Introduction to AtomsChapter 11Material on Midterm

Where is it?• How do we get information about items that we cannot see?

Section 1: Development of the Atomic Theory

•Objectives• Describe some of the experiments that

led to the current atomic theory• Compare the different models of the

atom• Explain how the atomic theory has

changed as scientists have discovered new information about the atom

History of the Atomic Theory

Section 1: Development of the Atomic Theory• The Beginning of Atomic Theory• Democritus• The particle that was so small

it could not be cut again• The atoms Is the smallest particle

into which an element can be divided and still be the same substance• Means not able to be divided

Section 1: Development of the Atomic Theory• The Beginning of

Atomic Theory• Democritus• Challenged by

Aristotle• Most believed him

(even though he was wrong!)

Section 1: Development of the Atomic Theory• Dalton’s Atomic Theory Based on

Experiments• Figured out that compounds are

made of elements that are made of single atoms

Section 1: Development of the Atomic Theory

• Dalton’s Atomic Theory Based on Experiments• Had three parts

1. All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed

2. Atoms of the same element are exactly alike, and atoms of different elements are different

3. Atoms join with other atoms to make new substances

Section 1: Development of the Atomic Theory

Section 1: Development of the Atomic Theory• Thomson’s Discovery of

Electrons• Thomson used a device

called a cathode ray tube• Determined that there are

negative particles inside of an atom

Section 1: Development of the Atomic Theory

Cathode Ray Tube Experiment

Section 1: Development of the Atomic Theory

• Electrons are subatomic particles that have a negative charge• Thomson created the Plum pudding model to

show where the electrons go

Section 1: Development of the Atomic Theory

• Rutherford’s Atomic “Shooting Gallery”• Rutherford wanted to test Thomson’s idea• Had a sheet of gold foil (SUPER THIN)• Had a detector on the outside• Shot a beam of positive particles at it

• What he expected: all of the particles would just go right through

Section 1: Development of the Atomic Theory

Section 1: Development of the Atomic Theory• Rutherford’s Atomic “Shooting Gallery”

• What really happened• Most went straight through (like he thought)• Some got bent at an angle• A few bounced right back!

“It was quite the most incredible event that has ever happened to me in my life. It was almost as if you fired a fifteen-inch shell into a piece of tissue paper and it came right back to you”

Section 1: Development of the Atomic Theory

• Where are the Electrons?• Plum-pudding model was wrong!• Rutherford realized that atoms were

mostly empty space• At the center is the nucleus• The nucleus is an atom’s central region, which is made up

of protons and neutrons

• Electrons move around it

Section 1: Development of the Atomic Theory• Bohr’s Electron Levels• Bohr figured out electrons

move around in paths• Like rungs on a ladder

Section 1: Development of the Atomic Theory

•Modern Atomic Theory• …it’s complicated• Schrodinger & Heisenberg and others helped• Electrons do not move in paths that are very

clear• We cannot predict where electrons are exactly• We’re likely to find them in electron clouds,

which are regions around the nucleus where electrons are likely to be found

Section 1: Development of the Atomic Theory• Section Summary• Please answer the objectives on your objective

summary sheet1. Describe some of the experiments that led to the

current atomic theory

2. Compare the different models of the atom

3. Explain how the atomic theory has changed as scientists have discovered new information about the atom

History of the Atomic Theory

Section 2: The Atom

• Objectives• Describe the size of an atom• Name and state information about the parts of

an atom• Describe the relationship between number of

protons and neutrons and atomic number• State how isotopes differ• Describe and calculate atomic masses• Describe the forces within an atom

Section 2: The Atom• How small is an atom?

Contains 2 x 1022

20,000,000,000,000,000,000,000

20 thousand billion billion atoms

Section 2: The Atom

•1 aluminum atom has a diameter of 0.00000003 cm

•How many atoms thick was our foil from chapter 1?

Section 2: The Atom• What is an Atom made of?• 3 major parts• The proton is the subatomic particle that has a positive

charge and that is found in the nucleus of an atom• The neutron is the subatomic particle that has no charge

and that is found in the nucleus of an atom• The electron is the subatomic particle that has a negative

charge and that is found in the electron cloud surrounding the atom

Section 2: The Atom

Section 2: The Atom• How big are these pieces• VERY SMALL!

• Scientists came up with a special term for them• The atomic mass unit is a unit of mass the describes the

mass of an atom or molecule• Proton: ~1 amu• Neutron: ~1amu• Electron: ~1/8000 amu

Section 2: The Atom

• An atom is normally neutral• Same number of protons and electrons

• If an atom changes this ratio, it becomes an ion

Section 2: The Atom

• How do Atoms of Different Elements Differ?• The atomic number is the number of protons in the

nucleus of an atom; the atomic number is the same for all atoms of an element

• An isotope is an atom that has the same number of protons/atomic number as other atoms of the same element but has a different number of neutrons (and this a different atomic mass)

• The mass number is the sum of the protons and neutrons in an atom

Section 2: The Atom

Subatomic particle Location Charge Key feature

Proton Nucleus Positive Atomic identity

Neutron Nucleus Neutral Isotopes

Electron Electron cloud Negative Charge

Section 2: The Atom

• Let’s do some practice!

• An atom has a mass number of 7. It’s atomic number is 3. What is…• The number of protons?• The number of neutrons?• The number of electrons?• The name of the element?

Section 2: The Atom• Let’s do some practice!

•An atom has a mass number of 32. It’s atomic number is 16. What is…• The number of protons?• The number of neutrons?• The number of electrons?• The name of the element?

Section 2: The Atom

• Let’s do some practice!

•An atom has a mass number of 1. It’s atomic number is 1. What is…• The number of protons?• The number of neutrons?• The number of electrons?• The name of the element?

Section 2: The Atom

Boron-10

• 5 protons• 5 neutrons• 5 electrons

• Atomic number:• Mass number:

Boron-11

• 5 protons• 6 neutrons• 5 electrons

• Atomic number:• Mass number:

• Naming isotopes• Remember, they are the SAME ELEMENT but differ in

neutron number

Section 2: The Atom• Calculating the mass of an element• The atomic mass is not really a whole number• The atomic mass is the mass of an atom expressed in

atomic mass units

• Hydrogen has an atomic mass of 1.000797• Where does this number come from?

Section 2: The Atom

•Calculating the Mass of an Element• The atomic mass is a weighted average

of the masses of all naturally occurring isotopes of the element• Takes into account that not all isotopes

are found equally on earth

Section 2: The Atom• Calculating the mass of an element• There are two different isotopes of copper• Copper-63 (69% of earth’s copper is this type)• Copper-65 (31% of earth’s copper is this type

• Steps1. Convert percentages into decimals2. Multiple decimal value by atomic mass3. Add these amounts together to find the mass

Section 2: The Atom

• Calculating the mass of an element• There are two different isotopes of copper• Copper-63 (69% of earth’s copper is this type)• Copper-65 (31% of earth’s copper is this type

• Steps1. Convert percentages into decimals• 69% 0.69• 31% 0.31

Section 2: The Atom• Calculating the mass of an element• There are two different isotopes of copper

• Copper-63 (69% of earth’s copper is this type)• Copper-65 (31% of earth’s copper is this type

• Steps2. Multiple decimal value by atomic mass

• (0.69 x 63) = 43.47• (0.31 x 65) = 20.15

Section 2: The Atom• Calculating the mass of an element• There are two different isotopes of copper

• Copper-63 (69% of earth’s copper is this type)• Copper-65 (31% of earth’s copper is this type

• Steps3. Add these amounts together to find the mass

• (43.47 + 20.15) = 63.62 amu

Section 2: The Atom• You try!• There are two different isotopes of chlorine

• Chlorine-35 (76% of earth’s chlorine is this type)• Chlorine-37 (24% of earth’s chlorine is this type

• Steps1. Convert percentages into decimals2. Multiple decimal value by atomic mass3. Add these amounts together to find the mass

Section 2: The Atom

• Forces in Atoms• Four basic forces that are working even

inside teeny atoms!• Gravitation Force• Electromagnetic Force• Strong Force•Weak Force

Section 2: The Atom• Section Summary• Please answer the objectives on your objective summary sheet1. Describe the size of an atom

2. Name and state information about the parts of an atom

3. Describe the relationship between number of protons and neutrons and atomic number

4. State how isotopes differ

5. Describe and calculate atomic masses

6. Describe the forces within an atom