Chapter 3“Atomic Structure”

Honors Special Topic:

Ch. 3-4 “Changes in the Nucleus”

OBJECTIVES

Describe the changes that accompany nuclear reactions.

Define radioactivity. Provide examples of nuclear equations

and applications.

Chapt. 3-4 Changes in the Nucleus Chemical reactions involve electrons, but nuclei

also can also change. Nuclear reactions change the composition of

the atom. Radioactivity – the spontaneous emission of

radiation from an atom. Radioisotope – an isotope that has an unstable

nucleus and undergoes radioactive decay. Alpha, beta and gamma radiation are produced

by nuclear changes (radioactive decay).

Nuclear Stability Recall our discussion about isotopes?

Why don’t the nuclei, which contain positively charged protons in a very confined space, fall apart spontaneously?

Strong nuclear force (resulting from the neutrons) are the ‘nuclear glue’.

Do atoms contain the same number of protons and neutrons? Stable nuclei for elements 1 – 20 generally have equal

numbers of protons and neutrons. (n/p =1/1) As the atom gets larger, more nuclear glue is needed to gain

stability. (Additional neutrons needed; 1.5/1. See Fig. 3-28 for the “band of stability.”)

Beyond bismuth (Z = 83), NO NUMBER of neutrons can hold the nucleus together.

All atoms with Z >83 are radioactive!

Nuclear Stability (cont’d) Over 1500 nuclei are known, but only 264 are stable!

These 264 nuclei are unchanged with time. Lead-206 (124 neutrons + 82 protons) is stable with a

n/p ratio of 1.5. Transmutation – the conversion of an atom of one

element into an atom of another element. Occurs by spontaneous radioactive decay of a

nucleus, or… …by artificial means (synthesis).

Too many neutrons can make an atom unstable as well. Neutrons decay into a proton and an electron.

Types of Radioactive Decay Alpha Decay (α)

Alpha particles are just helium nuclei. Mass number = 4 (4 amu) Charge = 2+ Low penetration power (Paper & clothing stop them.)

Beta Decay (β) Beta particles are just electrons. Mass number = 0 (1/1837 amu) Charge = -1 Medium penetration power (Metal foil stops them).

Gamma Decay (γ) Gamma radiation is high energy electromagnetic radiation. Mass number = 0 Charge = 0 High penetration power (Thick lead shield stops them.)

Nuclear Equation Rules

The sum of the mass numbers and atomic numbers are the SAME before and after a nuclear reaction.

Electrical charge of alpha particles is generally omitted.

Electrical charge of beta particles is shown as a subscript (where Z is usually shown).

Nuclear Equation Examples Examples:

Alpha decay of uranium-238

Alpha decay of gold-185

Beta decay of carbon-14

Beta decay of francium-223 Gamma rays often accompany alpha and beta

disintegration of a nucleus. Thorium-230 Radon-226 + α + γ Thorium-234 Protactinium-234 + β + γ

Rutherford bombarded nittrogen-14 with α- particles to form F-18, leading to O-17 & proton. Led to discovery of the proton!

Problems

A nuclear reaction produced magnesium-24 and beta radiation. What nucleus was responsible for this? (Sodium-24)

Mercury-200 and an alpha particle result from the radioactive decay of what nucleus? (Lead-204)

Sheet 3-4 PP (H/W)

Class Activity*

Collect 128 pennies, container, paper, graph paper, calculator, pen/pencil.

Make two-column table with “TRIAL” and “Number of Heads.”

Place pennies in container & shake them up. Pour pennies onto desk. (Keep them under control!) Pick out & count “Heads” and set them aside; record data. Place remaining pennies in container and shake them;

repeat the process five times. Repeat the entire experiment two more times. Graph “Heads” (y-axis) vs. “Trial” and note shape. Add new column “Log # Heads” and finish the table. Graph “Log # Heads’ vs. “Trial” and note shape.

*From Chemistry, Wilbraham, et al., Prentice-Hall, 2002, page 840.

Half-life (t1/2)

Half-life (t1/2) – the time required for one-half of the atoms of a radioactive isotope (radioisotope) to emit radiation and decay to products.

Simulating Radioactive Decay You just won $1,000, but… …you can only spend half of it in month 1… …half of the remainder in month 2, etc. After how many months would you be left with less

than $1? What is the half life for this prize?

Common Half-Lives & Radiation*

ISOTOPE HALF-LIFE RADIATION

Carbon-14 5,730 years Beta

Potassium-40 1,25 X 109 years Beta, gamma

Radon-222 3.8 days Alpha

Radium-226 1,600 years Alpha, gamma

Thorium-230 75,400 years Alpha, gamma

Thorium-234 24.1 days Beta, gamma

Uranium-235 7.0 X 108 years Alpha, gamma

Uranium-238 4.46 X 109 years Alpha

*From Chemistry, Wilbraham, et al., Prentice-Hall, 2002, page 847.

Applications of Nuclear Reactions Dating of ancient artifacts (Carbon-14). Smoke detectors (Americium-241). Radioactive tracers in medicine (Iodine-131,

barium-140, phosphorus-32). Cancer treatment (Cobalt-60). Electricity generation (Uranium-235). Artificial (lab-made) elements (beyond Z = 92). Bombs (Uranium-235). Fusion (Combining two small nuclei to form a

large nucleus.) Interested in learning more? See Chapter 24.

OBJECTIVES

Describe the changes that accompany nuclear reactions.

Define radioactivity. Provide examples of nuclear equations

and applications.