Chapter #3 Atoms: The Building Blocks of Matter Chapter #3 ATOMS: The Building Blocks of Matter.

Chapter #3

• Atoms: The Building Blocks of Matter

Chapter #3

ATOMS:

The Building Blocks of Matter

3-1 Early Atomic Theory

• Atoms are so small they cannot be observed directly. Scientists could use only experimental data to help describe the atom.

• Around 400 B.C., Democritus (a Greek philosopher) suggested that the world was made of two things - empty space and tiny particles called atoms.

• During the 1800's, a French Chemist (Antoine Lavoisier) discovered that chemical "changes" occurring in a closed system - the mass after a chemical change equaled the mass before the chemical change.

• He proposed that, in ordinary chemical reactions, matter can be changed in many ways, but it cannot be created or destroyed (Law of Conservation of Mass).

• Work by another French Chemist, Joseph Proust, had observed that specific substances always contain elements in the same ratio by mass (Law of Definite Proportions.)

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Foundations of Atomic Theory

• Law of Definite Proportions: The elements composing a compound are always found in the same ratio by mass.

• Law of Multiple Proportions: The masses of one element that combine with a fixed amount of another element to form more than one compound are in the ratio of small whole numbers. Example CO, CO2

For example: Oxygen can combine with Carbon to form Carbon Monoxide, CO, or form Carbon Dioxide,

CO2.

• Dalton was the founder of Atomic Theory.

Compound Mass of C in Sample

Mass of O in Sample

Ratio of O masses combined with constant mass

Carbon Monoxide, CO

12 g 16 g 1:1Carbon Dioxide, CO2 12g 32 g 2:1

Dalton’s Atomic Theory

1. All matter is composed of extremely small particles called atoms.

2. Atoms of a given element are identical in size, mass and other properties; atoms of different elements differ in size, mass, and other properties.

3. Atoms cannot be subdivided, created, or destroyed.

4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

5. In chemical reactions, atoms are combined, separated, or rearranged.

Modern Atomic Theory• Element have a characteristic average

mass which is unique to that element.• Atoms cannot be subdivided, created, or

destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!

• All matter is composed of atoms• Atoms of any one element differ in

properties from atoms of another element

Section 3-2• Atom- the smallest particle of an

element that retains the chemical properties of that element.

• Nucleus- is the positively charges, dense central portion of the atom that contains nearly all of its mass but takes up only an insignificant fraction of its volume.

Subatomic ParticlesElectrons e- Negatively charged particles

found around the nucleus in shells, energy level or electron clouds

Protons p+ Positively charged particles. Found in the nucleus

Neutrons N No charge. Found in the nucleus.

The Atomic ScaleThe Atomic Scale• Most of the mass of the atom is

in the nucleus (protons and neutrons)

• Electrons are found outside of the nucleus (the electron cloud). e- have very tiny mass.

• Most of the volume of the atom is empty space

Drawing atoms• In the nucleus 1. Symbol2. # of p+3. # of N• Outside the nucleus in the

energy shells/level1. electrons

Famous ScientistScientist Experiment

NameWhat it proved

JJ Thomson Cathode Ray Electrons have a negative chare

Robert Millikan Oil Drop Mass of an electron

Ernest Rutherford Metal Foil or gold foil

Nucleus contains positive charge

Discovery of the Discovery of the ElectronElectron

• In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle.

• Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

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• Thomson was awarded the Nobel Prize in 1906 for his "discovery" of the first sub-atomic particle; the electron. This discovery strongly implied that Dalton was wrong and that the atom was not the smallest particle of matter. It looked as if the atom could be broken down into even smaller pieces, and to Thomson these smaller pieces were his negatively charged electrons.

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Conclusions from the Conclusions from the Study of the ElectronStudy of the Electron

• Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.

• Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons

• Electrons have so little mass that atoms must contain other particles that account for most of the mass

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Thomson’s Atomic Thomson’s Atomic ModelModel

• Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

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Rutherford’s Gold Foil Rutherford’s Gold Foil ExperimentExperiment

• Alpha particles are helium nuclei • Particles were fired at a thin sheet of

gold foil • Particle hits on the detecting screen

(film) are recorded

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Rutherford’s Rutherford’s FindingsFindings

• Most of the particles passed right through

• A few particles were deflected

• VERY FEW were greatly deflected

• Conclusions:• The nucleus is small • The nucleus is dense • The nucleus is

positively charged

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Section 3-3• Atomic number (Z)

of an element is the number of protons in the nucleus of each atom of that element.

• The number of protons = the number of electrons

6 CCarbon 12.011

• Mass NumberMass Number• Mass number is the number of

protons and neutrons in the nucleus of an isotope.

• Mass # = p+ + n• SOooo the number of Neutrons

= • n= Mass # - p+

Isotopes• Isotopes are

atoms of the same element that have different masses. (number of neutrons)

Mass #

Atomic # Symbol

Nuclear Symbols

235

92

Mass number (p+ + n)

Atomic number (# of p+)

Mass number

(p+ + n)Element symbol

U235

92

Hyphen Notation

Sodium-23

(23 is the mass #) Sooo… 23- 11 (atomic #) = 12 for

the # of neutrons.

11 is the # of protons and electrons.

Isotopes of HIsotopes p

+e- n

Hydrogen–1 (protium)

1 1 0

Hydrogen-2 (deuterium)

1 1 1

Hydrogen-3 (tritium)

1 1 2

The MoleThe Mole

• 1 dozen =12• 1 gross = 144• 1 ream = 500• 1 mole = 6.022 x 1023

• There are exactly 12 grams of carbon-12 in one mole of carbon-12.

Calculations:Calculations:Converting moles to Converting moles to

gramsgrams• Given # of mole X ? g (look at periodic table)= g

of 1 mole

• How many grams of lithium are in 3.50 moles of lithium?

• 3.50 mole X 6.941 g = 24.29 g Li 1 mol

Calculations:Calculations:Converting grams to Converting grams to

molesmoles• Given # of g X 1 mol = mol of g (look at periodic table)

• How many moles of lithium are in 18.2 grams of lithium?

• 18.2 g X 1 mol Li = 2.622 mol Li 6.941 g

Avogadro’s NumberAvogadro’s Number• Is the number of particles in exactly

one mole of a pure substance.• 6.022 x 1023 is called “Avogadro’s

Number” in honor of the Italian chemist Amadeo Avogadro (1776-1855).

I didn’t discover it. Its just named after me!

Calculations:Calculations:Converting Moles to Converting Moles to

ParticlesParticles• Given # of mol x 6.022 x 1023 part= atoms

1 mol• How many atoms/particles/molecules

of lithium are in 3.50 moles of lithium?• 3.50 mole X 6.022 x 1023 = 2.11 x 1024 atoms of Li

1 mol

Calculations:Calculations:Converting Particles to Converting Particles to

MolesMoles• Given # of particles x 1 mole = mol 6.022 x 1023

• How many moles of lithium are there in 1.2044 x 1024 particles of Li?

• 1.2044 x 1024 part x 1 mole = 2.0 mol Li 6.022 x 1023 part

Calculations:Calculations:Converting grams to Converting grams to

particlesparticles• Given # of grams x 1 mol x 6.022 x 1023=

particles ? g 1 mol

• How many atoms/particles/molecules of lithium are in 18.2 g of lithium?

• 18.2 g x 1 mol x 6.022 x 1023 = 1.58 x 1024

particle Li 6.941g 1 mol

Calculations:Calculations:Converting particles to Converting particles to

gramsgrams• Given # of particles x 1 mol x ? g =

g

6.022 x 1023 1 mol• How many grams are there in 8.02 x 1025

particles of lithium?

Work Cited• “JJ Thomson”. Photo. July 28, 2006.

http://www.sciencemuseum.org.uk/online/electron/section2/recording.asp

• Cathode Ray Image and JJ Thomson Model. Image. July 28, 2006. http://www.brooklyn.cuny.edu/bc/ahp/LAD/C3/C3_Electrons.html

• “Gold Foil Experiment”. Image. July 28, 2006. http://www.avon-chemistry.com/atom_lecture.html

• “Rutherford”. Photo. July 28,2006. http://www.anthroposophie.net/bibliothek/nawi/physik/rutherford/bib_rutherford.htm

• “Mole”. Photo. Aug 8, 2006. http://www.mwt.net/~bionorse/chemistry.htm

• “Hydrogen Isotopes”. Picture. August 4, 2006. www.sr.bham.ac.uk/xmm/atom.html

• “Amedeo Avogadro”. avagadroc.jpg August 4, 2006. poohbah.cem.msu.edu/Portraits/PortraitsH...

• “Uranium symbol”. Picture. August 4, 2006. www.webelements.com/webelements/scholar/...

• Holt, Rinehart and Winston. Modern Chemistry. Harcourt Brace & Company. 1999.

• “Atom Comic Cover”. Photo. Aug. 12, 2006.

http://home.cfl.rr.com/fradford/Atom/Atom20.jpg