Unit 2

Chapter 3 & Chapter 19

The Structure of the Atom & Nuclear Chemistry

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Early Theories of Matter

Democritus (460-370 B.C.) proposed & believed that

1. Matter was not infinitely divisible

2. Made up of tiny particles called atomos

3. Atoms could not be created, destroyed, or further divided

- This sounds like the beginning of…..

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John Dalton

19th centuryDalton revised Democritus's ideas based

upon the results of scientific research he conducted

Dalton’s atomic theory

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Dalton’s Atomic TheoryElements are made of extremely small particles called

atomsAll atoms of a given element are identical

Same size, mass and chemical propertiesAtoms are indivisible in chemical processes.

They can neither be created nor destroyed in a chemical reaction

A chemical reaction simply changes the way the atom is grouped together

A compound has a constant composition of its elementsDifferent atoms combine in simple whole number

ratios to form compounds

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Dalton’s Atomic Theory

Dalton’s theory was supported by 2 Laws:

The Law of Conservation of Matter (Lavoisier)The Law of Definite or Constant Composition

(Proust)

Not all of his theory is correct!!

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What is an atom?

An atom is the smallest particle of an element that retains the properties of the element

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The Nuclear Atom

Ernest Rutherford developed a model of the atom.

His model consisted of the following ideas:An atom consists mostly of empty space through which

electrons moveElectrons are held within the atom by their attraction to

the positively charged nucleusTiny, dense region called the nucleus, which is located

in the center of the atom

Nucleus contains all of an atom’s positive charge and virtually all of its mass

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Three Subatomic Particles

Electron Proton Neutron

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Subatomic Particles in the NucleusPositively charged particles called

protonsProton’s charge: equal to but opposite that of an

electronWhy don’t the protons repel each other in the

nucleus?

Neutral particles called neutronsNeutron mass nearly equal to that of protonNeutron has no charge

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Outside the nucleusNegatively charged particles are called

electronsmove through empty space in atomNegatively charged

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Atomic Number

The number of Protons in the nucleus

Examples:Carbon (C) has 6 protons

Atomic number is 6Copper has 29 protons

Atomic number is 29

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Atomic NumberIn uncharged atom, atomic number is

also the number of electronsWhy? If an atom is charged, then it is an ion

Uncharged atom:Atomic number = # of protons = # of electrons

Take out your periodic table

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Mass Number

To find the Mass number

# protons + # neutrons

To find # neutrons mass number – proton (or atomic number)

Mass numbers are always WHOLE #’s!!

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Symbols for Atoms

X= symbol of element

A= mass number

Z= number of protons

X or XAZ

A

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Isotopes and Mass Number

C C carbon-12 carbon-13

Isotopes are atoms with the same number of protons but different number of neutrons

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6126

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Isotopes and Mass NumberExample: 3 types of Potassium

All 3 types contain 19 protons and __ electrons

# of Protons # of Neutrons Mass Number

19 20 19 21 19 22

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The End! February 2nd - Monday

Be sure to do your homework tonight!!First daily warm-up tomorrow!!

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What’s the difference between mass number and average atomic mass?

C carbon-12

But if you look on the periodic table, the number states 12.01…

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Mass of Individual Atoms

Mass of protons and neutrons:

1.67 x 10-24 g

Mass of electron is 1840 times smaller than that of protons and neutrons

9.11 x 10-28 g

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Atomic Mass Unit (amu)

Small mass #’s are not easy to work with, so the atomic mass unit (amu) was developed

One atomic mass unit (amu) is equal to 1/12 the mass of a carbon-12 atom

The mass of 1 amu is nearly equal to the mass of one proton or neutron

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Mass of Individual Atoms

Atomic mass:The weighted average mass of the isotopes of

an element

Example: ChlorineMixture of 75% chlorine-35 and 25% chlorine-37

Atomic mass = (0.75)*35 + (0.25)*37 = 35.5 amu

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What’s the difference between mass number and average atomic mass?

Mass number- specifically about one isotope

Average atomic mass- includes the masses of all the different isotopes for that atom

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The End! February 3rd - Tuesday

Go over test!Warm up practiceStart handout #2

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February 4th - Wednesday

You must complete the pre-lab questions before you can start on the labs!!

Be sure to do your homework tonight!

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Nuclear Reactions

Chemical reactionsWhat can NOT change in a chemical reaction?

Nuclear Reactions: changes that occur in the nucleus of an atom The nucleus is unstable!!

Most atoms have unstable nuclei.

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Why are some stable, while others are not?

Primary Reason:

ratio of the neutrons to the protons (n/p)

An atom is most stable when the ratio is 1:1The maximum ratio of stability is around 1.5 : 1

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Radioactivity

An unstable nucleus emits rays and particles, called radiation, to become stableThe process is called radioactivityGain stability by LOSING energy

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Radiation

Three types of radiation produced:AlphaBetaGamma

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3 Radioactive Particles

1. Alpha Particle (+)o It travels about 1/10 the speed of light (slowest)

o It is the largest, most massive particle

o It is the most dangerous if ingested

o It has the least penetrating ability - paper can stop this particle

He42

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3 Radioactive Particles

2. Beta Particle (-)o Fast accelerated electron

o Ejected when a neutron is converted to a proton in the nucleus

o Travels 1/4 the speed of light.

o It is lighter and faster than the alpha particle.

o Average penetrating ability - can be stopped by heavy clothing

01

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3 Radioactive Particles

3. Gamma Rayo Not really a particle; it is a form of energy from

the electromagnetic spectrum (EM)o Has no mass and no charge.o Always accompanies either beta or alpha

radiation.o Very high poweredo Travels at the speed of light.o Highest penetrating ability - can be stopped by

heavy shielding such as lead.

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Alpha Decay

An alpha particle is released A new atom is formed in the product in which:

a) the atomic number is lowered by 2

b) the mass number is lowered by 4

Example:

He42

HeRnRa 42

22286

22688

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Beta Decay

A neutron is converted in the nucleus of an atom resulting in:a) beta particle released

b) a new atom in the product whose atomic number increases by 1

c) mass number does not change

Example:

01

BNC 01

147

146

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Gamma Radiation

It is the “Energy” of the reactionAlways accompanies alpha and beta

emissions.Regardless of radiation or decay,

The Law of Conservation of Matter must be observed!!

The nuclear equation must be BALANCED!!

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HeRnRa 42

22286

22688

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RADIOACTIVE PARTICLES Summary

Alpha Particle

Positive Charge

Mass of a

Helium nucleus

1/10 the speed of light

Beta particle

Negative Charge

Mass of an

electron

1/4 the speed of light

Gamma ray

No charge

No Mass

Speed of Light, c

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Half Life

The half life of a radioisotope is the time it takes for half of the sample to decay.

Half-life is represented by “t1/2”

The number of half-lives that have passed is represented by “n”

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Amount RemainingYou can use two equations, depending on

what information you have

If you have the # of half-lives that have passed..

initial amount x (1/2) n

n= # of half-lives that have passed

If you DON”T have the # of half-lives that have passed..

initial amount x (1/2) P/ t1/2

p= time that has passed

t1/2= time of half-live38

Half-life problem

If the half life element A is 3 hours and you have 90 grams of it, how many grams would be left after 9 hours?

What percent of it is remaining?

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Handout #5 Equations to AddTo find the half-life

t1/2= time passed/ # of half-lives passed

To find the # of half lives that have passed…n= time passed/ one half-life

Total time passed= n times t1/2

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Problem 3

Given informationI= 64gr= 2.0gp=12.5 hoursn=?One half-life=??

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FUSION and FISSIONFission is the splitting of nuclei

resulting in a tremendous release of energyheavy atoms split so they can become more stable

Fusion is the combining of nuclei resulting in more energy being released than fissionThe sun produces energy as a result of nuclear

fusion2 H atoms combine to form Helium

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The End! February 6th - Friday

Finish Review sheet this weekend!Review on Monday!

Change of Schedule:Do half-live lab on Monday!

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