Section 19.1 Radioactivity 1.Students will be able to describe the factors that lead to nuclear stability. 2.Students will be able to describe types of.

Section 19.1

Radioactivity

1. Students will be able to describe the factors that lead to nuclear stability.

2. Students will be able to describe types of radioactive decay.

3. Students will be able to write equations to show radioactive decay.

Objectives

Section 19.1

Radioactivity

• nucleons – particles found in the nucleus of an atom – neutrons – protons

• atomic number (Z) – number of protons in the nucleus

• mass number (A) – sum of the number of protons and neutrons

• isotopes – atoms with identical atomic numbers but different mass numbers

• nuclide – each unique atom

A Review of Atomic Terms

Section 19.1

Radioactivity

Nuclear Stability

• Small atoms tend to have neutron-proton ratios ~ 1:1

• The ratio approaches 1.5:1 as the atomic number increases

• Explained by the relationship between strong nuclear forces and electrostatic repulsion

Section 19.1

Radioactivity

Nuclear Stability

• Strong nuclear forces act over very short distances

• As the # of protons increases, the electrostatic force between protons increases

• More neutrons are needed to stabilize the nucleus

Section 19.1

Radioactivity

A. Radioactive Decay

• radioactive – nucleus which spontaneously decomposes forming a different nucleus and producing one or more particles

• nuclear equation – shows the radioactive decomposition of an element

Section 19.1

Radioactivity

A. Radioactive Decay

• Alpha-particle decay • Alpha particle – helium nucleus

– Examples

Types of Radioactive Decay

• Common for heavy isotopes

Section 19.1

Radioactivity

A. Radioactive Decay

• Beta-particle decay

Types of Radioactive Decay

• Net effect is to change a neutron to a proton.

• Beta particle – electron – Examples

Section 19.1

Radioactivity

A. Radioactive Decay

• Gamma ray release

Types of Radioactive Decay

• Net effect is no change in mass number or atomic number.

• Gamma ray – high energy photon – Examples

Section 19.1

Radioactivity

A. Radioactive Decay

• Positron production

Types of Radioactive Decay

• Net effect is to change a proton to a neutron.

• Positron – particle with same mass as an electron but with a positive charge – Examples

Section 19.1

Radioactivity

A. Radioactive Decay

• Electron capture

Types of Radioactive Decay

– Example

Section 19.1

Radioactivity

A. Radioactive Decay

Section 19.1

Radioactivity

A. Radioactive Decay

Decay series

Section 19.1

Radioactivity

Sample Problems

1. Silver-116 undergoes beta particle decay.

2. Bismuth-211 undergoes alpha particle decay

3. An isotope undergoes alpha particle decay and produces the isotope Polonium-218

4. Silver 106 undergoes electron capture.

5. Potassium-38 undergoes positron emission.

Section 19.1

Radioactivity

Detection of Radioactivity and the Concept of Half- life

Objectives• Students will be able to define the term half-life and

explain how it relates to the stability of the nucleus. • Students will determine the half-life of an isotope or

the fraction of the isotope that remains given appropriate data.

Section 19.1

Radioactivity C. Detection of Radioactivity and the Concept of Half- life • Half-life – time required for half of the original sample of

radioactive nuclides to decay

Section 19.1

Radioactivity

Half Life Problems:

1. Phosphorus-32 has a half-life of 14.3 days. How many days will it take for a radioactive sample to decay to one-eighth its original size.

2. Iodine-131 has a half life of 8.0 days. How many grams of an original 160 mg sample will remain in 40 days?

3. Carbon-14 has a half life of 5715 years. It is used to determine the age of ancient objects. If a sample today contains 0.060 mg of carbon-14, how much carbon-14 must have been present in the sample 11,430 years ago>

Section 19.1

Radioactivity

B. Nuclear Transformations

• Nuclear transformation – change of one element to another

Section 19.1

Radioactivity

B. Nuclear Transformations

Discovery of Protons:

• Rutherford performed the first artificial transmutation, by bombarding nitrogen gas with alpha particles (1919)

HOHeN 11

178

42

147

Section 19.1

Radioactivity

B. Nuclear Transformations

Discovery of Neutrons

• Chadwick discovered the neutron by bombarding Be with alpha particles. (1932)

nCHeBe 10

126

42

94

Section 19.1

Radioactivity

B. Nuclear Transformations

Section 19.1

Radioactivity

B. Nuclear Transformations

Production of Neptunium and Plutonium

UnU 23992

10

23892

eNpU 01

23993

23992

ePuNp 01

23994

23993

Section 19.1

Radioactivity

B. Nuclear Transformations

• Transuranium elements – elements with atomic numbers greater than 92 which have been synthesized

Section 19.1

Radioactivity C. Detection of Radioactivity and the Concept of Half- life • Geiger-Muller counter – instrument which measures

radioactive decay by registering the ions and electrons produced as a radioactive particle passes through a gas-filled chamber

Section 19.1

Radioactivity C. Detection of Radioactivity and the Concept of Half- life • Scintillation counter – instrument which measures the

rate of radioactive decay by sensing flashes of light that the radiation produces in the detector