unit 3 atoms, sub-atomic particles & nuclear chemistry

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Unit 3 Atoms, Sub-Atomic Particles & Nuclear Chemistry

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Unit 3

Atoms, Sub-Atomic Particles & Nuclear Chemistry

The Particle Theory of Matter• In 400 B.C. DemocritusDemocritus, a Greek

philosopher, first proposed the idea of a basic particle of matter that could not be divided any further.

• He called this particle the atomatom, based on the Greek word atomosmeaning indivisible.

• This early theory was not backed up by experimental evidence and was ignored by the scientific community for nearly 2000 years.

Foundations of Atomic Theory

• By the late 1700s, experiments with chemical reactions led to the discovery of 3 basic laws:1. The Law of Conservation of Mass Law of Conservation of Mass.• This law states that mass

is neither created nor destroyed during ordinarychemical reactions or physical changes.

• Formulated by Antoine Lavoisier in 1789.

Foundations of Atomic TheoryThe Law of Conservation of MassThe Law of Conservation of Mass

Foundations of Atomic Theory

2. The Law of Definite Proportions Law of Definite Proportions• States that a chemical compound

contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound.

• Discovered by Joseph Proust in 1797.

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Foundations of Atomic Theory

3. The Law of Multiple ProportionsLaw of Multiple Proportions – • States that if different compounds are

composed of the same 2 elements, then the ratio of the masses of the elements is always a ratio of small whole numbers.

• Published by John Dalton in 1804.

Dalton’s Atomic Theory

• In 1808, John Dalton John Dalton proposed an explanation for the three laws. His atomic theory states:1. All matter is composed of atoms.2. Atoms of the same element are identical; atoms of

different elements are different.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.

Corrections to Dalton’s Theory

• Dalton turned Democritus’s idea into a scientific theory that could be tested by experiment.

• But not all aspects of Dalton’s theory have proven to be correct. We now know that:– Atoms are divisible into even smaller particles.– A given element can have atoms with different

masses.

Dalton’s Atomic Model• An atomatom is the smallest particle of an element

that has all the properties of that element.• Atoms are too small

to see…even through the most powerfulmicroscope!!

• Dalton thought atoms were solid balls of matter and were indivisible.

Discovery of the Electron

Thomson’s Cathode Ray Tube Experiment

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Charge and Mass of the Electron

• In 1909, Robert Millikan Robert Millikan measuredthe charge on the electron during his oil drop experimentoil drop experiment.

• Using the charge-to-mass ratio, scientists were able to figure out the mass of the electron: about 1/2000 the mass of a hydrogen atom.

Millikan’s Oil Drop Experiment

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Thomson’s Plum Pudding Model

• After the work of Thomson andMillikan, the accepted model of the atom was called the plum pudding modelplum pudding model.

• The atom was viewed as a ball of positively-charged material with

tiny negatively-charged electrons spread evenly throughout.

Discovery of the Atomic Nucleus

• More detail of the atom’s structure was provided in 1911 by Ernest Rutherford Ernest Rutherford and his associates Hans Geiger and Ernest Marsden.

• The results of their gold foil gold foil experiment experiment led to the discovery of a very densely packed bundle of matter with a positive electric charge.

• Rutherford called this positive bundle of matter the nucleusnucleus.

The Gold Foil Experiment

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Rutherford’s Atomic Model

• After Rutherford’s gold foil experiment, the accepted model of the atom looked like this:

• A small, positively-charged nucleus with negative electrons surrounding it at some distance away. Most of the atom is empty spaceempty space.

Structure of the Atom• Rutherford proposed that the nucleus

had particles with the same amount of charge as an electron but the opposite sign, called protonsprotons.–Relative charge = +1–Relative mass = 1 amu

• For an atom to be neutral there must be equal numbers of protons and electrons.

• Throughout the 1920’s scientists accepted an (incorrect) model of the atom composed of protons and electrons.

Some Problems• How could beryllium have 4 protons stuck

together in the nucleus?– shouldn’t they repel each other?

• If a beryllium atom has 4 protons, then it should weigh 4 amu; but it actually weighs 9.01 amu! Where is the extra mass coming from?– Each proton weighs approximately 1 amu.– The electron’s mass is only about 0.00055 amu

and Be has only 4 electrons, so they don’t account for the extra 5 amu of mass.

There Must Be Something Else There!

• These questions were answered in 1932 by James Chadwick (a student of Rutherford’s), who discovered another particle in the nucleus, which he called a neutronneutron.–Charge = 0 (no charge).–Relative mass = 1 amu.

Subatomic Particles

• The nucleus is made up of at least one positively charged particle called a protonproton and usually one or more neutral particles called neutronsneutrons.

• Protons, neutrons, and electrons are often referred to as subatomic particlessubatomic particles.

Atomic Number

• The atomic number atomic number (Z) of an element is the number of protons of each atom of that element.

• Atoms of the same element all have the same number of protons.

Mass Number

• The mass numbermass number is the total number of protons and neutrons in the nucleus of an atom.

• Atoms of the same element can havedifferentmass numbers.

Isotopes

• IsotopesIsotopes are atoms of the same element that have different masses.

•Isotopes have the same number of protons and electrons but different numbers of neutrons.

•Most of theelements consist of mixtures ofisotopes.

Designating Isotopes

• Hyphen notationHyphen notation: The mass number is written with a hyphen after the name of the element.

uranium-235• Nuclear symbolNuclear symbol: The superscript indicates the

mass number and the subscript indicates the atomic number.

U23592

Mass numberAtomic number

Calculating Neutrons

• The number of neutrons is found by subtracting the atomic number from the mass number.

mass number − atomic number = number of neutrons

• NuclideNuclide is a general term for a specific isotope of an element.

Calculating Subatomic ParticlesSample Problem

How many protons, electrons, and neutrons are there in an atom of chlorine-37?

Solution:Number of protonsprotons

Number of electronselectrons

Number of neutronsneutrons

1717

1717

2020

= atomic number (on periodic table)

= number of protons

= mass number - protons

The Atomic Mass Unit

• The standard used by scientists to compare units of atomic mass is the carbon-12carbon-12 atom.

• One atomic mass unitatomic mass unit, or 1 amuamu, is exactly 1/12 the mass of a carbon-12 atom.

• The atomic mass of any atom is determined by comparing it with the mass of the carbon-12 atom.

Average Atomic Mass

• Average atomic massAverage atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element.

•The average atomic mass of an element dependson both the

massand the relative abundance ofeach of the element’s isotopes.

Calculating a Weighted Average

If you have the following grades, what would your marking period average be?

• First, change percents to decimals.• Next, multiply each grade by its decimal percent.• Finally, add up all the products.

Category Percent of Grade Grade

Tests 45 72

Quizzes 10 78

Labs 25 84

HW/CW 10 98

Project 10 94

÷ 100 = 0.45

÷ 100 = 0.10

÷ 100 = 0.25

÷ 100 = 0.10

÷ 100 = 0.10

x = 32.4

x = 7.8

x = 21.0

x = 9.8

x = 9.4+80.4

Calculating Average Atomic MassSample Problem 1

Copper consists of 69.15% copper-63, which has an atomic mass of 62.929 601 amu, and 30.85% copper-65, which has an atomic mass of 64.927 794 amu. What is the Average Atomic Mass of Copper?Solution•Change percents to decimals.•Multiply the atomic mass of each isotope by its relative abundance.•Add up all of the products.

Relative Abundance

Mass

Cu-63 0.6915 62.93

Cu-65 0.3085 64.93

x = 43.52

x = 20.03+

63.55

Calculating Average Atomic MassSample Problem 2

A student believed that she had discovered a new element and named it mythium. Analysis found it contained two isotopes. The composition of the isotopes was 19.9% of atomic mass 10.013 and 80.1% of atomic mass 11.009. What is the average atomic mass, and do you think mythium was a new element?Solution:•Average Atomic Mass:(.199 x 10.013) + (.801 x 11.009) = 10.81110.811•Because the atomic mass is the same as the atomic mass of boronboron, mythium was not a new element.

10.8Round off to:

Forces in the Atom• Electrons and protons attract because of opposite

electrical chargeselectrical charges, but protons and protons repel since they have the same charge.

• The nucleus is heldtogether by a mysterious force called the strong strong nuclear force nuclear force which only exists between nucleons (protons and neutrons) whichare very close together.

Valley of Stability

for Z = 1 20, stable N/Z ≈ 1

for Z = 20 40, stable N/Z approaches 1.25

for Z = 40 80, stable N/Z approaches 1.5

for Z > 83, there are no stable nuclei

Naturally Radioactive Elements• All of the elements beyond atomic number 8383

are unstable and thus radioactive.

• Large, unstable nuclei spontaneously break apart to form smaller, more stable nuclei.

• A nuclear reaction nuclear reaction is a reaction that affects the nucleus of an atom.Example:

• A transmutationtransmutation is a change in the identity of a nucleus as a result of a change in the number of its protons.

Nuclear Reactions

ThHeU 23490

42

23892

Nuclear ReactionsSample Problem

Identify the products that balance the following nuclear reactions: a. b.

Solution:a.Atomic Mass: 212 = 4 + _____

Atomic Number: 84 = 2 + _____

b.Atomic Mass: 22 + ____ = 22Atomic Number: 11 + ____ = 10

____HePo 42

21284

Ne____Na 2210

2211

8282208208

00-1-1 e01

Pb20882

Radioactive Decay

• Radioactive decay Radioactive decay is the spontaneous disintegration of a nucleus into a lighter nucleus, accompanied by nuclear radiation.• Nuclear radiation Nuclear radiation is particles and/or

electromagnetic radiation emitted from the nucleus during radioactive decay.

He42

Types of Radioactive DecayAlpha EmissionAlpha Emission• An alpha particle (α) is two protons and two

neutrons bound together and is emitted from the nucleus during some kinds of radioactive decay.• • The atomic number

decreases by two and the mass number decreases by 4.

Types of Radioactive Decay (continued)

Beta EmissionBeta Emission• A beta particle (β) is an electron emitted from

the nucleus during some kinds of radioactive decay (a neutron can be converted into a proton and an electron.)• • The atomic number

increases by one and the mass number stays the same.

β01

Types of Radioactive Decay (continued)

Positron EmissionPositron Emission• A positron (β+) is a particle that has the

same mass as an electron, but has a positive charge (to decrease the number of protons, a proton can be converted into a neutron by emitting a positron.)

• • The atomic number

decreases by one and the mass number stays the same.

β01

Types of Radioactive Decay (continued)

Electron CaptureElectron Capture• In electron capture, an inner orbital electron

is captured by the nucleus of its own atom. (An inner orbital electron combines with a proton to form a neutron.)

• • The atomic number

decreases by one and the mass number stays the same.

e01

Types of Radioactive Decay (continued)

Gamma EmissionGamma Emission• Gamma rays () are high-energy electromagnetic

waves emitted from an unstable nucleus.• Atomic number and

mass number both stay the same because gamma rays have no charge and no mass.

• They are pure energy, and very dangerous to living things.

Comparing Alpha, Beta and Gamma

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Half-Life• Half-lifeHalf-life is the time

required for half the atoms of a radioactive nuclide to decay.• Each radioactive

nuclide has its own half-life. •More-stable nuclides

decay slowly and have longer half-lives.

Half-LifeSample Problem 1

The half-life of radon-222 is 4 days. After what time will ¼ of a given amount of radon remain?Solution:Determine the number of half-lives that it takes to cut a sample to ¼ of the original amount.Then multiply that number by the half-life.

2 x 4 days = 8 days8 days

Half-LifeSample Problem 2

Phosphorus-32 has a half-life of 14.3 days. How many milligrams of phosphorus-32 remain after 57.2 days if you start with 4.0 mg of the isotope?Solution:Determine the number of half-lives in 57.2 days.

For each half-life, multiply the original amount by ½:

4.0 mg x ½ = 0.25 mg0.25 mg

57.2 days ÷ 14.3 days = 4 half-lives

x ½ x ½ x ½

Uses of Radiation• Radioactive dating Radioactive dating – scientists can determine

the approximate age of an object based on the amount of certain radioactive nuclides present.

Uses of Radiation (continued)

•Radioactive tracers Radioactive tracers are radioactive atoms that are incorporated into substances so that movement of the substances can be followed by radiation detectors.–Radioactive tracers can

be used by doctors to diagnose diseases.–Radioactive tracers are

also used in agriculture to determine the effectiveness of fertilizers.

Uses of Radiation (continued)

• Irradiated Food Irradiated Food – nuclear radiation is used to prolong the shelf life of food.

Uses of Radiation (continued)

• Nuclear Power Plants Nuclear Power Plants – use energy as heat from nuclear reactors to produce electrical energy. They have five main components:

1. Shielding – radiation-absorbing material used to decrease exposure to radiation from nuclear reactors.

2. Fuel – usually Uranium-235.3. Coolant – usually water, it absorbs excess heat energy.4. Control rods – neutron-absorbing rods that limit the

number of free neutrons5. Moderator – used to slow down the fast neutrons

produced by fission.

Nuclear Power Plant

Whole-Body Radiation Exposure• RemRem – The unit used to measure the biological

effects of absorbed radiation in humans.

Radiation Detection• Film badges Film badges - use exposure of film to

measure the approximate exposure of people working with radiation.• Geiger-Müller counters Geiger-Müller counters - instruments that

detect radiation by counting electric pulses carried by gas ionized by radiation.• Scintillation counters Scintillation counters - instruments that

convert scintillating light to an electric signal for detecting radiation.

Fission vs. Fusion• Nuclear Fission Nuclear Fission – very heavy nuclei split into

smaller, more stable nuclei.–Can occur spontaneously or when

nuclei are bombarded by particles.–Controlled fission chain

reactions are used in nuclear power plants. Uncontrolled fission chain reactions are used in nuclear bombs.

Fission vs. Fusion (continued)

• Nuclear Fusion Nuclear Fusion – low-mass nuclei combine to form a heavier, more stable nucleus.–Fusion releases even more

energy per gram of fuel than fission.–Scientists are not yet able to control fusion

reactions, so we can’t use them in power plants.–Fusion is the primary process

that fuels our sun and the stars.

Nuclear Waste• Radioactive waste produced in nuclear reactors

can take hundreds of thousands of years to decay.• Disposal of nuclear waste

is done with the intentionof never retrieving it. • There are 77 disposal sites

around the country. A new one (Yucca Mountain) is being developed for the permanent disposal of much of our nuclear waste beginning in 2017.

Visual Concept