Defining the Atom

Defining the Atom

Democritus

460B.C. – 570 B.C.Termed the name atom from the Greek word

“atomos”. Philosopher

If you break down an object you will eventually reach a point where you can not break it down any further. He called the smallest unit “atomos”.

John Dalton

Father of the “Atomic Theory”

1.All elements are made up of atoms.2.Atoms of an element are identical.3.Atoms of different elements combine together in

whole number ratios (you will never see a ½ or 1/3, etc.)

– i.e. H2O, CO2, C6H12O6, etc.

4.In chemical reactions, atoms are not changed, they are only rearranged. If you change atoms, it is nuclear and explosive.

Structure of Nuclear Atom

Change in Dalton’s Atomic Theory is that atoms are divisible into subatomic particles: Electrons (e-) Protons (p+) Neutrons (n0)

J.J. Thompson

Found electron by cathode ray tube.

Robert Millikan

Discovered the mass of the electron through an oil drop test.

Mass of electron = 9.11 x 10-28 g or 0.0000000000000000000000000000911 g

Too small ---- Insignificant ----Basically zero!

Food for Thought

J.J. Thompson discovered there was a negative particle called the electron. Robert Millikan discovered this negative particle has a very very small mass.

Thought: If there is a negative, there must be a positive. If electrons are so small (relatively no mass), what in

the atom makes up its mass?

Eugene Goldstein

Discovered the positively charged proton and its mass of 1 amu.

Amu = atomic mass unit

James Chadwick

Discovered the neutron with no charge and its mass of 1 amu.

Plum Pudding Model

Thompson’s Atomic ModelProtons and Electrons are randomly

dispersed throughout.

Ernest Rutherford

Gold Foil ExperimentShot positive helium

atoms through a thin gold foil. Lots of the helium cations went through and only a few deflected back.

Found there was a concentration of positive (protons). He called this concentrated spot the nucleus.

Conclusions

1. Nucleus is small.2. Nucleus is dense.3. Nucleus is positively charged.4. Atom is mainly empty space.Thus, we no longer have the Plum Pudding

Model, instead it looks like this:

The Atom

Objectives

The Atom

Three Subatomic Particles

Neutron

Proton

Electron

Neutron

Has a mass of 1Has no charge (0)Found in the nucleus

Proton

Has a mass of 1Is positively charged (+1)Determines the identity of the atomFound in the nucleus

Electron

Has NO MassIs negatively charged (-1)Found on the outside of the nucleusAtoms gain and lose electrons

Element

Elements are made of atoms with the same number of protons.

There are many elements identified and scientists have placed them on the periodic table.

How do we read the Periodic Table?

Atomic Number Number of Protons

Atomic Mass Number of Protons and Neutrons

How do we read the Periodic Table?

How do we figure out the number of neutrons?

Take the atomic mass and subtract the atomic number.

Mass (neutrons + protons) – Atomic Number (protons)

Let’s Try Together

Atomic Weight – Atomic Number

Atomic Weight = 12.01 ~ 12Atomic Number = 612 - 6 = 6 Neutrons

Now You Try!

What is the atomic mass?

137.327~137What is the atomic #?

56How many neutrons

does Barium have?137 – 56 = 81 Neutrons

Let’s Reassess Our Knowledge

What does the atomic number tell us? Number of protons.

What does the atomic mass tell us? Number of protons and neutrons.

Which subatomic particle determines the identity of the element? Proton

Which subatomic particles have mass? Neutrons and Protons

Which subatomic particle do atoms gain and lose most often? Electrons

Ions

Objectives

Ions

An ion is a charged atom that is formed when an atom gains or loses an electron. Anion

A negatively charged atom An atom gains an electron It is gaining negatives, so it becomes negative

Cation A positively charged atom An atom loses an electron It is losing negatives, so it becomes positively charged

Identifying the Type of Ions

Ca+2

O-2

U6+

Sn4+

N-3

Cation

Anion

Cation

Cation

Anion

Lost Electrons

Gained Electrons

Lost Electrons

Lost Electrons

Gained Electrons

2

2

6

4

3

Determining the Number of Electrons

Ion Protons Gain or Lose

Electrons

Electrons Neutrons

Ca+2 20 Lose 18 20

O-2 8 Gain 10 8

U6+ 92 Lose 86 146

Sn4+ 50 Lose 46 69

N-3 7 Gain 10 7

Isotopes

Objectives

Isotopes

Atoms with the same number of protons, but a different number of neutrons.

Same element with a different atomic mass.

Carbon Isotopes

Determining the Number of Neutrons

Remember: To find the number of neutrons, you must take the atomic mass and subtract the atomic number.

Another way to write the element: Element Symbol – Atomic Mass C - 12

Determine the Neutrons in the Isotopes: Li-6 Li-7

6-3 = 3 Neutrons

7-3 = 4 Neutrons

Other Ways to Write

Carbon-12Carbon-13Carbon-14

C C C

12

14

13

Atomic Mass

6

6

6

Atomic NumberC

Average Atomic Mass

Objectives

Average Atomic Mass

Why does the mass have numbers after the decimal?

Elements contain atoms with different masses.

Same number of protons, but different number of neutrons (isotopes of the same element).

What is Average Atomic Mass?

Average Atomic mass Average of all atoms with the same number of

protons. Thus, it is the average of isotopes for that element.

Abundance Amount of that isotope in nature. Displayed in percentage.

How Do We Determine the Average Atomic Mass?

1. Write the abundance and the corresponding isotope mass.

2. Rewrite the percent abundance as a decimal by moving the decimal two places to the left.

3. Multiply abundance (decimal) of that isotope by the mass.

4. Repeat step 1-3 for all isotopes.5. Add all the numbers together.

Lets Try Together

Calculate the average atomic mass of iron if its abundance in nature is 15% iron-55 and 85% iron-56.

15% iron-55

85% iron-56

0.15

0.85 0.85 x 56

0.15 x 55 8.25

47.655.85 amu8.25 + 47.6

1. Write the percent abundance and corresponding isotope mass.

2. Rewrite the percent abundance as a decimal.

3. Multiply abundance (decimal) by the isotope mass.

4. Add the numbers together.

In-Class Practice #2

What is the average atomic mass of silicon if 92.21% of its atoms have a mass of 27.977 amu, 4.07% have a mass of 28.976 amu, and 3.09% have a mass of 29.974 amu?

92.21% Si-27.977

4.07% Si-28.976

3.09% Si-29.974

.9221

.0407

.0309

.9221 x 27.977

.0407 x 28.976

.0309 x 29.974

25.7975917

1.1793232

0.9261966

25.7975917 1.1793232+ 0.9261966 27.903 amu

1. Write the percent abundance and corresponding isotope mass.

2. Rewrite the percent abundance as a decimal.

3. Multiply abundance (decimal) by the isotope mass.

4. Add the numbers together.


Calculate the average atomic mass for neon if its abundance in nature is 90.5% neon-20 (19.922 amu), 0.3% neon-21 (20.994 amu), and 9.2% neon-22 (21.991 amu).

90.5% 19.922

9.2% 21.991

0.3% 20.994

0.905

0.003

0.092

0.905 x 19.922

0.003 x 20.994

0.092 x 21.991

18.02941

0.062982

2.023172

18.02941 0.062982+ 2.023172 20.116 amu


Calculate the average atomic mass of chromium.

4.35% 49.946

2.35% 53.939

83.8% 51.941

9.5% 52.941

x 49.946

x 51.941

x 52.941

x 53.939

0.0435

0.838

0.095

0.0235

2.172651

43.526558

5.029395

1.2675665

51.996 amu

Now You Try on Your Own!

Independent Practice On Average Atomic Mass

Nuclear Chemistry

Objectives

1. Define nuclear chemistry.2. Describe the two forces in the nucleus.3. Explain why nuclear reactions occur.4. Name five types of nuclear decay.

Nuclear Chemistry

Nuclear Chemistry Study of changes in structure of nuclei and

subsequent changes in chemistry.

When a nucleus spontaneously changes it structure and emits radiation, we call this radioactive nuclei.

What is in the nucleus?

Protons and Neutrons

Nuclear Versus Chemical Reactions

Differences between nuclear and chemical reactions. Involves the nucleus and not electrons Much larger release in energy in nuclear reaction. Nuclear reaction produces different elements. Rate of nuclear reaction not dependent upon the

chemical environment.

Nucleus

Nucleus Has two nucleons, protons and neutrons. Protons are positively charged. Neutrons are neutral or have no charge. The overall charge of the nucleus is positive.

But, what holds these nucleons (subatomic particles in the nucleus) together when there are so many positively charged particles in a small, dense space.

Wouldn’t they repel each other and fly apart?

Two Types of Forces

1. Electrostatic Force2. Strong Force

Electrostatic Force

Force that causes oppositely charged particles to attract/repel. Any element with more than 1 proton will have

electrostatic repulsion between the protons.

Strong Force

The force between the nucleons (protons and neutrons).

Keeps the nucleus from flying apartThe neutrons increase the strong force with

out increasing electrostatic repulsion between nucleons (the protons).

Neutron-Proton Ratios

Neutrons play a key role stabilizing the nucleus.

Therefore, the ratio of neutrons to protons is an important factor.

Neutron- Proton Ratio

Smaller nuclei are more stable because they have a neutron-to-proton ratio close to 1:1.

Small Nuclei Atomic number is less

than or equal to 20 Z 20

Neutron- Proton Ratio

As nuclei get larger (more protons = more repulsion), it takes a greater number of neutrons to stabilize the nucleus.

Belt of Stability

The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.

Radioactivity

If a nuclei is unstable (off the line of stability) it will decay in order for it to become stable again.

• Radioactive decay • Process in which a nucleus

spontaneously disintegrates, giving off radiation.

Types of Radioactive Decay

Objectives

Five Types of Radioactive Decay

1. Alpha Decay2. Beta Decay3. Gamma Emission4. Positron Emission5. Electron Capture

Alpha Decay

Loss of an -particle (a helium nucleus)

He42

U23892 Th

23490 He

42+

Beta Decay

Loss of a -particle (a high energy electron)

0

−1 e0

−1or

I13153 Xe

13154 + e0

−1

Gamma Emission

Loss of a -ray (high-energy radiation that almost always accompanies the loss of a

nuclear particle)

00

Positron Emission

Loss of a positron (a particle that has the same mass as but opposite charge than an

electron)

e0

+1

C11

6 B11

5 + e0

+1

Electron Capture/ K-Capture

Addition of an electron to a proton in the nucleus As a result, a proton is transformed into a neutron.

p11 + e

0−1 n

10

Writing and Balancing Nuclear Reactions

Objectives

1. Describe the difference between decay and capture.

2. Identify the type of decay in a reaction.3. Balance nuclear reactions.4. Predict the type of decay an atom will

undergo.

Nuclei above this belt have too many neutrons.

They tend to decay by emitting beta particles.

Stable Nuclei

Nuclei below the belt have too many protons.

They tend to become more stable by positron emission or electron capture.

Stable Nuclei

There are no stable nuclei with an atomic number greater than 83.

These nuclei tend to decay by alpha emission.

How Do We Identify Type of Nuclear Decay

1. Look for decay particles.

2. Emitted particles (decay) are on the right hand side of the arrow.

3. Captured particles are on the Left hand side of the arrow.

He42 e

0+1

0−1 e0

−1or 00

Decay

Captured

Let’s Try Together!

____ +_____

____ +_____

____ +_____

____ +_____

____ + _______

He42

e0

−1

e0

+1

00

e 0

−1

Write Nuclear Reactions

Uranium-235 undergoes alpha decay

235

92U 4

2He + 231

90Th

Predicting the Type of Decay

Objectives

Predict Type of Decay

If Atomic Number is greater than 83• Nucleus is too big• Undergoes alpha decay

Too many protons and neutrons Best way to get rid of the nucleons is to get rid of an

alpha particle or helium nucleus. This will remove 2 protons and 2 neutrons and reduce

the size of the nucleus.


If Atomic mass is greater than the periodic table. • Too many neutrons

• Above belt of Stability• Undergoes Beta decay To get rid of the neutron, the atom will split it into a

proton and electron.

o1n +-1

1p + -10e

The proton stays in the nucleus increasing the atomic number (changing the element) and releases the electron.

This reduces the # of neutrons and increases # of protons.


Atomic mass less than on the periodic table. oToo few neutrons or too many protons

Below belt of StabilityoTherefore positron emission or electron capture

Examples: 261104Rf, 57

25Mn, 116C

The atom will capture an electron and combine it with a proton to form a neutron. This decreases the # of protons and increases the # of neutrons. (electron capture)

-11p + -1

0e o1n

The atom will release a positron (positive electron) to reduce the number of protons and increase the number of neutrons.

Balance Nuclear Reactions

10n + 235

92U → 2 10n + 97

40Zr+ 13752Te

Balance Nuclear Reactions

84218Po 2

4He + ________

99253Es + 2

4He 01n + _________

61142Pm + ______ 60

142Nd

Importance of Radiation

Objectives

How Does This Effect US?

Alpha particles are large and are stopped by a piece of paper.

Beta particles are smaller. An aluminum plate will stop them.

Gamma radiation needs 4 meters of lead to be stopped because they have no mass or charge. Tungsten and its alloys

can stop gamma with less mass.

Energy in Nuclear Reactions

There is a tremendous amount of energy stored in nuclei.

Einstein’s famous equation, E = mc2, relates directly to the calculation of this energy. Mass is converted into Energy.

In chemical reactions the amount of mass converted to energy is minimal.

However, these energies are many thousands of times greater in nuclear reactions.

Uses of Radiation

Radiation is used in Medicine Academics Industry (generating electricity) Applications in

Agriculture Archaeology (carbon dating) Space exploration Law enforcement Geology (including mining) and many others

Medical Uses

X-rays Move through skin, but not bone because it is denser.

The shadow of the bones is printed on a film.

Radiation Therapy (detection/ treatment) Reduce tumors and treat cancer

Nuclear medicine to diagnose clinical conditions Conditions in kidney, pancreas, thyroid, liver and

brain. Use radioactive iodine for thyroid

Industrial

X-rays used to disinfect medical equipment (bandages, syringes, and surgical instruments) and food to make it much longer until it spoils.

Ultraviolet is used in some homes to disinfect their water supply. Nonstick cookware is treated with gamma rays to prevent our food

from sticking. Our clothes are treated with radiation before wrinkle-free and soil-

releasing chemicals on it. Polyethylene shrinkwrap has been treated with radiation so that

it can be heated above its usual melting point and wrapped around the foods to provide an airtight protective covering.

Radiation can be used to control insect populations, thereby decreasing the use of dangerous pesticides

Nuclear PowerPlants

Use fission and fusion to create energy

Half-Life

Objectives

Radioactive Decay Series

Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation.

They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).

Radioactive Decay Series

Half-Life

Half-life (t½)– Time required for half the atoms of a

radioactive nuclide to decay.– Shorter half-life = less stable.

20 g

10 g5 g

2.5 g

after 1 half-life

Start after 2 half-lives

after 3 half-lives

Half-Life

1.00 mg

0.875 mg

0.500 mg

0.250 mg0.125 mg

8.02 days0.00 days 16.04 days 24.06 days

131 53 I

131 53 I

0.500 mg0.750 mg131

54 Xe

I131

53 Xe131

540

-1+ +

Half-Life

0 1 2 3 4Number of half-lives

Rad

iois

otop

e re

mai

ning

(%

)

100

50

25

12.5

Half-life of Radiation

Initial amountof radioisotope

t1/2

t1/2

t1/2

After 1 half-life

After 2 half-lives

After 3 half-lives

Half-Life PlotA

mou

nt o

f Io

dine

-131

(g)

20

15

10

5

0

40 48 560 8

1 half-life

16

2 half-lives

24

3 half-lives

32

4 half-lives etc…

Time (days)

Half-life of iodine-131 is 8 days

Half-Life of Isotopes

Isotope Half-Live Radiation emitted

Half-Life and Radiation of Some Naturally Occurring Radioisotopes

Carbon-14 5.73 x 103 years

Potassium-40 1.25 x 109 years

Thorium-234 24.1 days

Radon-222 3.8 days

Radium-226 1.6 x 103 years

Thorium-230 7.54 x 104 years

Uranium-235 7.0 x 108 years

Uranium-238 4.46 x 109 years

How Much Remains?

After oneone half-life, of the original atoms remain.

After twotwo half-lives, ½ x ½ = 1/(22) = of the original atoms remain.

After threethree half-life, ½ x ½ x ½ = 1/(23) = of the original atoms remain.

After fourfour half-life, ½ x ½ x ½ x ½ = 1/(24) = of the original atoms remain.

After fivefive half-life, ½ x ½ x ½ x ½ x ½ = 1/(25) = of the original atoms remain.

After sixsix half-life, ½ x ½ x ½ x ½ x ½ x ½ = 1/(26) = of the original atoms remain.

14

12

18

116

132

164

1 half-life 2 half-lives 3 half-lives

12

14 1

8 116 1

32 164 1

128

Accumulating“daughter”

isotopes

4 half-life 5 half-lives 6 half-lives 7 half-lives

Surviving“parent”isotopes

Beginning

Fraction remaining = 1/(2n).

The iodine-131 nuclide has a half-life of 8 days. If you originally have a 625-g sample, after 2 months you will have approximately?

a. 40 gb. 20 gc. 10 gd. 5 ge. less than 1 g

625 g 312 g 156 g 78 g 39 g 20 g 10 g 5 g 2.5 g1.25 g

0 d 8 d 16 d 24 d 32 d 40 d 48 d 56 d 64 d 72 d

0 1 2 3 4 5 6 7 8 9

Data Table: Half-life Decay~ Amount Time # Half-Life

Assume 30 days = 1 month

60 days8 days = 7.5 half-life(s)

A = Ao(1/2)n

A = amount remainingAo = original amountn = # of half-life(s)

N = (625 g)(1/2)7.5

N = 3.45 g

Let’s Practice Together!

The half-life of carbon-14 is 5730 years. If a sample originally contained 3.36 g of C-14, how much is present after 22,920 years?

0.21 g C-14

Gold-191 has a half-life of 12.4 hours. After one day and 13.2 hours, 10.6 g of gold-19 remains in a sample. How much gold-191 was originally present in the sample?84.8 g Au-191

There are 3.29 g of iodine-126 remaining in a sample originally containing 26.3 g of iodine-126. The half-life of iodine-126 is 13 days. How old is the sample?

39 days old

A sample that originally contained 2.5 g of rubidium-87 now contains 1.25 g. The half-life of rubidium-87 is 6 x 1010 years. How old is the sample? Is this possible? Why or why not?

6 x 1010 years

22,930 years

The half-life of carbon-14 is 5730 years. If a sample originally contained 3.36 g of C-14, how much is present after 22,920 years?

3.36 g 1.68 g 0.84 g 0.42 g 0.21 g

0 y 5,730 y 11,460 y 17,190 y 22,920 y

0 1 2 3 4

Data Table: Half-life DecayAmount Time # Half-Lifet1/2 = 5730 years

n = 5,730 years

n = 4 half-life

(4 half-life)(5730 years) = age of sample

(# of half-life)(half-life) = age of sample

22,920 years

Fusion and Fission

Objectives

Nuclear Fission

A large nucleus is bombarded with a small particle.

Nucleus splits into smaller nuclei and several neutrons.

Large amounts of energy are released.

Nuclear Chain Reaction

Bombardment of the radioactive nuclide with a neutron starts the process.

Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.

This process continues in what we call a nuclear chain reaction.

If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.

Nuclear Fusion

occurs at extremely high temperatures (100 000 000°C). Why?

combines small nuclei into larger nuclei.releases large amounts of energy.occurs continuously in the sun and stars.

Nuclear Fusion

This type of Fusion is beingExamined asAn alternativeEnergy sourceOn Earth.

Nuclear Fusion

Fusion would be a superior method of generating power. The good news is that the

products of the reaction are not radioactive.

The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins.

Tokamak apparati like the one shown at the right show promise for carrying out these reactions.

They use magnetic fields to heat the material.

Documents

Defining the Atom