UEQ

Where does the concept and understanding of the atom originate?

LEQ

Who are the main players in the development of the concept of the atom?

Lecture NotesChapter 4-The Structure of the Atom

Early Ideas of the Atom:

Democritus: atomos: small cells

Aristotle: Rejected the concept of the atom. All things came from earth, wind and fire.

John Dalton: English teacher.

Dalton’s Atomic Theory: 1. All matter is made-up of tiny indivisible particles

called atoms.

2. The atom is the smallest form of matter.

3. Atoms are represented by symbols called elements.

4. Atoms are the foundation of the universe.

5. All atoms of the same elements will be the same.

6. All atoms of different elements will be different.

7. Atoms of different element can combine in definite proportions to form more compounds

Question?

Dalton was right about most of his theory. What part of Dalton’s Theory is not correct with today’s knowledge?

Answer: Atoms are divisible into small subatomic particles.

Sir William Crookes Developed the use of the cathode ray tube in

the study of the atom.

a. cathode ray tube: vacuum tube with a (+) end called the anode and a (-) end called the cathode.

b. Crookes study show movement from the cathode (-) to the anode (+).

From - +

JJ ThomsonUsed the cathode ray tube to show the charge to mass

ratio of the stream of light in the tube.

a. Compare the charge to mass ratio to other known values

b. Concluded that the charged particles are less than the mass of the lightest atom known.

c. Identified the first subatomic particle called the electron.

d. Concluded the atom formed a ‘plum pudding’ model.

Cathode Ray Tube

Robert Millikan

Use Thomson’s charge to mass ratio to define the mass of an electron.Mass of electron = 1/1840 the mass of hydrogen or one electron = 9.1 x 10 -28 g or 9.1 x 10-31 Kg

Milliken Oil Drop Experiment

Ernest Rutherford

Gold Foil Experiment.a. The atom has a dense center called the

nucleus.

b. The nucleus has a positive charge.

c. The positive charge is due to the protons.

d. The atom is mostly empty space.

(diagram: Link)

Rutherford Gold Fold Experiment

James Chadwick Discovered the neutron

a. Chadwick was a student of Rutherford.

b. Discovered that the nucleus is heavier than could be account for with just protons.

c. The extra mass called the neutron and had NO charge.

No charge = neutral = neutrons

LEQ

What is the structure of the atom?

Conclusion: Structure of the Atom

Within the nucleus:

*Protons: (+) charge; half the mass of the atom; number is fixed, unchanging (Rutherford)

*Neutrons: NO charge; the other half of the mass of the atom (Chadwick)

Outside the nucleus:

* Electrons: (-) charge; in the electron cloud about the nucleus; NO mass (JJ Thomson)

Conclusion: The Atom1. Every atom of the same atom has the same number of

protons. THE NUMBER OF PROTONS NEVER CHANGES.

2. The number of neutrons can change from atom to atom of the same element. This is called an isotope, (same atom but different number of neutrons). Note: Cesium-131 , Cesium-134

3. The number of protons NEVER changes for the same atom. Found in the nucleus.

4. Elements are identified by the number of protons.

5. In a neutral atom (net charge = 0),

the # of protons = # of electrons

6. Ion form when # of electrons changes (gain electron(s) yields (-) ion ; loss electron(s) yields (+) ion

6. All the mass of the atom is in the nucleus.

7. The # of neutrons + the # of protons = the mass number.

8. The # of protons = the atomic number

9. Periodic Table (Robert Mosley)

Reading the Periodic Table

50

SnTin

118.69

Element Atomic Number Protons Electron (neutral atom)

Fe16

19

Elements Atomic Number

Atomic MassMass #

Number of Neutrons

ClMgZnLa

Elements Atomic Number

Atomic Mass

MassNumber

Protons Neutrons

Br55

26.9815

QuestionsElement Symbol Atomic # Atomic Mass #e- for neutral atom

Oxygen

20

Br

Magnesium

83

Iodine

11

1.

Each atom is a neutral atomElement Symbol Atomic # Mass Number #p+ #no #e-

92

Si

17

Tin

16

55

Al

Note, the #e- will indicate for a neutral atom.

Indicate the magnitude of the ion Element Symbol Atomic # Mass Number Atomic Mass #p+ #no #e- Ion?

9 10

Zinc 65 65.39

38 36

Argon 39.948

K 39 18

Manganese 54.938

15 18

3. Note, the number of #e- may indicate a neutral atom or an ion. Indicate the magnitude of the ion for those elements indicated.

All atoms are indicated as neutralAtomic # Atomic Mass #p+ #no #e- Element

37

26

79

36

80

5

21

All the atoms listed here are neutral.

LEQ

What is the significant role played by each part of the atom?

Ions

Formed when there is a different number of electrons to protons (# of protons for an atom NEVER changes

Charged form of an atom brought about by the gain of electrons (-) or loss of electrons (+)

Ions are formed:

1. response to another atom

2. metals form (+) ions

3. non-metal form (-) ions

4. unlike charges attract; like charges repel.

Oxidation Numbers Ions will allow you to assign oxidation numbers to each element.

**Number of protons (+) NEVER changes

**Number of electrons (-) changes. No mass

**Difference in protons to electrons =

oxidation numbers

Mg: 12 p+ and 12 e- = ______ oxidation number

Mg: 12 p+ and 10 e- = ______ oxidation number

Element Atomic Number

Number of Protons

Number of electrons

Oxidation Number

O 2-20 18

F 1-56 2+

Isotopes

Same atoms with different number of neutron in the nucleus:

**Mass of one neutron = mass of one proton

**Accounts for different masses for the same atom.

**amu: atomic mass units, is the average of the mass in one mole.

**Accounts for some radioactivity

Germanium-70 Germanium-73 Germanium-76

ProtonsNeutrons

Electrons

Calculating the Average Atomic Mass

NOTE:

1. The relative abundance is the decimal form of the % abundance.

% Abundance Relative Abundance

56% 0.56

7% 0.07

Relative AbundanceHow much is present based on the amount of the

sample given.

-It is the decimal form of the percentage

-Used to find the amu.

Calculating the Average Atomic Mass

2. Mass of Isotope times the relative abundance will equal the mass contributed by that isotope to the total mass

3. The sum of each contributed mass will equal the amu.

Calculate the Average Atomic MassIsotope Percent Abundance Mass

Cr-50 4.35 % 49.946

Cr-52 83.79 % 51.941

Cr-53 9.5 % 52.941

Cr-54 2.36 % 53.939

SamplerIn a given sample of an element, 81.3% has a mass of

32.45 g and 18.7% has a mass of 30.39 g. What is the amu?

Sampler

1. Copper has two isotopes. Cu-63 has an amu of 62.930 occurring 69.17% and Cu-65 has an amu of 64.928 occurring 30.83% of the time is a sample. Find the atomic mass.

2. Find the element by calculating the amu.Isotope Abundance (%) Atomic Mass

X-70 21.23 69.924

X-72 27.66 71.922

X-73 7.73 72.923

X-74 35.94 73.921

X-76 7.44 75.921

3. Chlorine has two natural occurring isotopes. Cl-35 with an atomic mass of 34.969 occurring 75.77% of the time, and Cl-37 that occurs 24.23% of the time. What is the atomic mass of Cl-37?

Radioactivity

The spontaneous emission of radioactive (high energy) particles.

Radioactivity is caused by a proton to neutron ratio. The greater the difference between the p+ and no, the more unstable.

An unstable atom will emit energy until a more stable form is reached. This is called radioactive decay.

Types of Radiation, page 122

Alpha Decay

Alpha radiation: (α), contains two protons and two neutrons giving a 2+ charge and a change in the mass of 4. Note that the mass and the atomic number changes.

For Rn-222:222Rn86 218Po84 + 4He2

226Ra88 210Po84

Beta Decay

Beta radiation: (β), contains a single electron and carries a 1- charge. Note the mass does not change and the number of protons increase to balance the 1- charge.

For Bi- 241241Bi83 241Po84 + 0e1-

228Ra88 14C6

Gamma Decay

Gamma radiation: (γ), Very high energy particle with no mass and no charge. Accompanies a alpha or a beta decay.

For U-238238U92 234Th90 + 4He2 + 0γ0

244Pu94 200U92 +

209Bi83 209Po84 +

Radioactive Questions

1. Why are some atoms radioactive while some are not?

2. Write the symbols used to denote alpha, beta, and gamma radiation. Write the mass and charge of each.

3. What is the primary factor that determines if a nucleus is stable or unstable?

4. Boron-10 emits alpha particles and Cesum-137 emits beta particles. Write a balance equation.

Half LifeHalf Life is the time intervals it takes for ½ the amount of material to undergo radioactive decay.

100 % sample 50% 25% 12.5% 6.25% ………

Two ways to find Half Life Intervals (n):

1. n = t / t1/2

t = time of the decay

t1/2 = half life

2. Divide the original amount by 2 until the amount decay a and the end of time ‘t’ is reached.

Half LifeTo calculate the amount of material decayed:

A = Ao / 2n where A = amount that remains at the end of time ‘t’.

Ao = original or initial amount of radioactive material

n = number of half life intervals

Calculating Half Life1. How much of a 1 kg sample of K-39 remains after 151 years of decay? The

half life for K-39 is 30.2 years. t =

t1/2 =

A =

Ao =

2. Given 64 g of an unknown sample undergoes radioactive decay for 12.5 hours to yield an amount of 2 g. What is the half life of this sample?

t =

t1/2 =

A =

Ao =

Calculating Half Life3. Cobalt-60 has a half life of 5.26 years. How much time is needed for a 16 g

sample of Co-60 to decay to 1 g?

4. How much of Nb-259 will remain after 290 min if you begin with 125 g. The half life of Nb-259 is 58 minutes.