impossible to know both the position and momentum of an electron.
S Orbitals
P orbitals
D orbitals
F orbitals
An orbital is a mathematical (3D) graph of the solution to the quantum mechanical wave equation. It defines a region of space that has a high probability of containing up to 2 e-.
Movie visual
How do concepts of energy levels and orbitals fit together?
Each energy level is made of 1 or more sublevels:
Each sublevel is made of 1 or more orbitals:
Orbitals are filled from lowest to highest energy, in order of the periodic table
Electron Configurations
Aufbau Principle Electrons fill from
lowest energy to highest energy.
Electron Configurations
Pauli Exclusion Principle Paired electrons must have opposite
spins. Each orbital holds 2 electrons.
Electron Configurations
Hund’s Rule Electrons must be unpaired
before they are paired in a sublevel.
“Make sure that everyone gets a helping!”
RIGHTWRONG
Abbreviated Configurations
© 1998 by Harcourt Brace & Company
s p
d (n-1)
f (n-2)
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67
Abbreviated Configurations
[Ar]
1 2 3 4 5 6 7
4s2 3d10 4p2
Example - Germanium
Abbreviated Configurations
© 1998 by Harcourt Brace & Company
s p
d (n-1)
f (n-2)
1234567
67
THE PERIODIC TABLEChapter 6
Names and symbols
Symbol
Name
H HydrogenHe HeliumLi LithiumBe BerylliumB BoronC CarbonN NitrogenO OxygenF FluorineNe NeonNa SodiumMg MagnesiumAl AluminumSi SiliconP Phosphoru
s
Symbol
Name
S SulfurCl ChlorineAr ArgonK Potassiu
mCa CalciumFe IronCo CobaltCu CopperZn ZincAg SilverSn TinI IodineAu GoldHg MercuryPb Lead
Universe’s elements
Earth’s elements
Human Body Elements
Diatomic elements Label your
PT
Metals, nonmetals, metalloids
Label your PT
Periodic table organization
Groups or families = column, similar chemical properties Alkali metals Alkaline earth metals Halogens Noble gases
Period = row, chemical and physical trends repeat Other sections
Transition metals Metalloids Metals Nonmetals Lanthanide and actinide series (inner transition metals or rare
earth)
Trends based on # of electrons
Groups (columns) Elements in the same group have similar properties; why? They all have the same # of outer electrons= VALENCE ELECTRONS- Use the periodic table note valence electrons
Periods (rows) Elements in a period have valence electrons in the same
outer energy level. They all have the same # of inner electrons= CORE ELECTRONS- Use the periodic table note energy levels
Physical properties of elements
Physical state: • gas, solid, liquid
Conductivity: • Conductor, semiconductor
Physical qualities:
Label your
PT