History of Atomic TheoryHistory of Atomic Theory

Scientists B.C.Scientists B.C.

Democritus Aristotle

384-322 BC

Believed matter is continuous

400 BC

Coined the term “atom”

Dalton’s Atomic Theory Dalton’s Atomic Theory -early 1800’s--early 1800’s-

All matter is composed of tiny, indivisible All matter is composed of tiny, indivisible particles called atoms.particles called atoms.

Atoms of the same element have the same Atoms of the same element have the same properties (mass, size, etc.). properties (mass, size, etc.).

In a chemical reaction, matter cannot be In a chemical reaction, matter cannot be created or destroyed. (Law of Conservation of created or destroyed. (Law of Conservation of Mass)Mass)

Compounds always contain elements in the Compounds always contain elements in the same ratio by mass (Law of Definite same ratio by mass (Law of Definite Proportions)Proportions)

Atomic sizeAtomic size

A penny contains 2.4 x 10A penny contains 2.4 x 102222 atoms atoms Radius of one atom is around 2 x 10Radius of one atom is around 2 x 10--

1010mm

or .2 nmor .2 nm Scanning tunneling microscope can Scanning tunneling microscope can

generate images of individual atoms.generate images of individual atoms.

Thomson’s Cathode Ray TubeThomson’s Cathode Ray Tube -late 1800’s--late 1800’s-

Showed that electrons are negatively charged particles.Showed that electrons are negatively charged particles.Image from Addison Wesley ChemistryImage from Addison Wesley Chemistry

Thomson’s “plum pudding” Thomson’s “plum pudding” model model

Rutherford’s gold foil exp.Rutherford’s gold foil exp.-early 1900’s--early 1900’s-

Conclusion:

Most of an atom’s volume is empty space.

Rutherford’s “planetary” Rutherford’s “planetary” modelmodel

5 Models of the Atom

(a) Dalton's model (1803)

(b) Thomson's model (1897)

(c) Rutherford's model (1909)

(d) Bohr's model (1913) © Prentice-Hall, Inc.

(e) Electron-cloud model (present)

SubatomSubatomic ic particleparticle

chargchargee

locatiolocationn

massmass Other featureOther feature

protonproton ++ NucleuNucleuss

1 1 amuamu

Defines the Defines the elementelement

-atomic no.-atomic no.

neutronneutron 00 NucleuNucleuss

1 1 amuamu

Change no. to Change no. to form isotopes form isotopes

electronelectron -- ElectroElectron n

cloudcloud

~~0 0 atom’s atom’s volumevolume

-dictates -dictates reactivityreactivity

Nuclear ForcesNuclear Forces

Short-range forces that hold the Short-range forces that hold the nuclear particles together.nuclear particles together.

IsotopesIsotopes

Atoms of the same Atoms of the same element that differ in element that differ in massmass

Atomic no.=# protonsAtomic no.=# protons #protons=#electrons#protons=#electrons Mass no.=#protons + Mass no.=#protons +

# neutrons (nucleons)# neutrons (nucleons)

Num f neutrons

Isotopes of HydrogenIsotopes of Hydrogen

NuclideNuclide ProtonsProtons NeutronsNeutrons MassMass

NumberNumber

ProtiumProtium 11 00 11

DeuteriuDeuteriumm

11 11 22

tritiumtritium 11 22 33

Isotopes can be written two Isotopes can be written two waysways

Br

Pb

Ag

8035

20782

10847

or bromine-80

ElectronsElectrons

Found in an electron cloud outside of Found in an electron cloud outside of the nucleus (but not in paths like the the nucleus (but not in paths like the planets)planets)

11stst energy level holds 2 electrons energy level holds 2 electrons 22ndnd energy level holds up to 8 energy level holds up to 8 33rdrd energy level holds up to 18 energy level holds up to 18

Periodic TablePeriodic Table

Arranged by increasing atomic Arranged by increasing atomic numbernumber

Rows are called periodsRows are called periods Columns are called groupsColumns are called groups

Average Atomic MassAverage Atomic Mass

An element’s atomic mass is the An element’s atomic mass is the weighted average of its naturally weighted average of its naturally occurring isotopes.occurring isotopes.

Average Atomic MassAverage Atomic Mass

MultiplyMultiply the the massmass of each isotope by of each isotope by its its abundanceabundance to get the weighted to get the weighted average.average.

(% x mass)+ (% x mass) + . . . (% x mass)+ (% x mass) + . . .

100100

Ex.: Boron is 80.20% boron-11 (atomic Ex.: Boron is 80.20% boron-11 (atomic mass 11.01 amu) and 19.80% boron-10 mass 11.01 amu) and 19.80% boron-10 (atomic mass 10.01 amu). (atomic mass 10.01 amu).

What is the average atomic mass of What is the average atomic mass of boron? boron?

((11.01amu)(80.20) + (10.01amu)(19.80) =11.01amu)(80.20) + (10.01amu)(19.80) =

100100

=10.81 amu=10.81 amu

Sample ProblemSample Problem

ex.: Neon has 2 isotopes. ex.: Neon has 2 isotopes. Neon-20 has a mass of 19.992amu and Neon-20 has a mass of 19.992amu and neon-22 has a mass of 21.991amu. neon-22 has a mass of 21.991amu. In an average sample of neon atoms, 90% In an average sample of neon atoms, 90%

will be neon-20 and 10% will be neon-22. will be neon-20 and 10% will be neon-22. Calculate the average atomic mass. Calculate the average atomic mass. (90 x 19.992amu)+(10 x 21.991amu)(90 x 19.992amu)+(10 x 21.991amu) 100100= 20.192 amu= 20.192 amu