Chapter 2Chapter 2
Atoms and their structureAtoms and their structure
History of the atomHistory of the atom
Original idea of the atomOriginal idea of the atom: :
Ancient Greece (400 B.C.)Ancient Greece (400 B.C.) Greek philosophers Democritus Greek philosophers Democritus
and Leucippusand Leucippus Empty space and small particlesEmpty space and small particles
The Beginning of Atomic TheoryThe Beginning of Atomic Theory
• AtomosAtomos - not to be cut, indivisible - not to be cut, indivisible• AtomsAtoms - smallest particles of matter, - smallest particles of matter, different types of atoms exist for every type different types of atoms exist for every type of matterof matter
LavoisierLavoisier French chemist French chemist
(1743-1794) (1743-1794) Observed chemical Observed chemical
changes in sealed changes in sealed containerscontainers
Mass of reactants = Mass of productsMass of reactants = Mass of products
Conservation of MatterConservation of Matter
Who’s Next?Who’s Next? John DaltonJohn Dalton: A teacher in : A teacher in
Late 1700’s EnglandLate 1700’s England Summarized results of his Summarized results of his
experiments and those of experiments and those of other’s in:other’s in:
Dalton’s Atomic TheoryDalton’s Atomic Theory Combined ideas of Combined ideas of
elements with that of elements with that of atomsatoms
Dalton’s Atomic TheoryDalton’s Atomic Theory1. All 1. All mattermatter is made of tiny particles called is made of tiny particles called
atomsatoms..
2. Atoms are indestructible and cannot be 2. Atoms are indestructible and cannot be divided into smaller parts.divided into smaller parts.
3. Atoms of the same element are identical, 3. Atoms of the same element are identical, those of different atoms are different.those of different atoms are different.
Theory or Law?Theory or Law? A scientific A scientific law law is a fact of natureis a fact of nature
–Ex: the sun rises, conservation Ex: the sun rises, conservation of matterof matter
AA theory theory explains the law explains the law
–Dalton’s theory explains Dalton’s theory explains whywhy matter in conservedmatter in conserved
Parts of AtomsParts of Atoms J. J. ThomsonJ. J. Thomson - English physicist, 1897 - English physicist, 1897 Cathode ray tube experiment.Cathode ray tube experiment. Vacuum tube - all air has been pumped Vacuum tube - all air has been pumped
out.out.
Thomson’s ExperimentThomson’s Experiment
Voltage source
+-
Vacuum tube
Metal Disks
Thomson’s ExperimentThomson’s Experiment
Voltage source
+-
Thomson’s ExperimentThomson’s Experiment
Voltage source
+-
Thomson’s ExperimentThomson’s Experiment
Voltage source
+-
Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end
Thomson’s ExperimentThomson’s Experiment
Voltage source
+-
Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end
Thomson’s ExperimentThomson’s Experiment
Voltage source
+-
Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end
Thomson’s ExperimentThomson’s Experiment
Voltage source
+-
Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end
Thomson’s ExperimentThomson’s Experiment
Voltage source
+-
Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric field By adding an electric field
Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric field By adding an electric field
+
-
Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric field By adding an electric field
+
-
Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric field By adding an electric field
+
-
Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric field By adding an electric field
+
-
Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric field By adding an electric field
+
-
Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric field he found that the By adding an electric field he found that the moving pieces were negative moving pieces were negative
+
-
Thomson’s ModelThomson’s Model Found the Found the electronelectron Couldn’t find positive Couldn’t find positive
charge(for a while) charge(for a while) Said the atom was Said the atom was
like plum puddinglike plum pudding A bunch of positive A bunch of positive
stuff, with the stuff, with the electrons able to be electrons able to be removed removed
Other Subatomic ParticlesOther Subatomic Particles ProtonProton - positively charged particles - positively charged particles
– 1,840 times heavier than the electron1,840 times heavier than the electron NeutronNeutron - no charge, but the same - no charge, but the same
mass as a proton.mass as a proton. Where are the particles?Where are the particles?
Rutherford’s ExperimentRutherford’s Experiment Ernest RutherfordErnest Rutherford - English physicist, (1910) - English physicist, (1910) Used radioactivityUsed radioactivity Alpha particlesAlpha particles - positively charged He nuclei - positively charged He nuclei
given off by polonium given off by polonium Shot them at gold foil Shot them at gold foil
which can be made a which can be made a
few atoms thick few atoms thick
RadioactivityRadioactivity
Rutherford’s experimentRutherford’s experimentWhen the alpha particles hit a fluorescent When the alpha particles hit a fluorescent
screen, it glows.screen, it glows.
Americium 241, just over 1cm from the screen.
Lead block
Uranium
Gold Foil
Florescent Screen
He Expected…He Expected………The alpha particles to pass through The alpha particles to pass through
without changing direction very muchwithout changing direction very much Because…Because…
– The positive charges were spread out The positive charges were spread out evenly. Alone they were not enough evenly. Alone they were not enough to stop the alpha particlesto stop the alpha particles
What he expected
Because…
Because he thought the mass was evenly distributed in the atom
Because he thought the mass was evenly distributed in the atom
What he got
How Rutherford explained How Rutherford explained itit
+
Atom is mostly emptyAtom is mostly empty
Small, dense, positive Small, dense, positive particles at centerparticles at center
Alpha particles are Alpha particles are deflected by it if they deflected by it if they get closeget close enough enough (like repels like)(like repels like)
+
Subatomic ParticlesSubatomic Particles
Electron
Proton
Neutron
Name Symbol ChargeRelative mass
Actual mass (g)
e-
p+
n0
-1
+1
0
1/1840
1
1
9.11 x 10-28g
1.67 x 10-24g
1.67 x 10-24g
Structure of the AtomStructure of the AtomThere are two regions:There are two regions:1. The 1. The nucleusnucleus - protons and neutrons - protons and neutrons
- positive charge- positive charge- almost all the mass- almost all the mass
2. 2. Electron cloudElectron cloud - Most of the volume of an atom- Most of the volume of an atom- The region where the electron can - The region where the electron can
be foundbe found
Counting the PiecesCounting the Pieces Atomic Number Atomic Number = number of protons= number of protons
– # of protons determines kind of atom# of protons determines kind of atom
– the same as the number of electrons the same as the number of electrons in the in the neutralneutral atom atom
Mass Number = Mass Number = the number of protons the number of protons PLUS the number of neutronsPLUS the number of neutrons
– Includes all the things with massIncludes all the things with mass
IsotopesIsotopes Dalton was incorrect.Dalton was incorrect. Atoms of the same element can have Atoms of the same element can have
different numbers of neutrons!different numbers of neutrons!
– different mass numbersdifferent mass numbers
– called called isotopesisotopes
Periodic Table InfoPeriodic Table InfoContains the symbol of the element, the Contains the symbol of the element, the
atomic mass and the atomic numberatomic mass and the atomic number
– SymbolSymbol: letters in the middle: letters in the middle
– Atomic massAtomic mass: decimal number : decimal number (usually at bottom)(usually at bottom)
– Atomic numberAtomic number: whole number : whole number (usually at top) (usually at top)
Periodic Table InfoPeriodic Table InfoLabel the following diagram:Label the following diagram:
Atomic MassAtomic Mass How heavy is an atom of oxygen?How heavy is an atom of oxygen? There are different kinds of oxygen atoms.There are different kinds of oxygen atoms. Look at Look at Average Average atomic mass.atomic mass. Based on abundance of each element in Based on abundance of each element in
nature.nature.
Atomic MassAtomic Mass
Calculate the atomic mass of copper Calculate the atomic mass of copper if copper has two isotopes. if copper has two isotopes.
69.1% of Cu atoms have a mass of 69.1% of Cu atoms have a mass of 62.93 amu and the rest have a 62.93 amu and the rest have a mass of 64.93 amu.mass of 64.93 amu.
Bohr’s ModelBohr’s Model Why don’t electrons fall into the Why don’t electrons fall into the
nucleus?nucleus? Move like planets around the sun.Move like planets around the sun.
– Circular orbits at different levels.Circular orbits at different levels. Amounts of energy separate one Amounts of energy separate one
level from another.level from another.
Bohr’s ModelBohr’s Model
Nucleus
Electron
Orbit
Energy Levels
Bohr’s ModelBohr’s ModelIn
crea
sing
ene
rgy
Nucleus
First
Second
Third
Fourth
Fifth
} Further away Further away
from the nucleus from the nucleus means higher means higher energy.energy.
There is no “in There is no “in between” between” energyenergy
Energy Energy LevelsLevels