DEVELOPMENT OF THE DEVELOPMENT OF THE ATOMIC MODELATOMIC MODEL
From Democritus to RutherfordFrom Democritus to Rutherford
c.400 B.C.c.400 B.C.Ancient Greek PhilosophyAncient Greek Philosophy
Everything in the Everything in the universe is made of universe is made of one or more of the one or more of the basic “elements:”basic “elements:”
Earth, Fire, Water, AirEarth, Fire, Water, Air
What makes up the elements?What makes up the elements?
ARISTOTLEARISTOTLE Matter is infinitely Matter is infinitely
divisible; no matter divisible; no matter how small a piece is, it how small a piece is, it can always be divided can always be divided into smaller piecesinto smaller pieces
DEMOCRITUSDEMOCRITUS There exists a There exists a
“smallest piece” of “smallest piece” of matter, which cannot matter, which cannot be divided any further.be divided any further.
These pieces are These pieces are called “called “ατομοσατομοσ,” or ,” or “atoms”“atoms”
Whose Argument Prevails?Whose Argument Prevails?
Aristotle’s viewpoint enjoyed the support of Aristotle’s viewpoint enjoyed the support of most of the world because he was more most of the world because he was more well-known and because Democritus had well-known and because Democritus had no evidence to back up his claim since no evidence to back up his claim since these “atoms” would be too small to see.these “atoms” would be too small to see.
Antoine Lavoisier 1743-1794Antoine Lavoisier 1743-1794 Father of Modern ChemistryFather of Modern Chemistry Chemical Revolution – 1770-1790Chemical Revolution – 1770-1790 Stated the Law of Conservation of MassStated the Law of Conservation of Mass Oxygen for combustionOxygen for combustion Decomposed Water into two gases, hydrogen Decomposed Water into two gases, hydrogen
and oxygen and then reformed the exact same and oxygen and then reformed the exact same amount of water.amount of water.
1803: John Dalton1803: John Dalton
Dalton was an Dalton was an English schoolteacherEnglish schoolteacher
Began teaching Began teaching mathematics and mathematics and chemistry at the age chemistry at the age of 12of 12
Revived the idea of Revived the idea of Democritus’ “smallest Democritus’ “smallest piece” of matterpiece” of matter
Dalton’s Atomic TheoryDalton’s Atomic Theory
All matter is made of tiny particles called “atoms”All matter is made of tiny particles called “atoms” Atoms are indivisible and indestructibleAtoms are indivisible and indestructible Atoms of the same element are identicalAtoms of the same element are identical Atoms of different elements differ in some Atoms of different elements differ in some
fundamental wayfundamental way Atoms combine in simple whole number ratios to Atoms combine in simple whole number ratios to
form compoundsform compounds Atoms are rearranged in chemical reactions but Atoms are rearranged in chemical reactions but
cannot be created or destroyedcannot be created or destroyed
Three Laws Explained by the Atomic TheoryThree Laws Explained by the Atomic Theory
Law of Conservation of Mass (Antoine Lavoisier)Law of Conservation of Mass (Antoine Lavoisier) Law of Definite Proportions - compounds Law of Definite Proportions - compounds
always contained the same mass ratio of one always contained the same mass ratio of one element to another. (Joseph Proust)element to another. (Joseph Proust)
Law of Multiple Proportions – When elements Law of Multiple Proportions – When elements combine in different ratios, each new ratio is a combine in different ratios, each new ratio is a unique compound. (John Dalton)unique compound. (John Dalton)
So what?So what?
These results could only be explained by These results could only be explained by assuming that matter was made of atoms assuming that matter was made of atoms – tiny building blocks – and that these – tiny building blocks – and that these atoms only came in certain sizes. atoms only came in certain sizes.
Dalton’s View of an atom
1897: J.J. Thomson1897: J.J. Thomson
English physicistEnglish physicist Worked with Worked with
Cathode-Ray Tubes Cathode-Ray Tubes (CRTs)(CRTs)
Credited with the Credited with the discovery of the discovery of the electronelectron
CRTsCRTs
Mysterious particles Mysterious particles emanated from the emanated from the cathode endcathode end
These particles were These particles were deflected by magnetic deflected by magnetic and electric fieldsand electric fields
They were very small They were very small and negatively and negatively chargedcharged
These particles were called “electrons” and were assumed to be a part of all matter
Thomson’s Atomic ModelThomson’s Atomic Model
Electrons
Positively charged “goo”
A.K.A. the “Plum-Pudding Model”
1909-11: Robert Millikan1909-11: Robert Millikan
Set out to discover Set out to discover the charge of a single the charge of a single electronelectron
Famous experiment Famous experiment called the “oil-drop called the “oil-drop experiment”experiment”
Using his results and Using his results and the charge-to-mass the charge-to-mass ratio from Thomson, ratio from Thomson, the mass of an the mass of an electron was foundelectron was found
The Oil Drop ExperimentThe Oil Drop Experiment
To view an animation of this experiment click below
OIL DROP EXPERIMENT
1910: Ernest Rutherford1910: Ernest Rutherford
Expert in radiationExpert in radiation Famous “Gold-Foil Famous “Gold-Foil
Experiment”Experiment” Discovered the Discovered the
presence of the presence of the nucleus by firing nucleus by firing alpha particles at a alpha particles at a sheet of gold foilsheet of gold foil
The Gold Foil ExperimentThe Gold Foil Experiment
To view an animation of this experiment click below
GOLD FOIL EXPERIMENT
Rutherford’s AtomRutherford’s Atom
Rutherford’s Atomic ModelRutherford’s Atomic Model
Electrons
NucleusPositively chargedMade of “protons”
Empty Space
Gold Foil ConclusionsGold Foil Conclusions
The atom has a nucleusThe atom has a nucleusThe nucleus has a positive chargeThe nucleus has a positive chargeThe nucleus is very small and very denseThe nucleus is very small and very denseMost of the atom is empty spaceMost of the atom is empty spaceThe electron resides in the region outside The electron resides in the region outside
the nucleusthe nucleus
1932: James Chadwick1932: James Chadwick
A fellow researcher with Rutherford, A fellow researcher with Rutherford, Chadwick discovered years later that the Chadwick discovered years later that the nucleus was not made of only one particle nucleus was not made of only one particle – the proton – but of two particles.– the proton – but of two particles.
This second particle was called the This second particle was called the “neutron” because it had no electrical “neutron” because it had no electrical chargecharge
Subatomic ParticlesSubatomic Particles
Electron (eElectron (e--) – mass 1/1840 amu, charge -1, ) – mass 1/1840 amu, charge -1, found in space around the nucleusfound in space around the nucleus
Proton (pProton (p++) – mass of 1 amu, charge +1, ) – mass of 1 amu, charge +1, found in the nucleusfound in the nucleus
Neutron (n) – mass of 1 amu, no charge, Neutron (n) – mass of 1 amu, no charge, found in the nucleusfound in the nucleus
The AtomThe Atom
Made up of 3 fundamental subatomic Made up of 3 fundamental subatomic particles: protons, neutrons, and electronsparticles: protons, neutrons, and electrons
Very small and very dense nucleus Very small and very dense nucleus (nucleus make up over 99% of atom’s mass)(nucleus make up over 99% of atom’s mass)
Nucleus contains protons and neutronsNucleus contains protons and neutronsElectrons occupy the empty space outside Electrons occupy the empty space outside
the nucleusthe nucleus# of protons = # of electrons# of protons = # of electrons
What’s in the atom?What’s in the atom?
Nucleons – particles in the nucleus (protons and Nucleons – particles in the nucleus (protons and neutrons) neutrons)
The combined total of the protons and neutrons The combined total of the protons and neutrons is called the is called the mass numbermass number
The number of protons is called the The number of protons is called the atomic atomic numbernumber. .
The atomic number identifies the element.The atomic number identifies the element. Electrons found in the space outside the nucleusElectrons found in the space outside the nucleus Lots of empty spaceLots of empty space
IsotopesIsotopes
Particles with the same number of protons Particles with the same number of protons and electrons but different numbers of and electrons but different numbers of neutrons.neutrons.
Have different mass numbers.Have different mass numbers.Have different masses.Have different masses.React the same chemically.React the same chemically.
Rutherford’s DilemmaRutherford’s Dilemma
If the electron is in orbit around the nucleus, If the electron is in orbit around the nucleus, it should be emitting radiation, but it is not.it should be emitting radiation, but it is not.
What prevents the electron from being What prevents the electron from being pulled into the nucleus?pulled into the nucleus?
Electromagnetic PropertiesElectromagnetic Properties
Wavelength – distance between Wavelength – distance between consecutive waves.consecutive waves.
Frequency – the number of waves that Frequency – the number of waves that pass a point in a given amount of time, pass a point in a given amount of time, usually 1 s.usually 1 s.
Quantum TheoryQuantum Theory
Proposed by Max Planck Proposed by Max Planck Two main ideas:Two main ideas:
1.1. Energy changes are not continuous but Energy changes are not continuous but rather occur in small increments called rather occur in small increments called “Quantums”.“Quantums”.
2.2. The energy of a quantum is directly The energy of a quantum is directly proportional to the frequency of the proportional to the frequency of the radiation.radiation.
…….. Quantum theory continued... Quantum theory continued.
E = hE = hννE = energy of a quantumE = energy of a quantum
h= Planck’s constanth= Planck’s constant
(6.63 x 10(6.63 x 10-34-34 J J..s)s)
V = frequencyV = frequency
What is the energy of a quantum with a What is the energy of a quantum with a wavelength of 550 nm?wavelength of 550 nm?
Photoelectric EffectPhotoelectric Effect
Smart GuySmart Guy
In explaining the In explaining the photoelectric effect photoelectric effect Albert Einstein Albert Einstein showed that radiant showed that radiant energy, such as light, energy, such as light, can posses can posses particle-particle-likelike properties. properties.
Neils BohrNeils Bohr
Hydrogen’s SpectrumHydrogen’s Spectrum
Questions?Questions?
Why do the lines always occur at the same Why do the lines always occur at the same place and why only 4 lines?place and why only 4 lines?
Ans: Only specific energy changes are Ans: Only specific energy changes are possible in an atom. The lines are possible in an atom. The lines are representations of those energy changes.representations of those energy changes.
Bohr reasoned that if an electron could Bohr reasoned that if an electron could occupy an infinite number of possible occupy an infinite number of possible orbits, its jumps from these orbits should orbits, its jumps from these orbits should give rise to an infinite number of different give rise to an infinite number of different energy radiations……….THUSenergy radiations……….THUS
A continuous spectrumA continuous spectrum
Bohr’s InterpretationBohr’s Interpretation
Electrons in atoms can only occupy certain fixed Electrons in atoms can only occupy certain fixed orbits or “energy levels”.orbits or “energy levels”.
These energy positions are quantized, meaning These energy positions are quantized, meaning only certain values are possible within an atom.only certain values are possible within an atom.
To move from one energy orbit to another one, To move from one energy orbit to another one, an electron must absorb or emit a an electron must absorb or emit a quantumquantum of of energy exactly equal to the energy difference energy exactly equal to the energy difference between the two positions.between the two positions.
Electronic TransitionsElectronic Transitions
Since Bohr . . . The Modern ModelSince Bohr . . . The Modern Model
The Quantum Mechanical ModelThe Quantum Mechanical ModelHas a nucleusHas a nucleusElectrons are in a “cloud” of negative Electrons are in a “cloud” of negative
charge.charge.An electron “orbit” is an area where the An electron “orbit” is an area where the
electron is most “likely” to be.electron is most “likely” to be.
Quantum NumbersQuantum Numbers
Describe the properties of orbitals and Describe the properties of orbitals and electrons in those orbitals.electrons in those orbitals.
1.1. Principal q.n. (n) – designates the main Principal q.n. (n) – designates the main energy level or shell. (Bohr)energy level or shell. (Bohr)
Values: 1, 2, 3, 4, 5, …… Values: 1, 2, 3, 4, 5, ……
n=1; means the electron is located in the n=1; means the electron is located in the first energy level, which has the lowest first energy level, which has the lowest energy.energy.
2. Angular momentum q.n. – (l) designates 2. Angular momentum q.n. – (l) designates the shape of the atomic orbital.the shape of the atomic orbital.
values: 0, 1, 2, 3, . . . n-1 values: 0, 1, 2, 3, . . . n-1
so if n=3, l can be 0, 1, or 2.so if n=3, l can be 0, 1, or 2.
if l = 0, (s) then it is sphericalif l = 0, (s) then it is spherical
if l = 1, (p) then it is dumbbell if l = 1, (p) then it is dumbbell
if l = 2, (d) complexif l = 2, (d) complex
if l = 3, (f) complexif l = 3, (f) complex
Orbital ShapesOrbital Shapes
3. Magnetic q. n. (m3. Magnetic q. n. (mll) – designates the ) – designates the
orbital’s orientation in space.orbital’s orientation in space.
values – (from –l through 0 to +l)values – (from –l through 0 to +l)
ex. If l = 1 (p) then Mex. If l = 1 (p) then Mll can = -1, 0, +1 can = -1, 0, +1
-1 corresponds to p-1 corresponds to pxx
0 corresponds to p0 corresponds to pyy
+1 corresponds to p+1 corresponds to pz z
4. Spin q. n. (m4. Spin q. n. (mss) – describes the spin of the ) – describes the spin of the
electron on its axis; clockwise or counter electron on its axis; clockwise or counter clockwise.clockwise.
values - +1/2 or – 1/2values - +1/2 or – 1/2
Electron ConfigurationsElectron Configurations
Describes the arrangement of electrons in Describes the arrangement of electrons in an atom.an atom.
Each electron in an atom has a set of 4 q. Each electron in an atom has a set of 4 q. n. n.
Rules that govern orbital fillingRules that govern orbital filling
1.1. Aufbau principle – electrons enter orbitals of Aufbau principle – electrons enter orbitals of lowest energy first.lowest energy first.
2.2. Hund’s rule – when electrons enter orbitals of Hund’s rule – when electrons enter orbitals of equal energy, equal energy, degenerate orbitalsdegenerate orbitals, each orbital , each orbital receives one electron, with parallel spins before receives one electron, with parallel spins before any receive two.any receive two.
3.3. Pauli exclusion principle – no two electrons in Pauli exclusion principle – no two electrons in an atom can have the same set of 4 q.n. They an atom can have the same set of 4 q.n. They must have opposite spins.must have opposite spins.
Order of orbital fillingOrder of orbital filling
1869: Dmitri Mendeleev1869: Dmitri Mendeleev
Russian chemistRussian chemist Arranged elements in Arranged elements in
tabular form so that tabular form so that elements with similar elements with similar properties were in the properties were in the same columnsame column
When listed in order When listed in order by mass, elements by mass, elements generally repeat generally repeat properties in groups properties in groups of 8 (Law of Octaves)of 8 (Law of Octaves)
The First Periodic TableThe First Periodic Table Most tables at the time listed elements by massMost tables at the time listed elements by mass Mendeleev also arranged elements by mass, but Mendeleev also arranged elements by mass, but
left several “holes” in his table and occasionally left several “holes” in his table and occasionally reversed the order of elements to fit the reversed the order of elements to fit the properties of others in that columnproperties of others in that column
The “holes” were later filled in with newly The “holes” were later filled in with newly discovered elements that had the properties discovered elements that had the properties predicted by Mendeleev’s table.predicted by Mendeleev’s table.
The reason for the reversal of elements was The reason for the reversal of elements was explained later by Henry Moseley, who noted explained later by Henry Moseley, who noted that the elements were in order by atomic that the elements were in order by atomic number (number of protons) rather than by massnumber (number of protons) rather than by mass
Introducing the ElementsIntroducing the Elements
The Element SongThe Element Song
Periods- Horizontal Rows (7 periods)Periods- Horizontal Rows (7 periods)
Groups/families – Verticle columns (18 Groups/families – Verticle columns (18 groups)groups)
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