Atomic Structure

Early Atomic Theory

Democritus (400 BC) suggested that the world was made of 2 thing empty space and tiny, invisible particles called atoms.

AristotleAristotle believed matter was composed of 4 basic elements, water, fire, earth, and air. He was a well respected so everyone discounted Democritus’s atomic theory and believed Aristotle’s theory without question.

John Dalton (early 1800s) using the experimental observations of others, Lavoisier, and Proust proposed his own atomic theory.

Dalton’s Atomic Theory

1. All elements are composed of tiny

particles called atoms.2. Atoms of the same

element are identical. The atoms of any one element are different from those of another element.

Isotopes of an element are not identical.

3. Atoms cannot be divided, created, or destroyed. Atoms are

divisible by a nuclear charge.4. Atoms of different elements can

combine with one another in simple, whole number ratios to form compounds.

5. Chemical reactions occur when atoms are joined, separated, or rearranged. However, atoms of one element are not changed into atoms of another element.

Early Research on Atomic Particles

Thomson (1867) used a cathode ray tube to prove the existence of negatively charged subatomic particles called electrons.

Cathode Rays and the Electron

Thomson investigated cathode rays using a devise called a cathode ray tube or CRT. Currents of electricity were pumped into vacuum tubes causing the tube to fluoresce. Thomason used magnets to determine the identity of the particles making up the rays. He found them to be tiny, negatively charged particles.

Robert Millikan (1909) used his oil drop experiment to prove that the charge on of an electron is -1.

Rutherford

Using his gold foil experiment, Rutherford proposed that the atoms is composed of a lot of empty space with, a small dense, positively charged nucleus.

Thomson – used a modified cathode ray tube to prove the existence of a positively charged subatomic particle called a proton. It has equal but opposite charge to the electron (+1) and a mass 1840 times heavier than an electron.

Chadwick

Chadwick used a device to prove that the nucleus contained neutral particles of the same mass as the proton called neutron.

The atomic number is the number of protons in an atom. This number is unique for all elements and the atomic number is used to identify each element. Since atoms are electrically neutral,

THE NUMBER OF PROTONS EQUALS THE NUMBER OF ELECTRONS.

Isotopes are atoms of the same element that differ in the number of neutrons in the nucleus. Isotopes of the same element have the different chemical properties.The number of neutrons determines the particular isotope of the element.

Subatomic Particle Chart

Particle Charge Relative mass

Location

Electron -1 1/1840 Outside

Nucleus

Proton +1 1 Nucleus

Neutron 0 1 Nucleus

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

Mass Number = protons + neutrons

Mass number – protons = Number of

Neutrons

Calculating Atomic Mass

Atomic Mass is the weighted average of the masses of the isotopes of that element. A weighted average takes into consideration both the mass and the abundance of each isotope. The correct unit for atomic mass is amu.

To calculate relative atomic mass, multiply the mass number of each isotope by its percent abundance

changed to a decimal and total.

(Mass #)(isotope’s relative abundance) +

(Mass #)(another isotopes Rel. abund) =

Relative atomic mass of the element

EXAMPLE:Symbol Abundance Calculation Average

Atomic Mass

32S 95.00% 32 X 0.95 30.4

33S 0.76% 33 X 0.0076 0.2508

34S 4.22% 34 X 0.0422 1.4348

36S 0.014% 36 X 0.00014

0.00504

32.09384 amu

Example 2: Neon has 2 isotopes, Ne-20 with an abundance of 90% and Ne-22 with an abundance of 10%. Calculate the average atomic mass of neon.

Example 3: Carbon occurs in nature as a mixture of atoms of which 98.89% have a mass of 12.00 u and 1.11% have a mass of 13.00335 u. Calculate the atomic mass of carbon.

Radioactivity

Radioactivity was 1st

discovered by Antoine

Becquerel, when

a photographic plate

never exposed to

Sunlight in his lab had become exposed. The only possible culprit was a nearby uranium salt sitting on the bench top.

History of Radioactivity

The term radioactivity was 1st used by Marie Curie in 1898. Curie and her husband, Pierre, found that radioactive particles were emitted as either electrically negative which were called beta particles (ß) or positive particles called alpha particles (α).

Nuclear Chemistry

Nuclear reactions are reactions that affect the nucleus of the atom.

Radioactivity is the phenomenon of radiation (particles and/or energy) being ejected spontaneously by an unstable nucleus until it reaches a more stable arrangement.

Nuclear Stability is determined by the ratio of protons to neutrons in the nucleus.

There are forces in the nucleus that oppose each other, the "Strong" force holding Protons and Neutrons to each other and the electrostatic force of protons repelling other protons. 

Under certain arrangements of protons and neutrons the electrostatic force can cause instability in the nucleus causing it to decay.

 

It will continue to decay until it reaches a stable combination.

Radioactive decay is the process by which the unstable nuclei lose mass and/or energy by emitting radiation.

Eventually unstable nuclei achieve a more stable state when they are transformed into atoms of a different element.

This graph shows the stable nuclei in red.  There are several things to notice:

•There are no stable nuclei

with an atomic number

higher than 83 or a  neutron number

higher than 126.

•The more protons in the nuclei, the more neutrons are needed for stability.  Notice how the stability band pulls away from the P=N line.

•Stability is favored by even numbers of protons and even numbers of neutrons. 168 of the stable nuclei are even-even while only 4 of the stable nuclei are odd-odd.

Types of Radioactive Decay  When unstable nuclei decay, the reactions generally involve the

emission of a particle and or energy.

For each type of decay, the equation is balanced with regard to atomic number

and atomic mass. In other words, the total atomic number before and after the reaction are equal.  And the total atomic mass before and after the reaction are

also equal.

Transmutation

When particles break down in the nucleus in an atom of an element (radioactive decay), the element changes into another element. This is called transmutation.

TYPES OF RADIATION

Gamma emission is the high energy electromagnetic radiation given off in most nuclear reactions. GAMMA RAYS ARE NOT MATTER, THEY ARE ENERGY. Therefore, they are not involved in balancing the nuclear equation. They are very damaging and difficult to shield against.

Gamma Emission (λ)

Generally accompanies other radioactive radiation because it is the energy lost from settling within the nucleus after a change.

Alpha Emission (α)Happens when the atomic number is greater than 83The 2 p+ 2n ( ) loss brings the atom down and to the left toward the belt of stable nuclei.

He42

He42

Alpha Particle Emission (α)

HeThU 42

23490

23892

Uranium - 238 Thorium - 238 Alpha Particle (α)

BETA EMISSION A beta particle (a high energy electron, charge of -1) is generated in the nucleus as a neutron is converted into a proton.

eNC 01

147

146

Carbon - 14

Nitrogen - 14 BetaParticle

Beta Particle Emission (β)

Happens to nuclei with high neutron:proton ratioA neutron becomes a proton causing a shift down and to the right on the stability graph  

Positron Emission

A positron is an antimatter particle that is like an electron but has a positive charge. A positron is generated as a

proton is converted to a neutron.

eOF o1

188

189

Fluorine - 18 Oxygen - 18 Positron

Positron Emission

Happens to nuclei with a low neutron:proton ratioA proton becomes a neutron causing a shift up and to the left                                              

This graph shows all the trends of decay and the band of stable nuclei

Penetrating Power of Radiation

Nuclear Fission

Fission is the breaking apart of a very heavy nucleus into parts.

nBaKrU 10

14156

9236

23592 2

A nuclear reactor is a device for controlling nuclear fission to produce

energy for practical use.

The main fuels for nuclear reactors are U-235 and plutonium-239.

Fusion is the combining of 2 small nuclei into 1 larger one.

Fusion of hydrogen into helium occurs in the sun.

Fusions reactions should produce much more energy than fission and use much more accessible fuels. However, currently many problems exist in fusion reactions such as the extremely high temperature needed

for the reaction.

Decay Series for Uranium - 238

Half-Life

Half-Life (t1/2) is the time required for half of the atoms of a radioisotope to emit radiation and to decay to products.

Half-Life ExampleIt takes 4.5 X 109 years for one half of a sample of uranium-238 to decay to lead-206. Therefore, it would take another 4.5 X 109 years for one half of the remaining uranium to decay, et cetera, et cetera, et cetera.

PbU

PbU

PbU

PbU

20682

23892

20682

23892

20682

23892

20682

23892

100g 50g

50g

1 half-life

25g

25g12.5g

12.5g 6.25g

2nd half-life

3rd half-life

4th half life

How many atoms of a 2.97g. sample of molybdenum-91 would remain after 62 min. if the half-life of molybdenum-91 is 15.49 min.?How many ½ lives is this?

# Half-Lives Time Spent (min)

Amount Remaining (g)

0 0 2.97

1 15.49 1.49

2 30.98 0.74

3 46.47

0.37

4 61.96 0.19Answer = 0.19 g 4 half-lives

Radiation Detection

Film badges are used to monitor the amount of radiation exposure people have received.

Geiger Counter

Instrument that detects radiationby measuring current produced by gas particles ionized by radioactivity

Scintillation Counter

Instrument that converts light to an electric signal

for detecting radiation.

Uses for Nuclear Radiation

Since the physical and chemical properties of radioisotopes of an

element are the same as stable ones, many uses for radioactive nuclides are

possible.

In medicine radioactive nuclides are used to destroy cancer cells and as tracers to tract

substances through the body or identify cancer and other diseases.

Cobalt - 60 Radioactive Tracer

In agriculture, radioactive nuclides are used as tracers in fertilizer to determine the effectiveness or to prolong shelf life of food by irradiating to destroy microorganisms.

In dating radioactive nuclides are used to determine the age of objects. Example: Carbon -14 is used to date organic materials.

In energy production, currently nuclear fission is used to create energy. Example: Comanche Peak nuclear power plant in Glen Rose produces energy that is used by TXU.

Nuclear WasteNuclear fission produces radioactive wastes that must be contained and stored on-site (temporary) or disposed of (permanent).