PreAP Chemistry

Unit 4

Atomic Structure, the Periodic Table, and Nuclear Radiation

Democritus

• A Greek who lived ~400 BC, was the first to suggest the existence of atoms.

– He believed atoms to be indivisible and indestructible

– He had no experimental support

• Aristotle and Plato lived about the same time

– They were more famous

– They disagreed with Democritus

– So, there was no more thoughts about atoms for over 2000 years

John Dalton (2000 yrs later)

• Proposed atomic theory based on experimentation– All elements are composed of tiny indivisible particles

called atoms.

– Atoms of an element are identical. Atoms of one element differ from atoms of another.

– Atoms can form mixtures or chemically combine in whole number ratios to form compounds.

– Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element can never be changed into atoms of another element.

Atoms

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

• Atoms are composed of protons, neutrons, and electrons.

Electrons

• Electrons are negatively charged subatomic particles.

– Discovered by J.J. Thomson in 1897 while experimenting with a cathode ray tube.

– A cathode ray is a stream of tiny negatively charged particles moving at high speed.

– The mass of an electron has been determined to be 1/1840 times the mass of a proton.

The two electrodes were connected to some sort of sparking apparatus…keep in mind electricity was not readily available yet. Once connected the beam appeared to travel from one end of the tube to the other. Thomson determined that the beam was composed of negatively charged particles.

Protons

• Protons are positively charged subatomic particles.

– Discovered by E. Goldstein when he noticed that canal rays traveled in the opposite direction of cathode rays.

– These canal rays consisted of positively charged particles.

Neutrons

• Neutrons are subatomic particles that have no charge but have a mass nearly equal to a proton’s mass.

• Discovered by James Chadwick in 1932.

Let’s digress

• In the late 1800’s, Henri Bequerel was experimenting with an uranium ore.

– When placed in sunlight, the ore would fluoresce…then he would place it next to photographic film, which it would expose.

– One week, no sun, the ore did not fluoresce, but the ore exposed the film anyway.

– What was causing the film to be exposed? Along comes Rutherford to the rescue!

The α particles were attracted to the negative plate. The βparticles were attracted to the positive plate. The γ rays were not affected by charge.

Let’s digress again

• Now that scientists knew that atoms had positively and negatively charged particles within, how were these particles arranged.

• J.J. Thomson put forth the “raison bun theory”, also known as the “plum pudding model”, which I rechristened the “blueberry muffin model”

• Rutherford to the rescue again!!

Gold Foil Experiment

• Rutherford used the newly-found alpha particles to bombard gold atoms.

• If the atom was as Thomson believed, then the alpha particles would go straight through the atom with little deflection.

• But, Rutherford found much deflection at various angles. Which lead him to believe that the atom was mostly empty space.

This diagram represents the “Gold Foil Experiment” (also known as the “Alpha Scattering Experiment”). Rutherford was totally surprised when he found deflections at various angles….sometimes the alpha particles rebounded straight back. This lead him to believe that there was an extremely small center to the atom that held most of the mass of the atom and was positively charged. He called this center the nucleus and he said that the rest of the atom was “empty space” in which the electrons were located.

Gold Foil Experiment, cont.

• He defined the term nucleus, as the small, dense, core of the atom where most of the mass and all the positive charge are located.

• He said that the electrons were traveling within the remaining “empty space”.

• Football field analogy – If a pea were on the 50 yard line, then the outskirts of the stadium would represent the outskirts of the atom.

SummaryProperties of Subatomic Particles

Particle Location Symbol Charge Mass (amu)

ElectronOutside nucleus

e- 1- ~0

ProtonInside nucleus

p+ 1+ ~1

NeutronInside nucleus

n0 0 ~1

Review

• Atoms are composed of electrons, protons, and neutrons.– Protons and neutrons make up the small, dense

nucleus.

– Electrons surround the nucleus and occupy most of the volume of an atom.

• The atomic number is the number of protons in the nucleus. – This is the whole number found on the periodic table.

• The mass number is the total number of protons and neutrons in the nucleus.– This number is not found on a periodic table….it is

specific for a specific isotope.

Isotopes

• Isotopes of an element are atoms that have the same number of protons but different numbers of neutrons.

– Which means, that they have different masses

– Look at the three isotopes of hydrogen; hydrogen, deuterium, and tritium

Isotope Symbol

Isotope

Name # p+ # n0 # e-

Mass #

Hydrogen-1 1 0 1 1

Hydrogen-2 1 1 1 2

Hydrogen-3 1 2 1 3

H11

H21

H31

The top number represents the mass number (sum of p+ + n0). Notice that all three have the same number of protons (same atomic number).

Isotope Symbols

The top number is the mass number (sum of p+ + n0)

The bottom number is the atomic number (#of p+)

From this symbol, I know that the mass # is 32, atomic # is 15, which means 15 p, 15 e, and 17 n. The isotope name of that symbol is phosphorus-32. The isotope name is the name of the element with the mass # attached.

Worksheet examples

Isotope Symbol

Mass #

Atomic #

# p+

# n0

# e- Isotope Name

55 26 26 29 26 Iron-55

39 19 19 20 19 Potassium-39

To solve row 1: top # is mass #; bottom # is atomic #; atomic # tells you p+ and if the atom is neutral, it tells you e-; subtract p+ from mass # to get n0. The isotope name is the name of the element with the mass # attached.

To solve row 2; #p+ is the same as the #e- which is the same as the atomic #. Add the p and n to get the mass number. You can now create the isotope symbol by placing the mass # up top and the atomic # below….the atomic # identifies the element. The isotope name is the name of the element with the mass # attached.

More examples

Name the element that has

33 n0, 27 p+, 27 e-

How many protons does an atom of zinc have?

The important piece if info is the #p. 27 protons means element #27 or Cobalt.

Look up Zn on the periodic table. Since Zn’s atomic # is 30, Zn has 30 protons.

Average atomic mass

• The mass of an atom in amus is approximately the same as the sum of the p+ and n0

– An amu (atomic mass unit) is defined to be 1/12 the mass of a carbon-12 isotope (ie. 12 amus)

• The average mass of an element’s atoms is called the average atomic mass.

• The average atomic mass is the weighted average of all the isotopes of a given element in a sample of that element. (It will be the same no matter where in the universe the element is found

Example

• In any chlorine sample you will find 75.53% Cl-35 and 24.47% Cl-37. What is the average atomic mass of chlorine?

(you need to do a weighted average)

.7553 X 35 = 26.4355

.2447 X 37 = +9.0539

35.4894 amu

Periodic Table – A History

• Dmitri Mendeleev arranged the first working periodic table in the mid 1800’s.

– Arranged the known elements in columns according to similar properties.

– Left blanks in his table for elements he considered to be missing.

• He predicted masses and properties for those missing elements.

• The missing elements were eventually found.

Other chemists disagreed; why?

• Mendeleev did not arrange his elements by atomic mass, but by properties.

• Mendeleev switched several pairs of elements (such as Te and I)

– He claimed that some of the atomic masses were found incorrectly.

Who fixed the problem?

• In 1913, Henry Mosely determined that each element had an identifying atomic number.

• He ordered the elements according to the newly assigned atomic numbers, which proved that the switching of the element pairs by Mendeleev was correct.

Modern periodic table

• The periodic law states that when the elements are arranged in order of increasing atomic number, there is a periodic repetition of their physical and chemical properties.

• Blank periodic table.

Radioisotopes

• Radioisotopes are isotopes of elements that are radioactive.

– They gain stability by undergoing nuclear changes and by releasing energy.

– Nuclear reactions are not affected by changes in temperature, pressure, or the presence of a catalyst.

– These nuclear reactions cannot be slowed down, speeded up, or turned off.

Remember

• Henri Becquerel accidentally discovered that uranium ore gave off radiation.

• Lord Rutherford determined that there were three basic types of radiation.

– Alpha particles

– Beta particles

– Gamma rays

Alpha particles• Alpha radiation consists of helium nuclei that

have been emitted from a nucleus.– These particles contain 2 protons and 2 neutrons and

have a 2+ charge.– When an atom loses an alpha particle, the atomic

number is lowered by two and the mass number is lowered by four.

– Because of their large mass and charge, alpha particles do not travel very far and are not very penetrating.

– They are easily stopped by a sheet of paper or by the surface of your skin.

– However, alpha particles are dangerous if ingested or inhaled.

Beta particles• Beta radiation consists of fast-moving electrons

formed by the decomposition of a neutron (ie., )

– These fast-moving electrons are released by the nucleus and are called beta particles.

– They have effectively no mass and a charge of 1-.

– When an atom loses a beta particle, the atomic number is increased by one and the mass number remains the same.

– Much more penetrating than alpha particles.

– Stopped by sheet of metal foil or piece of plastic, like plexiglass.

epn 0

1

1

1

1

0 −+→

The Electromagnetic Spectrum

There are two basic types of electromagnetic energy: ionizing and nonionizing. Nonionizing electromagnetic radiation is not harmful to the human body. Anything below visible light (including visible light) is considered safe. UV radiation and above will cause damage to the human body.

Gamma rays

• Gamma radiation is high-energy electro-magnetic radiation that is given off by the nucleus…similar to X-rays.

– Gamma rays have no mass and no charge.

– Thus the emission of a gamma ray does not alter atomic number or mass number.

– Extremely penetrating; very dangerous

– Can only be stopped by several inches of lead or several feet of concrete.

Summary of Radiation

Property Alpha Beta Gamma

Composition Helium nucleus Electron High energy ER

Symbol

Charge 2+ 1- 0

Mass (amu) 4 0 0

Penetrating power

Low Medium high

shielding Paper, clothing Al foil, plastic Lead, concrete

He4

2,α e0

1,−β γγ 0

0,

How to predict which radioactive particle is emitted?

• Those isotopes that are too big, will most likely emit an alpha particle.

• Those isotopes that have too many neutrons, will most likely emit a beta particle.

• Those isotopes that have too much pent up energy, will most likely emit a gamma ray.

How to predict which radioactive particle is emitted?

• Those isotopes that are too big, will most likely emit an alpha particle.

• Those isotopes that have too many neutrons, will most likely emit a beta particle.

• Those isotopes that have too much pent up energy, will most likely emit a gamma ray.

Nuclear Stability

About 1500 different nuclei are known;

• Of these, only 264 are stable

• The stability of a nucleus depends upon its neutron to proton ratio.

– For atomic numbers under 20, this ratio is 1:1

• Ex. Argon has 18 protons…it would like 18 neutrons

– This ratio tops out at about 1.5 neutrons to every proton for atoms with atomic numbers >80

• For example; a neutral atom of mercury has 80 protons and 120 neutrons (1.5 X 80 = 120)

Two Important atomic #’s

• Any nucleus with an atomic number >83 is radioactive.

– In other words, Bismuth (#83) is the last element on the periodic table that can possibly have a stable nucleus.

• Those elements with atomic numbers greater than 92 (Uranium) are known as the transuranium elements.

– The artificial elements were created by bombarding uranium with particles or by bombarding elements made from uranium with particles.

Nuclear Decay equations

• Alpha Decay of Californium-252

• Beta decay of potassium-41

CmHeCf 248

96

4

2

252

98+→

CaeK 41

20

0

1

41

19+→ −

Fusion

• Fusion involves combining two small nuclei into one larger nucleus.– This occurs on our sun and other stars.

– The product is usually not radioactive.

– Fusion occurs at very high temperatures.

– Need a magnetic field to contain the plasma required for fusion to occur.

– Researchers have not as yet been able to control a magnetic field on Earth. If they could, then we could have fusion reactors instead of fission reactors.

Fission

• Fission involves the splitting of one large nucleus into two smaller nuclei.

– This occurs in nuclear power plants.

– The products are highly radioactive.

– Three fissionable nuclei.

• U-235, Pu-239, and Cf-252

– Fission occurs when one of the above nuclei are bombarded with neutrons.

Half-life

• Half-life is the time required for ½ of a radioactive isotope to decay

– Some half-lives are very short and are useful in medicine.

– Some half-lives are very long and are useful in dating artifacts.

– Each radioactive isotope has its own half-life which never changes….it is like a finger print

Half-life problems

• Half-life problems concern themselves with four ideas

– Total time – the overall time covered by the problem

– Half-life – symbol is t1/2

– Beginning mass (or beginning # of particles)

– Ending mass (or ending # of particles)

• Two examples….

Example 1Phosphorus-32 has a half-life of 28 days. How

much time is needed for 84 g of the phosphorus-32 to decay to 5.25 g?

T1/2 = 28 daysTotal time = ? DaysBeg mass = 84 gEnd mass = 525 g

Any time you have a beginning and an ending mass, you will complete a decay chain. You will cut the masses in half each time. You stop the decay chain when you reach the end mass.

84 g 42 g 21 g 10.5 g 5.25 g

Count the arrows. 4 arrows means 4 half-lives occurred. Each half-life took 28 days. So

4 x 28 days = 112 days

Example 2Hydrogen-3 has a half-life of 12.3 years. If

you begin with 100 g of hydrogen-3, how much is left after 36.9 years?

T1/2 = 12.3 yearsTotal time = 36.9 yrsBeg mass = 100 gEnd mass = ? g