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